Vhdl cookbook

The
VHDL
Cookbook
First Edition

Peter J. Ashenden

The VHDL Cookbook
First Edition
July, 1990

Peter J. Ashenden
Dept. Computer Science
University of Adelaide
South Australia

© 1990, Peter J. Ashenden

Contents iii

Contents

1. Introduction............................................................................ 1-1
1.1. Describing Structure ....................................................... 1-2
1.2. Describing Behaviour ...................................................... 1-2
1.3. Discrete Event Time Model............................................... 1-3
1.4. A Quick Example............................................................ 1-3

2. VHDL is Like a Programming Language ................................... 2-1
2.1. Lexical Elements ............................................................ 2-1
2.1.1. Comments .......................................................... 2-1
2.1.2. Identifiers........................................................... 2-1
2.1.3. Numbers ............................................................ 2-1
2.1.4. Characters.......................................................... 2-2
2.1.5. Strings ............................................................... 2-2
2.1.6. Bit Strings........................................................... 2-2
2.2. Data Types and Objects .................................................... 2-2
2.2.1. Integer Types ...................................................... 2-3
2.2.2. Physical Types..................................................... 2-3
2.2.3. Floating Point Types............................................. 2-4
2.2.4. Enumeration Types.............................................. 2-4
2.2.5. Arrays................................................................ 2-5
2.2.6. Records .............................................................. 2-7
2.2.7. Subtypes ............................................................. 2-7
2.2.8. Object Declarations .............................................. 2-8
2.2.9. Attributes ........................................................... 2-8
2.3. Expressions and Operators .............................................. 2-9
2.4. Sequential Statements ....................................................2-10
2.4.1. Variable Assignment..........................................2-10
2.4.2. If Statement .......................................................2-11
2.4.3. Case Statement...................................................2-11
2.4.4. Loop Statements .................................................2-12
2.4.5. Null Statement ...................................................2-13
2.4.6. Assertions .........................................................2-13
2.5. Subprograms and Packages ............................................2-13
2.5.1. Procedures and Functions ...................................2-14
2.5.2. Overloading .......................................................2-16
2.5.3. Package and Package Body Declarations ...............2-17
2.5.4. Package Use and Name Visibility .........................2-18

iv The VHDL Cookbook

Contents (cont'd)

3. VHDL Describes Structure ........................................................3-1
3.1. Entity Declarations ..........................................................3-1
3.2. Architecture Declarations ................................................3-3
3.2.1. Signal Declarations ..............................................3-3
3.2.2. Blocks .................................................................3-4
3.2.3. Component Declarations.......................................3-5
3.2.4. Component Instantiation ......................................3-6

4. VHDL Describes Behaviour .......................................................4-1
4.1. Signal Assignment..........................................................4-1
4.2. Processes and the Wait Statement .....................................4-2
4.3. Concurrent Signal Assignment Statements........................4-4
4.3.1. Conditional Signal Assignment .............................4-5
4.3.2. Selected Signal Assignment ..................................4-6

5. Model Organisation ..................................................................5-1
5.1. Design Units and Libraries...............................................5-1
5.2. Configurations................................................................5-2
5.3. Complete Design Example................................................5-5

6. Advanced VHDL ......................................................................6-1
6.1. Signal Resolution and Buses .............................................6-1
6.2. Null Transactions ...........................................................6-2
6.3. Generate Statements........................................................6-2
6.4. Concurrent Assertions and Procedure Calls.......................6-3
6.5. Entity Statements ............................................................6-4

7. Sample Models: The DP32 Processor...........................................7-1
7.1. Instruction Set Architecture.............................................7-1
7.2. Bus Architecture.............................................................7-4
7.3. Types and Entity..............................................................7-6
7.4. Behavioural Description...................................................7-9
7.5. Test Bench.................................................................... 7-18
7.6. Register Transfer Architecture....................................... 7-24
7.6.1. Multiplexor ....................................................... 7-25
7.6.2. Transparent Latch ............................................. 7-25
7.6.3. Buffer ............................................................... 7-26
7.6.4. Sign Extending Buffer......................................... 7-28
7.6.5. Latching Buffer.................................................. 7-28
7.6.6. Program Counter Register .................................. 7-28
7.6.7. Register File ...................................................... 7-29

Contents v

Contents (cont'd)

7.6.8. Arithmetic & Logic Unit ......................................7-30
7.6.9. Condition Code Comparator .................................7-34
7.6.10. Structural Architecture of the DP32 ......................7-34

1 . Introduction

VHDL is a language for describing digital electronic systems. It arose
out of the United States Government’s Very High Speed Integrated Circuits
(VHSIC) program, initiated in 1980. In the course of this program, it
became clear that there was a need for a standard language for describing
the structure and function of integrated circuits (ICs). Hence the VHSIC
Hardware Description Language (VHDL) was developed, and subsequently
adopted as a standard by the Institute of Electrical and Electronic
Engineers (IEEE) in the US.
VHDL is designed to fill a number of needs in the design process.
Firstly, it allows description of the structure of a design, that is how it is
decomposed into sub-designs, and how those sub-designs are
interconnected. Secondly, it allows the specification of the function of
designs using familiar programming language forms. Thirdly, as a
result, it allows a design to be simulated before being manufactured, so that
designers can quickly compare alternatives and test for correctness without
the delay and expense of hardware prototyping.
The purpose of this booklet is to give you a quick introduction to VHDL.
This is done by informally describing the facilities provided by the
language, and using examples to illustrate them. This booklet does not
fully describe every aspect of the language. For such fine details, you
should consult the IEEE Standard VHDL Language Reference Manual.
However, be warned: the standard is like a legal document, and is very
difficult to read unless you are already familiar with the language. This
booklet does cover enough of the language for substantial model writing. It
assumes you know how to write computer programs using a conventional
programming language such as Pascal, C or Ada.
The remaining chapters of this booklet describe the various aspects of
VHDL in a bottom-up manner. Chapter2 describes the facilities of VHDL
which most resemble normal sequential programming languages. These
include data types, variables, expressions, sequential statements and
subprograms. Chapter3 then examines the facilities for describing the
structure of a module and how it it decomposed into sub-modules.
Chapter4 covers aspects of VHDL that integrate the programming
language features with a discrete event timing model to allow simulation of
behaviour. Chapter5 is a key chapter that shows how all these facilities are
combined to form a complete model of a system. Then Chapter6 is a pot-
pourri of more advanced features which you may find useful for modeling
more complex systems.
Throughout this booklet, the syntax of language features is presented in
Backus-Naur Form (BNF). The syntax specifications are drawn from the
IEEE VHDL Standard. Concrete examples are also given to illustrate the
language features. In some cases, some alternatives are omitted from BNF

1-1

1-2 The VHDL Cookbook

A A F
Y
B G
A A
Y Y Y
F B I
B
A
Y
(a) B B H

(b)

Figure 1-1. Example of a structural description.

productions where they are not directly relevant to the context. For this
reason, the full syntax is included in AppendixA, and should be consulted
as a reference.

1.1. Describing Structure
A digital electronic system can be described as a module with inputs
and/or outputs. The electrical values on the outputs are some function of
the values on the inputs. Figure1-1(a) shows an example of this view of a
digital system. The module F has two inputs, A and B, and an output Y.
Using VHDL terminology, we call the module F a design entity, and the
inputs and outputs are called ports.
One way of describing the function of a module is to describe how it is
composed of sub-modules. Each of the sub-modules is an instance of some
entity, and the ports of the instances are connected using signals.
Figure1-1(b) shows how the entity F might be composed of instances of
entities G, H and I. This kind of description is called a structural
description. Note that each of the entities G, H and I might also have a
structural description.

1.2. Describing Behaviour
In many cases, it is not appropriate to describe a module structurally.
One such case is a module which is at the bottom of the hierarchy of some
other structural description. For example, if you are designing a system
using IC packages bought from an IC shop, you do not need to describe the
internal structure of an IC. In such cases, a description of the function
performed by the module is required, without reference to its actual
internal structure. Such a description is called a functional or behavioural
description.
To illustrate this, suppose that the function of the entity F in
Figure1-1(a) is the exclusive-or function. Then a behavioural description of
F could be the Boolean function
Y= A . B+ A. B
More complex behaviours cannot be described purely as a function of
inputs. In systems with feedback, the outputs are also a function of time.
VHDL solves this problem by allowing description of behaviour in the form

1. Introduction 1-3

of an executable program. Chapters2 and4 describe the programming
language facilities.

1.3. Discrete Event Time Model
Once the structure and behaviour of a module have been specified, it is
possible to simulate the module by executing its bevioural description. This
is done by simulating the passage of time in discrete steps. At some
simulation time, a module input may be stimulated by changing the value
on an input port. The module reacts by running the code of its behavioural
description and scheduling new values to be placed on the signals
connected to its output ports at some later simulated time. This is called
scheduling a transaction on that signal. If the new value is different from
the previous value on the signal, an event occurs, and other modules with
input ports connected to the signal may be activated.
The simulation starts with an initialisation phase, and then proceeds by
repeating a two-stage simulation cycle. In the initialisation phase, all
signals are given initial values, the simulation time is set to zero, and each
module’s behaviour program is executed. This usually results in
transactions being scheduled on output signals for some later time.
In the first stage of a simulation cycle, the simulated time is advanced to
the earliest time at which a transaction has been scheduled. All
transactions scheduled for that time are executed, and this may cause
events to occur on some signals.
In the second stage, all modules which react to events occurring in the
first stage have their behaviour program executed. These programs will
usually schedule further transactions on their output signals. When all of
the behaviour programs have finished executing, the simulation cycle
repeats. If there are no more scheduled transactions, the whole simulation
is completed.
The purpose of the simulation is to gather information about the
changes in system state over time. This can be done by running the
simulation under the control of a simulation monitor. The monitor allows
signals and other state information to be viewed or stored in a trace file for
later analysis. It may also allow interactive stepping of the simulation
process, much like an interactive program debugger.

1.4. A Quick Example
In this section we will look at a small example of a VHDL description of
a two-bit counter to give you a feel for the language and how it is used. We
start the description of an entity by specifying its external interface, which
includes a description of its ports. So the counter might be defined as:
entity count2 is
generic (prop_delay : Time := 10 ns);
port (clock : in bit;
q1, q0 : out bit);
end count2;
This specifies that the entity count2 has one input and two outputs, all of
which are bit values, that is, they can take on the values '0' or '1'. It also
defines a generic constant called prop_delay which can be used to control the
operation of the entity (in this case its propagation delay). If no value is


BIT_0 COUNT2
T_FLIPFLOP
CLOCK Q0
FF0
CK Q

BIT_1
INV T_FLIPFLOP
INVERTER Q1
INV_FF0 FF1
A Y CK Q

Figure1-2. Structure of count2.

explicitly given for this value when the entity is used in a design, the default
value of 10ns will be used.
An implementation of the entity is described in an architecture body.
There may be more than one architecture body corresponding to a single
entity specification, each of which describes a different view of the entity.
For example, a behavioural description of the counter could be written as:
architecture behaviour of count2 is
begin
count_up: process (clock)
variable count_value : natural := 0;
begin
if clock = '1' then
count_value := (count_value + 1) mod 4;
q0 <= bit'val(count_value mod 2) after prop_delay;
q1 <= bit'val(count_value / 2) after prop_delay;
end if;
end process count_up;
end behaviour;
In this description of the counter, the behaviour is implemented by a
process called count_up, which is sensitive to the input clock. A process is a
body of code which is executed whenever any of the signals it is sensitive to
changes value. This process has a variable called count_value to store the
current state of the counter. The variable is initialized to zero at the start of
simulation, and retains its value between activations of the process. When
the clock input changes from '0' to '1', the state variable is incremented, and
transactions are scheduled on the two output ports based on the new value.
The assignments use the generic constant prop_delay to determine how long
after the clock change the transaction should be scheduled. When control
reaches the end of the process body, the process is suspended until another
change occurs on clock.
The two-bit counter might also be described as a circuit composed of two
T-flip-flops and an inverter, as shown in Figure1-2. This can be written in
VHDL as:

1. Introduction 1-5

architecture structure of count2 is
component t_flipflop
port (ck : in bit; q : out bit);
end component;
component inverter
port (a : in bit; y : out bit);
end component;
signal ff0, ff1, inv_ff0 : bit;
begin
bit_0 : t_flipflop port map (ck => clock, q => ff0);
inv : inverter port map (a => ff0, y => inv_ff0);
bit_1 : t_flipflop port map (ck => inv_ff0, q => ff1);
q0 <= ff0;
q1 <= ff1;
end structure;
In this architecture, two component types are declared, t_flipflop and
inverter, and three internal signals are declared. Each of the components is
then instantiated, and the ports of the instances are mapped onto signals
and ports of the entity. For example, bit_0 is an instance of the t_flipflop
component, with its ck port connected to the clock port of the count2 entity,
and its q port connected to the internal signal ff0. The last two signal
assignments update the entity ports whenever the values on the internal
signals change.

2. VHDL is Like a Programming Language

As mentioned in Section 1.2, the behaviour of a module may be described
in programming language form. This chapter describes the facilities in
VHDL which are drawn from the familiar programming language
repertoire. If you are familiar with the Ada programming language, you
will notice the similarity with that language. This is both a convenience
and a nuisance. The convenience is that you don’t have much to learn to
use these VHDL facilities. The problem is that the facilities are not as
comprehensive as those of Ada, though they are certainly adequate for most
modeling purposes.

2.1. Lexical Elements
2.1.1. Comments
Comments in VHDL start with two adjacent hyphens (‘--’) and extend to
the end of the line. They have no part in the meaning of a VHDL
description.
2.1.2. Identifiers
Identifiers in VHDL are used as reserved words and as programmer
defined names. They must conform to the rule:
identifier ::= letter { [ underline ] letter_or_digit }
Note that case of letters is not considered significant, so the identifiers cat
and Cat are the same. Underline characters in identifiers are significant,
so This_Name and ThisName are different identifiers.
2.1.3. Numbers
Literal numbers may be expressed either in decimal or in a base
between two and sixteen. If the literal includes a point, it represents a real
number, otherwise it represents an integer. Decimal literals are defined
by:
decimal_literal ::= integer [ . integer ] [ exponent ]
integer ::= digit { [ underline ] digit }
exponent ::= E [ + ] integer | E - integer
Some examples are:
0 1 123_456_789 987E6 -- integer literals
0.0 0.5 2.718_28 12.4E-9 -- real literals
Based literal numbers are defined by:
based_literal ::= base # based_integer [ . based_integer ] # [ exponent ]
base ::= integer
based_integer ::= extended_digit { [ underline ] extended_digit }

2-1


extended_digit ::= digit | letter
The base and the exponent are expressed in decimal. The exponent
indicates the power of the base by which the literal is multiplied. The
letters A to F (upper or lower case) are used as extended digits to represent
10 to 15. Some examples:
2#1100_0100# 16#C4# 4#301#E1 -- the integer 196
2#1.1111_1111_111#E+11 16#F.FF#E2 -- the real number 4095.0

2.1.4. Characters
Literal characters are formed by enclosing an ASCII character in
single-quote marks. For example:
'A' '*' ''' ' '

2.1.5. Strings
Literal strings of characters are formed by enclosing the characters in
double-quote marks. To include a double-quote mark itself in a string, a
pair of double-quote marks must be put together. A string can be used as a
value for an object which is an array of characters. Examples of strings:
"A string"
"" -- empty string
"A string in a string: ""A string"". " -- contains quote marks

2.1.6. Bit Strings
VHDL provides a convenient way of specifying literal values for arrays of
type bit ('0's and '1's, see Section 2.2.5). The syntax is:
bit_string_literal ::= base_specifier " bit_value "
base_specifier ::= B | O | X
bit_value ::= extended_digit { [ underline ] extended_digit }
Base specifier B stands for binary, O for octal and X for hexadecimal. Some
examples:
B"1010110" -- length is 7
O"126" -- length is 9, equivalent to B"001_010_110"
X"56" -- length is 8, equivalent to B"0101_0110"

2.2. Data Types and Objects
VHDL provides a number of basic, or scalar, types, and a means of
forming composite types. The scalar types include numbers, physical
quantities, and enumerations (including enumerations of characters), and
there are a number of standard predefined basic types. The composite types
provided are arrays and records. VHDL also provides access types
(pointers) and files, although these will not be fully described in this booklet.
A data type can be defined by a type declaration:
full_type_declaration ::= type identifier is type_definition ;
type_definition ::=
scalar_type_definition
| composite_type_definition
| access_type_definition
| file_type_definition
scalar_type_definition ::=
enumeration_type_definition | integer_type_definition
| floating_type_definition | physical_type_definition

2. VHDL is Like a Programming Language 2-3

composite_type_definition ::=
array_type_definition
| record_type_definition
Examples of different kinds of type declarations are given in the following
sections.
2.2.1. Integer Types
An integer type is a range of integer values within a specified range.
The syntax for specifying integer types is:
integer_type_definition ::= range_constraint
range_constraint ::= range range
range ::= simple_expression direction simple_expression
direction ::= to | downto
The expressions that specify the range must of course evaluate to integer
numbers. Types declared with the keyword to are called ascending ranges,
and those declared with the keyword downto are called descending ranges.
The VHDL standard allows an implementation to restrict the range, but
requires that it must at least allow the range –2147483647 to +2147483647.
Some examples of integer type declarations:
type byte_int is range 0 to 255;
type signed_word_int is range –32768 to 32767;
type bit_index is range 31 downto 0;
There is a predefined integer type called integer. The range of this type is
implementation defined, though it is guaranteed to include –2147483647 to
+2147483647.
2.2.2. Physical Types
A physical type is a numeric type for representing some physical
quantity, such as mass, length, time or voltage. The declaration of a
physical type includes the specification of a base unit, and possibly a
number of secondary units, being multiples of the base unit. The syntax for
declaring physical types is:
physical_type_definition ::=
range_constraint
units
base_unit_declaration
{ secondary_unit_declaration }
end units
base_unit_declaration ::= identifier ;
secondary_unit_declaration ::= identifier = physical_literal ;
physical_literal ::= [ abstract_literal ] unit_name
Some examples of physical type declarations:


type length is range 0 to 1E9
units
um;
mm = 1000 um;
cm = 10 mm;
m = 1000 mm;
in = 25.4 mm;
ft = 12 in;
yd = 3 ft;
rod = 198 in;
chain = 22 yd;
furlong = 10 chain;
end units;
type resistance is range 0 to 1E8
units
ohms;
kohms = 1000 ohms;
Mohms = 1E6 ohms;
end units;
The predefined physical type time is important in VHDL, as it is used
extensively to specify delays in simulations. Its definition is:
type time is range implementation_defined
units
fs;
ps = 1000 fs;
ns = 1000 ps;
us = 1000 ns;
ms = 1000 us;
sec = 1000 ms;
min = 60 sec;
hr = 60 min;
end units;
To write a value of some physical type, you write the number followed by
the unit. For example:
10 mm 1 rod 1200 ohm 23 ns

2.2.3. Floating Point Types
A floating point type is a discrete approximation to the set of real
numbers in a specified range. The precision of the approximation is not
defined by the VHDL language standard, but must be at least six decimal
digits. The range must include at least –1E38 to +1E38. A floating point
type is declared using the syntax:
floating_type_definition := range_constraint
Some examples are:
type signal_level is range –10.00 to +10.00;
type probability is range 0.0 to 1.0;
There is a predefined floating point type called real. The range of this
type is implementation defined, though it is guaranteed to include –1E38 to
+1E38.
2.2.4. Enumeration Types
An enumeration type is an ordered set of identifiers or characters. The
identifiers and characters within a single enumeration type must be
distinct, however they may be reused in several different enumeration
types.


The syntax for declaring an enumeration type is:
enumeration_type_definition ::= ( enumeration_literal { , enumeration_literal } )
enumeration_literal ::= identifier | character_literal
Some examples are:
type logic_level is (unknown, low, undriven, high);
type alu_function is (disable, pass, add, subtract, multiply, divide);
type octal_digit is ('0', '1', '2', '3', '4', '5', '6', '7');
There are a number of predefined enumeration types, defined as follows:
type severity_level is (note, warning, error, failure);
type boolean is (false, true);
type bit is ('0', '1');
type character is (
NUL, SOH, STX, ETX, EOT, ENQ, ACK, BEL,
BS, HT, LF, VT, FF, CR, SO, SI,
DLE, DC1, DC2, DC3, DC4, NAK, SYN, ETB,
CAN, EM, SUB, ESC, FSP, GSP, RSP, USP,
' ', '!', '"', '#', '$', '%', '&', ''',
'(', ')', '*', '+', ',', '-', '.', '/',
'0', '1', '2', '3', '4', '5', '6', '7',
'8', '9', ':', ';', '<', '=', '>', '?',
'@', 'A', 'B', 'C', 'D', 'E', 'F', 'G',
'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O',
'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W',
'X', 'Y', 'Z', '[', '', ']', '^', '_',
'`', 'a', 'b', 'c', 'd', 'e', 'f', 'g',
'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o',
'p', 'q', 'r', 's', 't', 'u', 'v', 'w',
'x', 'y', 'z', '{', '|', '}', '~', DEL);
Note that type character is an example of an enumeration type containing a
mixture of identifiers and characters. Also, the characters '0' and '1' are
members of both bit and character . Where '0' or '1' occur in a program, the
context will be used to determine which type is being used.
2.2.5. Arrays
An array in VHDL is an indexed collection of elements all of the same
type. Arrays may be one-dimensional (with one index) or multi-
dimensional (with a number of indices). In addition, an array type may be
constrained, in which the bounds for an index are established when the
type is defined, or unconstrained, in which the bounds are established
subsequently.
The syntax for declaring an array type is:
array_type_definition ::=
unconstrained_array_definition | constrained_array_definition
unconstrained_array_definition ::=
array ( index_subtype_definition { , index_subtype_definition } )
of element_subtype_indication
constrained_array_definition ::=
array index_constraint of element_subtype_indication
index_subtype_definition ::= type_mark range <>
index_constraint ::= ( discrete_range { , discrete_range } )
discrete_range ::= discrete_subtype_indication | range


Subtypes, referred to in this syntax specification, will be discussed in detail
in Section2.2.7.
Some examples of constrained array type declarations:
type word is array (31 downto 0) of bit;
type memory is array (address) of word;
type transform is array (1 to 4, 1 to 4) of real;
type register_bank is array (byte range 0 to 132) of integer;
An example of an unconstrained array type declaration:
type vector is array (integer range <>) of real;
The symbol ‘<>’ (called a box) can be thought of as a place-holder for the
index range, which will be filled in later when the array type is used. For
example, an object might be declared to be a vector of 20 elements by giving
its type as:
vector(1 to 20)
There are two predefined array types, both of which are unconstrained.
They are defined as:
type string is array (positive range <>) of character;
type bit_vector is array (natural range <>) of bit;
The types positive and natural are subtypes of integer, defined in Section2.2.7
below. The type bit_vector is particularly useful in modeling binary coded
representations of values in simulations of digital systems.
An element of an array object can referred to by indexing the name of
the object. For example, suppose a and b are one- and two-dimensional
array objects respectively. Then the indexed names a(1) and b(1, 1) refer to
elements of these arrays. Furthermore, a contiguous slice of a one-
dimensional array can be referred to by using a range as an index. For
example a(8 to 15) is an eight-element array which is part of the array a.
Sometimes you may need to write a literal value of an array type. This
can be done using an array aggregate, which is a list of element values.
Suppose we have an array type declared as:
type a is array (1 to 4) of character;
and we want to write a value of this type containing the elements 'f', 'o', 'o',
'd' in that order. We could write an aggregate with positional association
as follows:
('f', 'o', 'o', 'd')
in which the elements are listed in the order of the index range, starting
with the left bound of the range. Alternatively, we could write an aggregate
with named association:
(1 => 'f', 3 => 'o', 4 => 'd', 2 => 'o')
In this case, the index for each element is explicitly given, so the elements
can be in any order. Positional and named association can be mixed within
an aggregate, provided all the positional associations come first. Also, the
word others can be used in place of an index in a named association,
indicating a value to be used for all elements not explicitly mentioned. For
example, the same value as above could be written as:
('f', 4 => 'd', others => 'o')


2.2.6. Records
VHDL provides basic facilities for records, which are collections of
named elements of possibly different types. The syntax for declaring record
types is:
record_type_definition ::=
record
element_declaration
{ element_declaration }
end record
element_declaration ::= identifier_list : element_subtype_definition ;
identifier_list ::= identifier { , identifier )
element_subtype_definition ::= subtype_indication
An example record type declaration:
type instruction is
record
op_code : processor_op;
address_mode : mode;
operand1, operand2: integer range 0 to 15;
end record;
When you need to refer to a field of a record object, you use a selected
name. For example, suppose that r is a record object containing a field
called f. Then the name r.f refers to that field.
As for arrays, aggregates can be used to write literal values for records.
Both positional and named association can be used, and the same rules
apply, with record field names being used in place of array index names.
2.2.7. Subtypes
The use of a subtype allows the values taken on by an object to be
restricted or constrained subset of some base type. The syntax for declaring
a subtype is:
subtype_declaration ::= subtype identifier is subtype_indication ;
subtype_indication ::= [ resolution_function_name ] type_mark [ constraint ]
type_mark ::= type_name | subtype_name
constraint ::= range_constraint | index_constraint
There are two cases of subtypes. Firstly a subtype may constrain values
from a scalar type to be within a specified range (a range constraint). For
example:
subtype pin_count is integer range 0 to 400;
subtype digits is character range '0' to '9';
Secondly, a subtype may constrain an otherwise unconstrained array
type by specifying bounds for the indices. For example:
subtype id is string(1 to 20);
subtype word is bit_vector(31 downto 0);
There are two predefined numeric subtypes, defined as:
subtype natural is integer range 0 to highest_integer
subtype positive is integer range 1 to highest_integer


2.2.8. Object Declarations
An object is a named item in a VHDL description which has a value of a
specified type. There are three classes of objects: constants, variables and
signals. Only the first two will be discusses in this section; signals will be
covered in Section3.2.1. Declaration and use of constants and variables is
very much like their use in programming languages.
A constant is an object which is initialised to a specified value when it is
created, and which may not be subsequently modified. The syntax of a
constant declaration is:
constant_declaration ::=
constant identifier_list : subtype_indication [ := expression ] ;
Constant declarations with the initialising expression missing are called
deferred constants, and may only appear in package declarations (see
Section2.5.3). The initial value must be given in the corresponding package
body. Some examples:
constant e : real := 2.71828;
constant delay : Time := 5 ns;
constant max_size : natural;
A variable is an object whose value may be changed after it is created.
The syntax for declaring variables is:
variable_declaration ::=
variable identifier_list : subtype_indication [ := expression ] ;
The initial value expression, if present, is evaluated and assigned to the
variable when it is created. If the expression is absent, a default value is
assigned when the variable is created. The default value for scalar types is
the leftmost value for the type, that is the first in the list of an enumeration
type, the lowest in an ascending range, or the highest in a descending
range. If the variable is a composite type, the default value is the
composition of the default values for each element, based on the element
types.
Some examples of variable declarations:
variable count : natural := 0;
variable trace : trace_array;
Assuming the type trace_array is an array of boolean, then the initial value of
the variable trace is an array with all elements having the value false.
Given an existing object, it is possible to give an alternate name to the
object or part of it. This is done using and alias declaration. The syntax is:
alias_declaration ::= alias identifier : subtype_indication is name ;
A reference to an alias is interpreted as a reference to the object or part
corresponding to the alias. For example:
variable instr : bit_vector(31 downto 0);
alias op_code : bit_vector(7 downto 0) is instr(31 downto 24);
declares the name op_code to be an alias for the left-most eight bits of instr.
2.2.9. Attributes
Types and objects declared in a VHDL description can have additional
information, called attributes, associated with them. There are a number
of standard pre-defined attributes, and some of those for types and arrays


are discussed here. An attribute is referenced using the ‘'’ notation. For
example,
thing'attr
refers to the attribute attr of the type or object thing.
Firstly, for any scalar type or subtype T, the following attributes can be
used:
Attribute Result
T'left Left bound of T
T'right Right bound of T
T'low Lower bound of T
T'high Upper bound of T
For an ascending range, T'left = T'low, and T'right = T'high. For a
descending range, T'left = T'high, and T'right = T'low.
Secondly, for any discrete or physical type or subtype T, X a member of T,
and N an integer, the following attributes can be used:
Attribute Result
T'pos(X) Position number of X in T
T'val(N) Value at position N in T
T'leftof(X) Value in T which is one position left from X
T'rightof(X) Value in T which is one position right from X
T'pred(X) Value in T which is one position lower than X
T'succ(X) Value in T which is one position higher than X
For an ascending range, T'leftof(X) = T'pred(X), and T'rightof(X) =
T'succ(X). For a descending range, T'leftof(X) = T'succ(X), and T'rightof(X)
= T'pred(X).
Thirdly, for any array type or object A, and N an integer between 1 and
the number of dimensions of A, the following attributes can be used:
Attribute Result
A'left(N) Left bound of index range of dim’n N of A
A'right(N) Right bound of index range of dim’n N of A
A'low(N) Lower bound of index range of dim’n N of A
A'high(N) Upper bound of index range of dim’n N of A
A'range(N) Index range of dim’n N of A
A'reverse_range(N) Reverse of index range of dim’n N of A
A'length(N) Length of index range of dim’n N of A

2.3. Expressions and Operators
Expressions in VHDL are much like expressions in other programming
languages. An expression is a formula combining primaries with
operators. Primaries include names of objects, literals, function calls and
parenthesized expressions. Operators are listed in Table 2-1 in order of
decreasing precedence.
The logical operators and, or, nand, nor, xor and not operate on values of
type bit or boolean, and also on one-dimensional arrays of these types. For
array operands, the operation is applied between corresponding elements of
each array, yielding an array of the same length as the result. For bit and


Highest precedence: ** abs not
* / mod rem
+ (sign) – (sign)
+ – &
= /= < <= > >=
Lowest precedence: and or nand nor xor

Table 7-1. Operators and precedence.

boolean operands, and , or, nand, and nor are ‘short-circuit’ operators, that
is they only evaluate their right operand if the left operand does not
determine the result. So and and nand only evaluate the right operand if
the left operand is true or '1', and or and nor only evaluate the right
operand if the left operand is false or '0'.
The relational operators =, /=, <, <=, > and >= must have both operands
of the same type, and yield boolean results. The equality operators (= and /=)
can have operands of any type. For composite types, two values are equal if
all of their corresponding elements are equal. The remaining operators
must have operands which are scalar types or one-dimensional arrays of
discrete types.
The sign operators (+ and –) and the addition (+) and subtraction (–)
operators have their usual meaning on numeric operands. The
concatenation operator (&) operates on one-dimensional arrays to form a
new array with the contents of the right operand following the contents of
the left operand. It can also concatenate a single new element to an array,
or two individual elements to form an array. The concatenation operator is
most commonly used with strings.
The multiplication (*) and division (/) operators work on integer, floating
point and physical types types. The modulus (mod) and remainder (rem)
operators only work on integer types. The absolute value (abs) operator
works on any numeric type. Finally, the exponentiation (**) operator can
have an integer or floating point left operand, but must have an integer
right operand. A negative right operand is only allowed if the left operand
is a floating point number.

2.4. Sequential Statements
VHDL contains a number of facilities for modifying the state of objects
and controlling the flow of execution of models. These are discussed in this
section.
2.4.1. Variable Assignment
As in other programming languages, a variable is given a new value
using an assignment statement. The syntax is:
variable_assignment_statement ::= target := expression ;
target ::= name | aggregate
In the simplest case, the target of the assignment is an object name, and
the value of the expression is given to the named object. The object and the
value must have the same base type.


If the target of the assignment is an aggregate, then the elements listed
must be object names, and the value of the expression must be a composite
value of the same type as the aggregate. Firstly, all the names in the
aggregate are evaluated, then the expression is evaluated, and lastly the
components of the expression value are assigned to the named variables.
This is effectively a parallel assignment. For example, if a variable r is a
record with two fields a and b, then they could be exchanged by writing
(a => r.b, b => r.a) := r
(Note that this is an example to illustrate how such an assignment works;
it is not an example of good programming practice!)
2.4.2. If Statement
The if statement allows selection of statements to execute depending on
one or more conditions. The syntax is:
if_statement ::=
if condition then
sequence_of_statements
{ elsif condition then
sequence_of_statements }
[ else
sequence_of_statements ]
end if ;
The conditions are expressions resulting in boolean values. The
conditions are evaluated successively until one found that yields the value
true. In that case the corresponding statement list is executed. Otherwise,
if the else clause is present, its statement list is executed.
2.4.3. Case Statement
The case statement allows selection of statements to execute depending
on the value of a selection expression. The syntax is:
case_statement ::=
case expression is
case_statement_alternative
{ case_statement_alternative }
end case ;
case_statement_alternative ::=
when choices =>
choices ::= choice { | choice }
choice ::=
simple_expression
| discrete_range
| element_simple_name
| others
The selection expression must result in either a discrete type, or a one-
dimensional array of characters. The alternative whose choice list
includes the value of the expression is selected and the statement list
executed. Note that all the choices must be distinct, that is, no value may be
duplicated. Furthermore, all values must be represented in the choice
lists, or the special choice others must be included as the last alternative. If
no choice list includes the value of the expression, the others alternative is
selected. If the expression results in an array, then the choices may be
strings or bit strings.


Some examples of case statements:
case element_colour of
when red =>
statements for red;
when green | blue =>
statements for green or blue;
when orange to turquoise =>
statements for these colours;
end case;
case opcode of
when X"00" => perform_add;
when X"01" => perform_subtract;
when others => signal_illegal_opcode;
end case;

2.4.4. Loop Statements
VHDL has a basic loop statement, which can be augmented to form the
usual while and for loops seen in other programming languages. The
syntax of the loop statement is:
loop_statement ::=
[ loop_label : ]
[ iteration_scheme ] loop
end loop [ loop_label ] ;
iteration_scheme ::=
while condition
| for loop_parameter_specification
parameter_specification ::=
identifier in discrete_range
If the iteration scheme is omitted, we get a loop which will repeat the
enclosed statements indefinitely. An example of such a basic loop is:
loop
do_something;
end loop;
The while iteration scheme allows a test condition to be evaluated before
each iteration. The iteration only proceeds if the test evaluates to true. If
the test is false, the loop statement terminates. An example:
while index < length and str(index) /= ' ' loop
index := index + 1;
end loop;
The for iteration scheme allows a specified number of iterations. The
loop parameter specification declares an object which takes on successive
values from the given range for each iteration of the loop. Within the
statements enclosed in the loop, the object is treated as a constant, and so
may not be assigned to. The object does not exist beyond execution of the
loop statement. An example:
for item in 1 to last_item loop
table(item) := 0;
end loop;
There are two additional statements which can be used inside a loop to
modify the basic pattern of iteration. The ‘next’ statement terminates
execution of the current iteration and starts the subsequent iteration. The


‘exit’ statement terminates execution of the current iteration and
terminates the loop. The syntax of these statements is:
next_statement ::= next [ loop_label ] [ when condition ] ;
exit_statement ::= exit [ loop_label ] [ when condition ] ;
If the loop label is omitted, the statement applies to the inner-most
enclosing loop, otherwise it applies to the named loop. If the when clause is
present but the condition is false, the iteration continues normally. Some
examples:
for i in 1 to max_str_len loop
a(i) := buf(i);
exit when buf(i) = NUL;
end loop;
outer_loop : loop
inner_loop : loop
do_something;
next outer_loop when temp = 0;
do_something_else;
end loop inner_loop;
end loop outer_loop;

2.4.5. Null Statement
The null statement has no effect. It may be used to explicitly show that
no action is required in certain cases. It is most often used in case
statements, where all possible values of the selection expression must be
listed as choices, but for some choices no action is required. For example:
case controller_command is
when forward => engage_motor_forward;
when reverse => engage_motor_reverse;
when idle => null;
end case;

2.4.6. Assertions
An assertion statement is used to verify a specified condition and to
report if the condition is violated. The syntax is:
assertion_statement ::=
assert condition
[ report expression ]
[ severity expression ] ;
If the report clause is present, the result of the expression must be a string.
This is a message which will be reported if the condition is false. If it is
omitted, the default message is "Assertion violation". If the severity clause
is present the expression must be of the type severity_level. If it is omitted,
the default is error. A simulator may terminate execution if an assertion
violation occurs and the severity value is greater than some
implementation dependent threshold. Usually the threshold will be under
user control.

2.5. Subprograms and Packages
Like other programming languages, VHDL provides subprogram
facilities in the form of procedures and functions. VHDL also provided a
package facility for collecting declarations and objects into modular units.
Packages also provide a measure of data abstraction and information
hiding.


2.5.1. Procedures and Functions
Procedure and function subprograms are declared using the syntax:
subprogram_declaration ::= subprogram_specification ;
subprogram_specification ::=
procedure designator [ ( formal_parameter_list ) ]
| function designator [ ( formal_parameter_list ) ] return type_mark
A subprogram declaration in this form simply names the subprogram and
specifies the parameters required. The body of statements defining the
behaviour of the subprogram is deferred. For function subprograms, the
declaration also specifies the type of the result returned when the function
is called. This form of subprogram declaration is typically used in package
specifications (see Section 2.5.3), where the subprogram body is given in the
package body, or to define mutually recursive procedures.
The syntax for specifying the formal parameters of a subprogram is:
formal_parameter_list ::= parameter_interface_list
interface_list ::= interface_element { ; interface_element }
interface_element ::= interface_declaration
interface_declaration ::=
interface_constant_declaration
| interface_signal_declaration
| interface_variable_declaration
interface_constant_declaration ::=
[ constant ] identifier_list : [ in ] subtype_indication [ := static_expression ]
interface_variable_declaration ::=
[ variable ] identifier_list : [ mode ] subtype_indication [ := static_expression ]
For now we will only consider constant and variable parameters, although
signals can also be used(see Chapter3). Some examples will clarify this
syntax. Firstly, a simple example of a procedure with no parameters:
procedure reset;
This simply defines reset as a procedure with no parameters, whose
statement body will be given subsequently in the VHDL program. A
procedure call to reset would be:
reset;
Secondly, here is a declaration of a procedure with some parameters:
procedure increment_reg(variable reg : inout word_32;
constant incr : in integer := 1);
In this example, the procedure increment_reg has two parameters, the
first called reg and the second called incr. Reg is a variable parameter,
which means that in the subprogram body, it is treated as a variable object
and may be assigned to. This means that when the procedure is called, the
actual parameter associated with reg must itself be a variable. The mode of
reg is inout, which means that reg can be both read and assigned to. Other
possible modes for subprogram parameters are in, which means that the
parameter may only be read, and out, which means that the parameter
may only be assigned to. If the mode is inout or out, then the word variable
can be omitted and is assumed.
The second parameter, incr, is a constant parameter, which means that
it is treated as a constant object in the subprogram statement body, and may
not be assigned to. The actual parameter associated with incr when the
procedure is called must be an expression. Given the mode of the


parameter, in, the word constant could be omitted and assumed. The
expression after the assignment operator is a default expression, which is
used if no actual parameter is associated with incr in a call to the procedure.
A call to a subprogram includes a list of actual parameters to be
associated with the formal parameters. This association list can be
position, named, or a combination of both. (Compare this with the format of
aggregates for values of composite types.) A call with positional association
lists the actual parameters in the same order as the formals. For example:
increment_reg(index_reg, offset–2); -- add value to index_reg
increment_reg(prog_counter); -- add 1 (default) to prog_counter
A call with named association explicitly gives the formal parameter name
to be associated with each actual parameter, so the parameters can be in
any order. For example:
increment_reg(incr => offset–2, reg => index_reg);
increment_reg(reg => prog_counter);
Note that the second call in each example does not give a value for the
formal parameter incr, so the default value is used.
Thirdly, here is an example of function subprogram declaration:
function byte_to_int(byte : word_8) return integer;
The function has one parameter. For functions, the parameter mode must
be in, and this is assumed if not explicitly specified. If the parameter class
is not specified it is assumed to be constant. The value returned by the body
of this function must be an integer.
When the body of a subprogram is specified, the syntax used is:
subprogram_body ::=
subprogram_specification is
subprogram_declarative_part
begin
subprogram_statement_part
end [ designator ] ;
subprogram_declarative_part ::= { subprogram_declarative_item }
subprogram_statement_part ::= { sequential_statement }
subprogram_declarative_item ::=
subprogram_declaration
| subprogram_body
| type_declaration
| subtype_declaration
| constant_declaration
| variable_declaration
| alias_declaration
The declarative items listed after the subprogram specification declare
things which are to be used locally within the subprogram body. The
names of these items are not visible outside of the subprogram, but are
visible inside locally declared subprograms. Furthermore, these items
shadow any things with the same names declared outside the subprogram.
When the subprogram is called, the statements in the body are executed
until either the end of the statement list is encountered, or a return
statement is executed. The syntax of a return statement is:
return_statement ::= return [ expression ] ;


If a return statement occurs in a procedure body, it must not include an
expression. There must be at least one return statement in a function body,
it must have an expression, and the function must complete by executing a
return statement. The value of the expression is the valued returned to the
function call.
Another point to note about function subprograms is that they may not
have any side-effects. This means that no visible variable declared outside
the function body may be assigned to or altered by the function. This
includes passing a non-local variable to a procedure as a variable
parameter with mode out or inout. The important result of this rule is that
functions can be called without them having any effect on the environment
of the call.
An example of a function body:
function byte_to_int(byte : word_8) return integer is
variable result : integer := 0;
begin
for index in 0 to 7 loop
result := result*2 + bit'pos(byte(index));
end loop;
return result;
end byte_to_int;

2.5.2. Overloading
VHDL allows two subprograms to have the same name, provided the
number or base types of parameters differs. The subprogram name is then
said to be overloaded. When a subprogram call is made using an
overloaded name, the number of actual parameters, their order, their base
types and the corresponding formal parameter names (if named
association is used) are used to determine which subprogram is meant. If
the call is a function call, the result type is also used. For example, suppose
we declared the two subprograms:
function check_limit(value : integer) return boolean;
function check_limit(value : word_32) return boolean;
Then which of the two functions is called depends on whether a value of
type integer or word_8 is used as the actual parameter. So
test := check_limit(4095)
would call the first function, and
test := check_limit(X"0000_0FFF")
would call the second function.
The designator used to define a subprogram can be either an identifier
or a string representing any of the operator symbols listed in Section2.3.
The latter case allows extra operand types to be defined for those operators.
For example, the addition operator might be overloaded to add word_32
operands by declaring a function:
function "+" (a, b : word_32) return word_32 is
begin
return int_to_word_32( word_32_to_int(a) + word_32_to_int(b) );
end "+";
Within the body of this function, the addition operator is used to add
integers, since its operands are both integers. However, in the expression:
X"1000_0010" + X"0000_FFD0"


the newly declared function is called, since the operands to the addition
operator are both of type word_32. Note that it is also possible to call
operators using the prefix notation used for ordinary subprogram calls, for
example:
"+" (X"1000_0010", X"0000_FFD0")

2.5.3. Package and Package Body Declarations
A package is a collection of types, constants, subprograms and possibly
other things, usually intended to implement some particular service or to
isolate a group of related items. In particular, the details of constant values
and subprogram bodies can be hidden from users of a package, with only
their interfaces made visible.
A package may be split into two parts: a package declaration, which
defines its interface, and a package body, which defines the deferred
details. The body part may be omitted if there are no deferred details. The
syntax of a package declaration is:
package_declaration ::=
package identifier is
package_declarative_part
end [ package_simple_name ] ;
package_declarative_part ::= { package_declarative_item }
package_declarative_item ::=
| type_declaration
| alias_declaration
| use_clause
The declarations define things which are to be visible to users of the
package, and which are also visible inside the package body. (There are
also other kinds of declarations which can be included, but they are not
discussed here.)
An example of a package declaration:
package data_types is
subtype address is bit_vector(24 downto 0);
subtype data is bit_vector(15 downto 0);
constant vector_table_loc : address;
function data_to_int(value : data) return integer;
function int_to_data(value : integer) return data;
end data_types;
In this example, the value of the constant vector_table_loc and the bodies of
the two functions are deferred, so a package body needs to be given.
The syntax for a package body is:
package_body ::=
package body package_simple_name is
package_body_declarative_part
end [ package_simple_name ] ;
package_body_declarative_part ::= { package_body_declarative_item }


package_body_declarative_item ::=
| subprogram_body
| type_declaration
| alias_declaration
| use_clause
Note that subprogram bodies may be included in a package body, whereas
only subprogram interface declarations may be included in the package
interface declaration.
The body for the package data_types shown above might be written as:
package body data_types is
constant vector_table_loc : address := X"FFFF00";
function data_to_int(value : data) return integer is
body of data_to_int
end data_to_int;
function int_to_data(value : integer) return data is
body of int_to_data
end int_to_data;
end data_types;
In this package body, the value for the constant is specified, and the
function bodies are given. The subtype declarations are not repeated, as
those in the package declarations are visible in the package body.
2.5.4. Package Use and Name Visibility
Once a package has been declared, items declared within it can be used
by prefixing their names with the package name. For example, given the
package declaration in Section2.4.3 above, the items declared might be used
as follows:
variable PC : data_types.address;
int_vector_loc := data_types.vector_table_loc + 4*int_level;
offset := data_types.data_to_int(offset_reg);
Often it is convenient to be able to refer to names from a package without
having to qualify each use with the package name. This may be done using
a use clause in a declaration region. The syntax is:
use_clause ::= use selected_name { , selected_name } ;
selected_name ::= prefix . suffix
The effect of the use clause is that all of the listed names can subsequently
be used without having to prefix them. If all of the declared names in a
package are to be used in this way, you can use the special suffix all, for
example:
use data_types.all;

3. VHDL Describes Structure

In Section 1.1 we introduced some terminology for describing the
structure of a digital system. In this chapter, we will look at how structure
is described in VHDL.

3.1. Entity Declarations
A digital system is usually designed as a hierarchical collection of
modules. Each module has a set of ports which constitute its interface to
the outside world. In VHDL, an entity is such a module which may be used
as a component in a design, or which may be the top level module of the
design.
The syntax for declaring an entity is:
entity_declaration ::=
entity identifier is
entity_header
entity_declarative_part
[ begin
entity_statement_part ]
end [ entity_simple_name ] ;
entity_header ::=
[ formal_generic_clause ]
[ formal_port_clause ]
generic_clause ::= generic ( generic_list ) ;
generic_list ::= generic_interface_list
port_clause ::= port ( port_list ) ;
port_list ::= port_interface_list
entity_declarative_part ::= { entity_declarative_item }
The entity declarative part may be used to declare items which are to be
used in the implementation of the entity. Usually such declarations will be
included in the implementation itself, so they are only mentioned here for
completeness. Also, the optional statements in the entity declaration may
be used to define some special behaviour for monitoring operation of the
entity. Discussion of these will be deferred until Section6.5.
The entity header is the most important part of the entity declaration. It
may include specification of generic constants, which can be used to control
the structure and behaviour of the entity, and ports, which channel
information into and out of the entity.
The generic constants are specified using an interface list similar to
that of a subprogram declaration. All of the items must be of class
constant. As a reminder, the syntax of an interface constant declaration is:
interface_constant_declaration ::=
[ constant ] identifier_list : [ in ] subtype_indication [ := static_expression ]

3-1


The actual value for each generic constant is passed in when the entity is
used as a component in a design.
The entity ports are also specified using an interface list, but the items
in the list must all be of class signal. This is a new kind of interface item
not previously discussed. The syntax is:
interface_signal_declaration ::=
[ signal ] identifier_list : [ mode ] subtype_indication [ bus ]
[ := static_expression ]
Since the class must be signal, the word signal can be omitted and is
assumed. The word bus may be used if the port is to be connected to more
than one output (see Sections 6.1 and 6.2). As with generic constants the
actual signals to be connected to the ports are specified when the entity is
used as a component in a design.
To clarify this discussion, here are some examples of entity
declarations:
entity processor is
generic (max_clock_freq : frequency := 30 MHz);
address : out integer;
data : inout word_32;
control : out proc_control;
ready : in bit);
end processor;
In this case, the generic constant max_clock_freq is used to specify the timing
behaviour of the entity. The code describing the entity's behaviour would
use this value to determine delays in changing signal values.
Next, an example showing how generic parameters can be used to
specify a class of entities with varying structure:
entity ROM is
generic (width, depth : positive);
port (enable : in bit;
address : in bit_vector(depth–1 downto 0);
data : out bit_vector(width–1 downto 0) );
end ROM;
Here, the two generic constants are used to specify the number of data bits
and address bits respectively for the read-only memory. Note that no
default value is given for either of these constants. This means that when
the entity is used as a component, actual values must be supplied for them.
Finally an example of an entity declaration with no generic constants or

TEST_BENCH

Y A A Y

Z TG B B DUT Z

Figure 3-1. Test bench circuit.

3. VHDL Describes Structure 3-3

ports:
entity test_bench is
end test_bench;
Though this might at first seem to be a pointless example, in fact it
illustrates a common use of entities, shown in Figure3-1. A top-level entity
for a design under test (DUT) is used as a component in a test bench circuit
with another entity (TG) whose purpose is to generate test values. The
values on signals can be traced using a simulation monitor, or checked
directly by the test generator. No external connections from the test bench
are needed, hence it has no ports.

3.2. Architecture Declarations
Once an entity has had its interface specified in an entity declaration,
one or more implementations of the entity can be described in architecture
bodies. Each architecture body can describe a different view of the entity.
For example, one architecture body may purely describe the behaviour
using the facilities covered in Chapters 2 and 4, whereas others may
describe the structure of the entity as a hierarchically composed collection
of components. In this section, we will only cover structural descriptions,
deferring behaviour descriptions until Chapter4.
An architecture body is declared using the syntax:
architecture_body ::=
architecture identifier of entity_name is
architecture_declarative_part
begin
architecture_statement_part
end [ architecture_simple_name ] ;
architecture_declarative_part ::= { block_declarative_item }
architecture_statement_part ::= { concurrent_statement }
block_declarative_item ::=
| subprogram_body
| type_declaration
| signal_declaration
| alias_declaration
| component_declaration
| configuration_specification
| use_clause
concurrent_statement ::=
block_statement
| component_instantiation_statement
The declarations in the architecture body define items that will be used to
construct the design description. In particular, signals and components
may be declared here and used to construct a structural description in
terms of component instances, as illustrated in Section1.4. These are
discussed in more detail in the next sections.
3.2.1. Signal Declarations
Signals are used to connect submodules in a design. They are declared
using the syntax:


signal_declaration ::=
signal identifier_list : subtype_indication [ signal_kind ] [ := expression ] ;
signal_kind ::= register | bus
Use of the signal kind specification is covered in Section6.2. Omitting the
signal kind results in an ordinary signal of the subtype specified. The
expression in the declaration is used to give the signal an initial value
during the initialization phase of simulation. If the expression is omitted,
a default initial value will be assigned.
One important point to note is that ports of an object are treated exactly
as signals within that object.
3.2.2. Blocks
The submodules in an architecture body can be described as blocks. A
block is a unit of module structure, with its own interface, connected to
other blocks or ports by signals. A block is specified using the syntax:
block_statement ::=
block_label :
block [ ( guard_expression ) ]
block_header
block_declarative_part
begin
block_statement_part
end block [ block_label ] ;
block_header ::=
[ generic_clause
[ generic_map_aspect ; ] ]
[ port_clause
[ port_map_aspect ; ] ]
generic_map_aspect ::= generic map ( generic_association_list )
port_map_aspect ::= port map ( port_association_list )
block_declarative_part ::= { block_declarative_item }
block_statement_part ::= { concurrent_statement }
The guard expression is not covered in this booklet, and may be omitted.
The block header defines the interface to the block in much the same way as
an entity header defines the interface to an entity. The generic association
list specifies values for the generic constants, evaluated in the context of the
enclosing block or architecture body. The port map association list specifies
which actual signals or ports from the enclosing block or architecture body
are connected to the block’s ports. Note that a block statement part may also
contain block statements, so a design can be composed of a hierarchy of
blocks, with behavioural descriptions at the bottom level of the hierarchy.
As an example, suppose we want to describe a structural architecture of
the processor entity example in Section3.1. If we separate the processor
into a control unit and a data path section, we can write a description as a
pair of interconnected blocks, as shown in Figure3-2.
The control unit block has ports clk, bus_control and bus_ready, which are
connected to the processor entity ports. It also has an output port for
controlling the data path, which is connected to a signal declared in the
architecture. That signal is also connected to a control port on the data
path block. The address and data ports of the data path block are connected
to the corresponding entity ports. The advantage of this modular
decomposition is that each of the blocks can then be developed

3. VHDL Describes Structure 3-5

architecture block_structure of processor is
type data_path_control is … ;
signal internal_control : data_path_control;
begin
control_unit : block
port (clk : in bit;
bus_control : out proc_control;
bus_ready : in bit;
control : out data_path_control);
port map (clk => clock,
bus_control => control, bus_ready => ready;
control => internal_control);
declarations for control_unit
begin
statements for control_unit
end block control_unit;
data_path : block
port (address : out integer;
data : inout word_32;
control : in data_path_control);
port map (address => address, data => data,
control => internal_control);
declarations for data_path
begin
statements for data_path
end block data_path;
end block_structure;

Figure3-2. Structural architecture of processor example.

independently, with the only effects on other blocks being well defined
through their interfaces.
3.2.3. Component Declarations
An architecture body can also make use of other entities described
separately and placed in design libraries. In order to do this, the
architecture must declare a component, which can be thought of as a
template defining a virtual design entity, to be instantiated within the
architecture. Later, a configuration specification (see Section3.3) can be
used to specify a matching library entity to use. The syntax of a component
declaration is:
component_declaration ::=
component identifier
[ local_generic_clause ]
[ local_port_clause ]
end component ;
Some examples of component declarations:
component nand3
generic (Tpd : Time := 1 ns);
port (a, b, c : in logic_level;
y : out logic_level);
end component;


component read_only_memory
generic (data_bits, addr_bits : positive);
port (en : in bit;
addr : in bit_vector(depth–1 downto 0);
data : out bit_vector(width–1 downto 0) );
end component;
The first example declares a three-input gate with a generic parameter
specifying its propagation delay. Different instances can later be used with
possibly different propagation delays. The second example declares a read-
only memory component with address depth and data width dependent on
generic constants. This component could act as a template for the ROM
entity described in Section3.1.
3.2.4. Component Instantiation
A component defined in an architecture may be instantiated using the
syntax:
component_instantiation_statement ::=
instantiation_label :
component_name
[ generic_map_aspect ]
[ port_map_aspect ] ;
This indicates that the architecture contains an instance of the named
component, with actual values specified for generic constants, and with the
component ports connected to actual signals or entity ports.
The example components declared in the previous section might be
instantiated as:
enable_gate: nand3
port map (a => en1, b => en2, c => int_req, y => interrupt);
parameter_rom: read_only_memory
generic map (data_bits => 16, addr_bits => 8);
port map (en => rom_sel, data => param, addr => a(7 downto 0);
In the first instance, no generic map specification is given, so the default
value for the generic constant Tpd is used. In the second instance, values
are specified for the address and data port sizes. Note that the actual signal
associated with the port addr is a slice of an array signal. This illustrates
that a port which is an array can be connected to part of a signal which is a
larger array, a very common practice with bus signals.

4. VHDL Describes Behaviour

In Section 1.2 we stated that the behaviour of a digital system could be
described in terms of programming language notation. The familiar
sequential programming language aspects of VHDL were covered in detail
in Chapter 2. In this chapter, we describe how these are extended to
include statements for modifying values on signals, and means of
responding to the changing signal values.

4.1. Signal Assignment
A signal assignment schedules one or more transactions to a signal (or
port). The syntax of a signal assignment is:
signal_assignment_statement ::= target <= [ transport ] waveform ;
target ::= name | aggregate
waveform ::= waveform_element { , waveform_element }
waveform_element ::=
value_expression [ after time_expression ]
| null [ after time_expression ]
The target must represent a signal, or be an aggregate of signals (see also
variable assignments, Section 2.4.1). If the time expression for the delay is
omitted, it defaults to 0 fs. This means that the transaction will be
scheduled for the same time as the assignment is executed, but during the
next simulation cycle.
Each signal has associated with it a projected output waveform, which
is a list of transactions giving future values for the signal. A signal
assignment adds transactions to this waveform. So, for example, the
signal assignment:
s <= '0' after 10 ns;
will cause the signal enable to assume the value true 10 ns after the
assignment is executed. We can represent the projected output waveform
graphically by showing the transactions along a time axis. So if the above
assignment were executed at time 5 ns, the projected waveform would be:
15ns
'0'

When simulation time reaches 15 ns, this transaction will be processed and
the signal updated.
Suppose then at time 16 ns, the assignment:
s <= '1' after 4 ns, '0' after 20 ns;
were executed. The two new transactions are added to the projected output
waveform:

4-1


20ns 36ns
'1' '0'

Note that when multiple transactions are listed in a signal assignment, the
delay times specified must be in ascending order.
If a signal assignment is executed, and there are already old
transactions from a previous assignmenton the projected output waveform,
then some of the old transactions may be deleted. The way this is done
depends on whether the word transport is included in the new assignment.
If it is included, the assignment is said to use transport delay. In this case,
all old transactions scheduled to occur after the first new transaction are
deleted before the new transactions are added. It is as though the new
transactions supercede the old ones. So given the projected output
waveform shown immediately above, if the assignment:
s <= transport 'Z' after 10 ns;
were executed at time 18 ns, then the transaction scheduled for 36 ns would
be deleted, and the projected output waveform would become:
20ns 28ns
'1' 'Z'

The second kind of delay, inertial delay, is used to model devices which
do not respond to input pulses shorter than their output delay. An intertial
delay is specified by omitting the word transport from the signal
assignment. When an inertial delay transaction is added to a projected
output waveform, firstly all old transactions scheduled to occur after the
new transaction are deleted, and the new transaction is added, as in the
case of transport delay. Next, all old transactions scheduled to occur before
the new transaction are examined. If there are any with a different value
from the new transaction, then all transactions up to the last one with a
different value are deleted. The remaining transactions with the same
value are left.
To illustrate this, suppose the projected output waveform at time 0 ns is:
10ns 15ns 20ns 30ns
'1' '0' '1' 'Z'

and the assignment:
s <= '1' after 25 ns;
is executed also at 0 ns. Then the new projected ouptut waveform is:
20ns 25ns
'1' '1'

When a signal assignment with multiple waveform elements is
specified with intertial delay, only the first transaction uses inertial delay;
the rest are treated as being transport delay transactions.

4.2. Processes and the Wait Statement
The primary unit of behavioural description in VHDL is the process. A
process is a sequential body of code which can be activated in response to
changes in state. When more than one process is activated at the same

4. VHDL Describes Behaviour 4-3

time, they execute concurrently. A process is specified in a process
statement, with the syntax:
process_statement ::=
[ process_label : ]
process [ ( sensitivity_list ) ]
process_declarative_part
begin
process_statement_part
end process [ process_label ] ;
process_declarative_part ::= { process_declarative_item }
process_declarative_item ::=
| subprogram_body
| type_declaration
| variable_declaration
| alias_declaration
| use_clause
process_statement_part ::= { sequential_statement }
sequential_statement ::=
wait_statement
| assertion_statement
| signal_assignment_statement
| variable_assignment_statement
| procedure_call_statement
| if_statement
| case_statement
| loop_statement
| next_statement
| exit_statement
| return_statement
| null_statement
A process statement is a concurrent statement which can be used in an
architecture body or block. The declarations define items which can be
used locally within the process. Note that variables may be defined here
and used to store state in a model.
A process may contain a number of signal assignment statements for a
given signal, which together form a driver for the signal. Normally there
may only be one driver for a signal, and so the code which determines a
signals value is confined to one process.
A process is activated initially during the initialisation phase of
simulation. It executes all of the sequential statements, and then repeats,
starting again with the first statement. A process may suspended itself by
executing a wait statement. This is of the form:
wait_statement ::=
wait [ sensitivity_clause ] [ condition_clause ] [ timeout_clause ] ;
sensitivity_clause ::= on sensitivity_list
sensitivity_list ::= signal_name { , signal_name }
condition_clause ::= until condition
timeout_clause ::= for time_expression
The sensitivity list of the wait statement specifies a set of signals to
which the process is sensitive while it is suspended. When an event occurs


on any of these signals (that is, the value of the signal changes), the process
resumes and evaluates the condition. If it is true or if the condition is
omitted, execution procedes with the next statement, otherwise the process
resuspends. If the sensitivity clause is omitted, then the process is
sensitive to all of the signals mentioned in the condition expression. The
timeout expression must evaluate to a positive duration, and indicates the
maximum time for which the process will wait. If it is omitted, the process
may wait indefinitely.
If a sensitivity list is included in the header of a process statement, then
the process is assumed to have an implicit wait statement at the end of its
statement part. The sensitivity list of this implicit wait statement is the
same as that in the process header. In this case the process may not
contain any explicit wait statements.
An example of a process statements with a sensitivity list:
process (reset, clock)
variable state : bit := false;
begin
if reset then
state := false;
elsif clock = true then
state := not state;
end if;
q <= state after prop_delay;
-- implicit wait on reset, clock
end process;
During the initialization phase of simulation, the process is activated and
assigns the initial value of state to the signal q. It then suspends at the
implicit wait statement indicated in the comment. When either reset or
clock change value, the process is resumed, and execution repeats from the
beginning.
The next example describes the behaviour of a synchronization device
called a Muller-C element used to construct asynchronous logic. The
output of the device starts at the value '0', and stays at this value until both
inputs are '1', at which time the output changes to '1'. The output then
stays '1' until both inputs are '0', at which time the output changes back to
'0'.
muller_c_2 : process
begin
wait until a = '1' and b = '1';
q <= '1';
wait until a = '0' and b = '0';
q <= '0';
end process muller_c_2 ;
This process does not include a sensitivity list, so explicit wait statements
are used to control the suspension and activation of the process. In both
wait statements, the sensitivity list is the set of signals a and b, determined
from the condition expression.

4.3. Concurrent Signal Assignment Statements
Often a process describing a driver for a signal contains only one signal
assignment statement. VHDL provides a convenient short-hand notation,
called a concurrent signal assignment statement, for expressing such
processes. The syntax is:


concurrent_signal_assignment_statement ::=
[ label : ] conditional_signal_assignment
| [ label : ] selected_signal_assignment
For each kind of concurrent signal assignment, there is a
corresponding process statement with the same meaning.
4.3.1. Conditional Signal Assignment
A conditional signal assignment statement is a shorthand for a process
containing signal assignments in an if statement. The syntax is:
conditional_signal_assignment ::= target <= options conditional_waveforms ;
options ::= [ guarded ] [ transport ]
conditional_waveforms ::=
{ waveform when condition else }
waveform
Use of the word guarded is not covered in this booklet. If the word transport
is included, then the signal assignments in the equivalent process use
transport delay.
Suppose we have a conditional signal assignment:
s <= waveform_1 when condition_1 else
waveform_2 when condition_2 else
…
waveform_n;
Then the equivalent process is:
process
if condition_1 then
s <= waveform_1;
elsif condition_2 then
s <= waveform_2;
elsif …
else
s <= waveform_n;
wait [ sensitivity_clause ];
end process;
If none of the waveform value expressions or conditions contains a
reference to a signal, then the wait statement at the end of the equivalent
process has no sensitivity clause. This means that after the assignment is
made, the process suspends indefinitely. For example, the conditional
assignment:
reset <= '1', '0' after 10 ns when short_pulse_required else
'1', '0' after 50 ns;
schedules two transactions on the signal reset, then suspends for the rest of
the simulation.
On the other hand, if there are references to signals in the waveform
value expressions or conditions, then the wait statement has a sensitivity
list consisting of all of the signals referenced. So the conditional
assignment:
mux_out <= 'Z' after Tpd when en = '0' else
in_0 after Tpd when sel = '0' else
in_1 after Tpd;
is sensitive to the signals en and sel. The process is activated during the
initialization phase, and thereafter whenever either of en or sel changes
value.


The degenerate case of a conditional signal assignment, containing no
conditional parts, is equivalent to a process containing just a signal
assignment statement. So:
s <= waveform;
is equivalent to:
process
s <= waveform;
end process;

4.3.2. Selected Signal Assignment
A selected signal assignment statement is a shorthand for a process
containing signal assignments in a case statement. The syntax is:
selected_signal_assignment ::=
with expression select
target <= options selected_waveforms ;
selected_waveforms ::=
{ waveform when choices , }
waveform when choices
choices ::= choice { | choice }
The options part is the same as for a conditional signal assignment. So if
the word transport is included, then the signal assignments in the
equivalent process use transport delay.
Suppose we have a selected signal assignment:
with expression select
s <= waveform_1 when choice_list_1,
waveform_2 when choice_list_2,
…
waveform_n when choice_list_n;
Then the equivalent process is:
process
case expression is
when choice_list_1=>
s <= waveform_1;
when choice_list_2=>
s <= waveform_2;
…
when choice_list_n=>
s <= waveform_n;
end case;
end process;
The sensitivity list for the wait statement is determined in the same way as
for a conditional signal assignment. That is, if no signals are referenced in
the selected signal assignment expression or waveforms, the wait
statement has no sensitivity clause. Otherwise the sensitivity clause
contains all the signals referenced in the expression and waveforms.
An example of a selected signal assignment statement:
with alu_function select
alu_result <= op1 + op2 when alu_add | alu_incr,
op1 – op2 when alu_subtract,
op1 and op2 when alu_and,
op1 or op2 when alu_or,
op1 and not op2 when alu_mask;


In this example, the value of the signal alu_function is used to select which
signal assignment to alu_result to execute. The statement is sensitive to the
signals alu_function, op1 and op2, so whenever any of these change value, the
selected signal assignment is resumed.

5. Model Organisation

The previous chapters have described the various facilities of VHDL
somewhat in isolation. The purpose of this chapter is to show how they are
all tied together to form a complete VHDL description of a digital system.

5.1. Design Units and Libraries
When you write VHDL descriptions, you write them in a design file,
then invoke a compiler to analyse them and insert them into a design
library. A number of VHDL constructs may be separately analysed for
inclusion in a design library. These constructs are called library units.
The primary library units are entity declarations, package declarations and
configuration declarations (see Section 5.2). The secondary library units
are architecture bodies and package bodies. These library units depend on
the specification of their interface in a corresponding primary library unit,
so the primary unit must be analysed before any corresponding secondary
unit.
A design file may contain a number of library units. The structure of a
design file can be specified by the syntax:
design_file ::= design_unit { design_unit }
design_unit ::= context_clause library_unit
context_clause ::= { context_item }
context_item ::= library_clause | use_clause
library_clause ::= library logical_name_list ;
logical_name_list ::= logical_name { , logical_name }
library_unit ::= primary_unit | secondary_unit
primary_unit ::=
entity_declaration | configuration_declaration | package_declaration
secondary_unit ::= architecture_body | package_body
Libraries are referred to using identifiers called logical names. This
name must be translated by the host operating system into an
implementation dependent storage name. For example, design libraries
may be implemented as database files, and the logical name might be used
to determine the database file name. Library units in a given library can be
referred to by prefixing their name with the library logical name. So for
example, ttl_lib.ttl_10 would refer to the unit ttl_10 in library ttl_lib.
The context clause preceding each library unit specifies which other
libraries it references and which packages it uses. The scope of the names
made visible by the context clause extends until the end of the design unit.
There are two special libraries which are implicitly available to all
design units, and so do not need to be named in a library clause. The first of
these is called work, and refers to the working design library into which the

5-1


current design units will be placed by the analyser. Hence in a design unit,
the previously analysed design units in a design file can be referred to
using the library name work.
The second special libary is called std, and contains the packages
standard and textio. Standard contains all of the predefined types and
functions. All of the items in this package are implicitly visible, so no use
clause is necessary to access them.

5.2. Configurations
In Sections 3.2.3 and 3.2.4 we showed how a structural description can
declare a component specification and create instances of components. We
mentioned that a component declared can be thought of as a template for a
design entity. The binding of an entity to this template is achieved through
a configuration declaration. This declaration can also be used to specify
actual generic constants for components and blocks. So the configuration
declaration plays a pivotal role in organising a design description in
preparation for simulation or other processing.
The syntax of a configuration declaration is:
configuration_declaration ::=
configuration identifier of entity_name is
configuration_declarative_part
block_configuration
end [ configuration_simple_name ] ;
configuration_declarative_part ::= { configuration_declarative_item }
configuration_declarative_item ::= use_clause
block_configuration ::=
for block_specification
{ use_clause }
{ configuration_item }
end for ;
block_specification ::= architecture_name | block_statement_label
configuration_item ::= block_configuration | component_configuration
component_configuration ::=
for component_specification
[ use binding_indication ; ]
[ block_configuration ]
end for ;
component_specification ::= instantiation_list : component_name
instantiation_list ::=
instantiation_label { , instantiation_label )
| others
| all
binding_indication ::=
entity_aspect
[ generic_map_aspect ]
[ port_map_aspect ]
entity_aspect ::=
entity entity_name [ ( architecture_identifier ) ]
| configuration configuration_name
| open
generic_map_aspect ::= generic map ( generic_association_list )

5. Model Organisation 5-3

entity processor is
generic (max_clock_speed : frequency := 30 MHz);
port ( port list );
end processor;
architecture block_structure of processor is
declarations
begin
control_unit : block
port ( port list );
port map ( association list );
declarations for control_unit
begin
statements for control_unit
end block control_unit;
data_path : block
port ( port list );
declarations for data_path
begin
statements for data_path
end block_structure;

Figure 5-1. Example processor entity and architecture body.

port_map_aspect ::= port map ( port_association_list )
The declarative part of the configuration declaration allows the
configuration to use items from libraries and packages. The outermost
block configuration in the configuration declaration defines the
configuration for an architecture of the named entity. For example, in
Chapter 3 we had an example of a processor entity and architecture,
outlined again in Figure5-1. The overall structure of a configuration
declaration for this architecture might be:
configuration test_config of processor is
use work.processor_types.all
for block_structure
configuration items
end for;
end test_config;
In this example, the contents of a package called processor_types in the
current working library are made visible, and the block configuration
refers to the architecture block_structure of the entity processor.
Within the block configuration for the architecture, the submodules of
the architecture may be configured. These submodules include blocks and
component instances. A block is configured with a nested block
configuration. For example, the blocks in the above architecture can be
configured as shown in Figure5-2.
Where a submodule is an instance of a component, a component
configuration is used to bind an entity to the component instance. To
illustrate, suppose the data_path block in the above example contained an


configuration test_config of processor is
use work.processor_types.all
for block_structure
for control_unit
configuration items
end for;
for data_path
configuration items
end for;
end for;
end test_config;

Figure5-2. Configuration of processor example.

data_path : block
port ( port list );
component alu
port (function : in alu_function;
op1, op2 : in bit_vector_32;
result : out bit_vector_32);
end component;
other declarations for data_path
begin
data_alu : alu
port map (function => alu_fn, op1 => b1, op2 => b2, result => alu_r);
other statements for data_path

Figure5-3. Structure of processor data-path block.

instance of the component alu, declared as shown in Figure5-3. Suppose
also that a library project_cells contains an entity called alu_cell defined as:
entity alu_cell is
generic (width : positive);
port (function_code : in alu_function;
operand1, operand2 : in bit_vector(width-1 downto 0);
result : out bit_vector(width-1 downto 0);
flags : out alu_flags);
end alu_cell;
with an architecture called behaviour. This entity matches the alu
component template, since its operand and result ports can be constrained
to match those of the component, and the flags port can be left unconnected.
A block configuration for data_path could be specified as shown in
Figure5-4.
Alternatively, if the library also contained a configuration called
alu_struct for an architecture structure of the entity alu_cell, then the block
configuration could use this, as shown in Figure5-5.


for data_path
for data_alu : alu
use entity project_cells.alu_cell(behaviour)
generic map (width => 32)
port map (function_code => function, operand1 => op1, operand2 => op2,
result => result, flags => open);
end for;
other configuration items
end for;

Figure5-4. Block configuration using library entity.

for data_path
for data_alu : alu
use configuration project_cells.alu_struct
port map (function_code => function, operand1 => op1, operand2 => op2,
result => result, flags => open);
end for;
other configuration items
end for;

Figure5-5. Block configuration using another configuration.

5.3. Complete Design Example
To illustrate the overall structure of a design description, a complete
design file for the example in Section1.4 is shown in Figure5-6. The design
file contains a number of design units which are analysed in order. The
first design unit is the entity declaration of count2. Following it are two
secondary units, architectures of the count2 entity. These must follow the
entity declaration, as they are dependent on it. Next is another entity
declaration, this being a test bench for the counter. It is followed by a
secondary unit dependent on it, a structural description of the test bench.
Following this is a configuration declaration for the test bench. It refers to
the previously defined library units in the working library, so no library
clause is needed. Notice that the count2 entity is referred to in the
configuration as work.count2, using the library name. Lastly, there is a
configuration declaration for the test bench using the structural
architecture of count2. It uses two library units from a separate reference
library, misc. Hence a library clause is included before the configuration
declaration. The library units from this library are referred to in the
configuration as misc.t_flipflop and misc.inverter.
This design description includes all of the design units in one file. It is
equally possible to separate them into a number of files, with the opposite
extreme being one design unit per file. If multiple files are used, you need
to take care that you compile the files in the correct order, and re-compile
dependent files if changes are made to one design unit. Source code control
systems can be of use in automating this process.


-- primary unit: entity declaration of count2
entity count2 is
generic (prop_delay : Time := 10 ns);
q1, q0 : out bit);
end count2;
-- secondary unit: a behavioural architecture body of count2
architecture behaviour of count2 is
begin
count_up: process (clock)
variable count_value : natural := 0;
begin
if clock = '1' then
count_value := (count_value + 1) mod 4;
q0 <= bit'val(count_value mod 2) after prop_delay;
q1 <= bit'val(count_value / 2) after prop_delay;
end if;
end process count_up;
end behaviour;
-- secondary unit: a structural architecture body of count2
architecture structure of count2 is
component t_flipflop
port (ck : in bit; q : out bit);
end component;
component inverter
port (a : in bit; y : out bit);
end component;
signal ff0, ff1, inv_ff0 : bit;
begin
bit_0 : t_flipflop port map (ck => clock, q => ff0);
inv : inverter port map (a => ff0, y => inv_ff0);
bit_1 : t_flipflop port map (ck => inv_ff0, q => ff1);
q0 <= ff0;
q1 <= ff1;
end structure;

Figure5-6. Complete design file.


-- primary unit: entity declaration of test bench
entity test_count2 is
end test_count2;
-- secondary unit: structural architecture body of test bench
architecture structure of test_count2 is
signal clock, q0, q1 : bit;
component count2
q1, q0 : out bit);
end component;
begin
counter : count2
port map (clock => clock, q0 => q0, q1 => q1);
clock_driver : process
begin
clock <= '0', '1' after 50 ns;
wait for 100 ns;
end process clock_driver;
end structure;

-- primary unit: configuration using behavioural architecture
configuration test_count2_behaviour of test_count2 is
for structure -- of test_count2
for counter : count2
use entity work.count2(behaviour);
end for;
end for;
end test_count2_behaviour;

-- primary unit: configuration using structural architecture
library misc;
configuration test_count2_structure of test_count2 is
for structure -- of test_count2
for counter : count2
use entity work.count2(structure);
for structure -- of count_2
for all : t_flipflop
use entity misc.t_flipflop(behaviour);
end for;
for all : inverter
use entity misc.inverter(behaviour);
end for;
end for;
end for;
end for;
end test_count2_structure;

Figure5-6 (continued).

6. Advanced VHDL

This chapter describes some more advanced facilities offered in VHDL.
Although you can write many models using just the parts of the language
covered in the previous chapters, you will find the features described here
will significantly extend your model writing abilities.

6.1. Signal Resolution and Buses
In many digital sytems, buses are used to connect a number of output
drivers to a common signal. For example, if open-collector or open-drain
output drivers are used with a pull-up load on a signal, the signal can be
pulled low by any driver, and is only pulled high by the load when all
drivers are off. This is called a wired-or or wired-and connection. On the
other hand, if tri-state drivers are used, at most one driver may be active at
a time, and it determines the signal value.
VHDL normally allows only one driver for a signal. (Recall that a driver
is defined by the signal assignments in a process.) In order to model
signals with multiple drivers, VHDL uses the notion of resolved types for
signals. A resolved type includes in its definition a resolution function,
which takes the values of all the drivers contributing to a signal, and
combines them to determine the final signal value.
A resolved type for a signal is declared using the syntax for a subtype:
subtype_indication ::= [ resolution_function_name ] type_mark [ constraint ]
The resolution function name is the name of a function previously defined.
The function must take a parameter which is an unconstrained array of
values of the signal subtype, and must return a result of that subtype. To
illustrate, consider the declarations:
type logic_level is (L, Z, H);
type logic_array is array (integer range <>) of logic_level;
function resolve_logic (drivers : in logic_array) return logic_level;
subtype resolved_level is resolve_logic logic_level;
In this example, the type logic_level represents three possible states for a
digital signal: low (L), high-impedance (Z) and high (H). The subtype
resolved_level can be used to declare a resolved signal of this type. The
resolution function might be implemented as shown in Figure6-1.
This function iterates over the array of drivers, and if any is found to have
the value L, the function returns L. Otherwise the function returns H, since
all drivers are either Z or H. This models a wired-or signal with a pull-up.
Note that in some cases a resolution function may be called with an empty
array as the parameter, and should handle that case appropriately. The
example above handles it by returning the value H, the pulled-up value.

6-1


function resolve_logic (drivers : in logic_array) return logic_level;
begin
for index in drivers'range loop
if drivers(index) = L then
return L;
end if;
end loop;
return H;
end resolve_logic;

Figure 7-1. Resolution function for three-state logic

6.2. Null Transactions
VHDL provides a facility to model outputs which may be turned off (for
example tri-state drivers). A signal assignment may specify that no value
is to be assigned to a resolved signal, that is, that the driver should be
disconnected. This is done with a null waveform element. Recall that the
syntax for a waveform element is:
waveform_element ::=
value_expression [ after time_expression ]
| null [ after time_expression ]
So an example of such a signal assignment is:
d_out <= null after Toz;
If all of the drivers of a resolved signal are disconnected, the question of
the resulting signal value arises. There are two possibilities, depending on
whether the signal was declared with signal kind register or bus. For
register kind signals, the most recently determined value remains on the
signal. This can be used to model charge storage nodes in MOS logic
families. For bus kind signals, the resolution function must determine the
value for the signal when no drivers are contributing to it. This is how tri-
state, open-collector and open-drain buses would typically be modeled.

6.3. Generate Statements
VHDL has an additional concurrent statement which can be used in
architecture bodies to describe regular structures, such as arrays of blocks,
component instances or processes. The syntax is:
generate_statement ::=
generate_label :
generation_scheme generate
{ concurrent_statement }
end generate [ generate_label ] ;
generation_scheme ::=
for generate_parameter_specification
| if condition
The for generation scheme describes structures which have a repeating
pattern. The if generation scheme is usually used to handle exception
cases within the structure, such as occur at the boundaries. This is best
illustrated by example. Suppose we want to describe the structure of an

6. Advanced VHDL 6-3

adder : for i in 0 to width-1 generate
ls_bit : if i = 0 generate
ls_cell : half_adder port map (a(0), b(0), sum(0), c_in(1));
end generate lsbit;
middle_bit : if i > 0 and i < width-1 generate
middle_cell : full_adder port map (a(i), b(i), c_in(i), sum(i), c_in(i+1));
end generate middle_bit;
ms_bit : if i = width-1 generate
ms_cell : full_adder port map (a(i), b(i), c_in(i), sum(i), carry);
end generate ms_bit;
end generate adder;

Figure6-2. Generate statement for adder.

adder constructed out of full-adder cells, with the exception of the least
significant bit, which is consists of a half-adder. A generate statement to
achieve this is shown in Figure6-2.
The outer generate statement iterates with i taking on values from 0 to
width-1. For the least significant bit (i=0), an instance of a half adder
component is generated. The input bits are connected to the least
significant bits of a and b, the output bit is connected to the least significant
bit of sum, and the carry bit is connectected to the carry in of the next stage.
For intermediate bits, an instance of a full adder component is generated
with inputs and outputs connected similarly to the first stage. For the most
significant bit (i=width-1), an instance of the half adder is also generated, but
its carry output bit is connected to the signal carry.

6.4. Concurrent Assertions and Procedure Calls
There are two kinds of concurrent statement which were not covered in
previous chapters: concurrent assertions and concurrent procedure calls.
A concurrent assertion statement is equivalent to a process containing only
an assertion statement followed by a wait statement. The syntax is:
concurrent_assertion_statement ::= [ label : ] assertion_statement
The concurrent signal assertion:
L : assert condition report error_string severity severity_value;
is equivalent to the process:
L : process
begin
assert condition report error_string severity severity_value;
wait [ sensitivity_clause ] ;
end process L;
The sensitivity clause includes all the signals which are referred to in
the condition expression. If no signals are referenced, the process is
activated once at simulation initialisation, checks the condition, and then
suspends indefinitely.
The other concurrent statement, the concurrent procedure call, is
equivalent to a process containing only a procedure call followed by a wait
statement. The syntax is:


concurrent_procedure_call ::= [ label : ] procedure_call_statement
The procedure may not have any formal parameters of class variable,
since it is not possible for a variable to be visible at any place where a
concurrent statement may be used. The sensitivity list of the wait
statement in the process includes all the signals which are actual
parameters of mode in or inout in the procedure call. These are the only
signals which can be read by the called procedure.
Concurrent procedure calls are useful for defining process behaviour
that may be reused in several places or in different models. For example,
suppose a package bit_vect_arith declares the procedure:
procedure add(signal a, b : in bit_vector; signal result : out bit_vector);
Then an example of a concurrent procedure call using this procedure is:
adder : bit_vect_arith.add (sample, old_accum, new_accum);
This would be equivalent to the process:
adder : process
begin
bit_vect_arith.add (sample, old_accum, new_accum);
wait on sample, old_accum;
end process adder;

6.5. Entity Statements
In Section3.1, it was mentioned that an entity declaration may include
statements for monitoring operation of the entity. Recall that the syntax for
an entity declaration is:
entity_declaration ::=
entity identifier is
entity_header
entity_declarative_part
[ begin
entity_statement_part ]
end [ entity_simple_name ] ;
The syntax for the statement part is:
entity_statement_part ::= { entity_statement }
entity_statement ::=
concurrent_assertion_statement
| passive_concurrent_procedure_call
| passive_process_statement
The concurrent statement that are allowed in an entity declaration must
be passive, that is, they may not contain any signal assignments. (This
includes signal assignments inside nested procedures of a process.) A
result of this rule is that such processes cannot modify the state of the
entity, or any circuit the entity may be used in. However, they can fully
monitor the state, and so may be used to report erroneous operating
conditions, or to trace the behavior of the design.

7. Sample Models: The DP32 Processor

This chapter contains an extended example, a description of a
hypothetical processor called the DP32. The processor instruction set and
bus architectures are first described, and then a behavioural description is
given. A test bench model is constructed, and the model checked with a
small test program. Next, the processor is decomposed into components at
the register transfer level. A number of components are described, and a
structural description of the processor is constructed using these
components. The same test bench is used, but this time with the structural
architecture.

7.1. Instruction Set Architecture
The DP32 is a 32-bit processor with a simple instruction set. It has a
number of registers, shown in Figure 7-1. There are 256 general purpose
registers (R0–R255), a program counter (PC) and a condition code register
(CC). The general purpose registers are addressable by software, whereas
the PC and CC registers are not.
On reset, the PC is initialised to zero, and all other registers are
undefined. By convention, R0 is read-only and contains zero. This is not
enforced by hardware, and the zero value must be loaded by software after
reset.
The memory accessible to the DP32 consists of 32-bit words, addressed by
a 32-bit word-address. Instructions are all multiples of 32-bit words, and
are stored in this memory. The PC register contains the address of the next
instruction to be executed. After each instruction word is fetched, the PC is
incremented by one to point to the next word.
The three CC register bits are updated after each arithmetic or logical
instruction. The Z (zero) bit is set if the result is zero. The N (negative) bit
is set if the result of an arithmetic instruction is negative, and is undefined
after logical instructions. The V(overflow) bit is set if the result of an
arithmetic instruction exceeds the bounds of representable integers, and is

31 0 31 0

R0 PC
•
•
31
• 0 CC
R255 VN Z

Figure 7-1. DP32 registers.

7-1


Instruction Name Function opcode
Add add r3 ← r1 + r2 X“00”
Sub subtract r3 ← r1 − r2 X“01”
Mul multiply r3 ← r1 × r2 X“02”
Div divide r3 ← r1 ÷ r2 X“03”
Addq add quick r3 ← r1 + i8 X“10”
Subq subtract quick r3 ← r1 − i8 X“11”
Mulq multiply quick r3 ← r1 × i8 X“12”
Divq divide quick r3 ← r1 ÷ i8 X“13”
Land logical and r3 ← r1 & r2 X“04”
Lor logical or r3 ← r1 | r2 X“05”
Lxor logical exclusive or r3 ← r1 ⊕ r2 X“06”
Lmask logical mask r3 ← r1 & ~r2 X“07”

Table 7-1. DP32 arithmetic and logic instructions.

undefined after logical instructions.
The DP32 instruction set is divided into a number of encoding formats.
Firstly, arithmetic and logical instructions are all one 32-bit word long,
formatted as follows:
31 24 23 16 15 8 7 0

(Addr): op r3 r1 r2/i8
The op field is the op-code, r3 is the destination register address, r1 and r2
are source register addresses, and i8 is an immediate two-compliment
integer operand. The arithmetic and logical instructions are listed in
Table7-1.
Memory load and store instructions have two formats, depending on
whether a long or short displacement value is used. The format for a long
displacement is:
31 24 23 16 15 8 7 0

(Addr): op r3 r1 ignored

(Addr+1): disp

The format for a short displacement is:
31 24 23 16 15 8 7 0

(Addr): op r3 r1 i8

The op field is the op-code, r3 specifies the register to be loaded or stored, r1
is used as an index register, disp is a long immediate displacement, and i8
is a short immediate displacement. The load and store instructions are
listed in Table7-2.

7. Sample Models: The DP32 Processor 7-3

Ld load r3 ← M[r1 + disp32] X“20”
St store M[r1 + disp32] ← r3 X“21”
Ldq load quick r3 ← M[r1 + i8] X“30”
Stq store quick M[r1 + i8] ← r3 X“31”

Table7-2. DP32 load and store instructions.

Br-ivnz branch if cond then X“40”
PC ← PC + disp32
Brq-ivnz branch quick if cond then X“51”
PC ← PC + i8
Bi-ivnz branch indexed if cond then X“41”
PC ← r1 + disp32
Biq-ivnz branch indexed if cond then X“51”
quick PC ← r1 + i8

Table7-3. DP32 load and store instructions.

Finally, there are four branch instructions, listed in Table7-3, each with
a slightly different format. The format of the ordinary brach is:
31 24 23 20 19 16 15 8 7 0

(Addr): op xxxx ivnz xxxx xxxx

(Addr+1): disp

The format of a quick branch is:
31 24 23 20 19 16 15 8 7 0

(Addr): op xxxx ivnz xxxx i8

The format of an indexed branch
31 24 23 20 19 16 15 8 7 0

(Addr): op xxxx ivnz r1 xxxx

(Addr+1): disp

The format of a quick indexed branch
31 24 23 20 19 16 15 8 7 0

(Addr): op xxxx ivnz r1 i8

The op field is the op-code, disp is a long immediate displacement, i8 is a
short immediate displacement, r1 is used as an index register, and ivnz is
a the condition mask. The branch is taken if
cond ≡ ((V & v) | (N & n) | (Z & z)) = i.


DP32
PHI1 FETCH
PHI2 READ
RESET WRITE
phi1

READY A_BUS
phi2
D_BUS

Figure7-2. DP32 port diagram. Figure7-3. DP32 clock waveforms.

7.2. Bus Architecture
The DP32 processor communicates with its memory over synchronous
32-bit address and data buses. The external ports of the DP32 are shown in
Figure7-2.
The two clock inputs, phi1 and phi2, provide a two-phase non-overlapping
clock for the processor. The clock waveforms are shown in Figure7-3.
Each cycle of the phi1 clock defines a bus state, one of Ti (idle), T1 or T2. Bus
transactions consist of a T1 state followed by one or more T2 states, with Ti
states between transactions.
The port a_bus is a 32-bit address bus, and d_bus is a 32-bit bidirection
data bus. The read and write ports control bus read and write transactions.
The fetch port is a status signal indicating that a bus read in progress is an
instruction fetch. The ready input is used by a memory device to indicate
that read data is available or write data has been accepted.
The timing for a bus read transaction is show in Figure7-4. During an
idle state, Ti, the processor places the memory address on the address bus
to start the transaction. The next state is a T1 state. After the leading edge
of the phi1 clock, the processor asserts the read control signal, indicating
that the address is valid and the memory should start the read transaction.
The processor also asserts the fetch signal if it is reading instructions. It
always leaves the write signal negated during read transactions. During the
T1 state and the following T2 state, the memory accesses the requested data,
and places it on the data bus. If it has completed the data access by the end
of the T2 state, it asserts ready. The processor accepts the data, and
completes the transaction. On the other hand, if the memory has not yet
supplied the data by the end of the T2 state, it leaves ready false. The
processor then repeats T2 states until it detects ready true. By this means, a
slow memory can extend the transaction until it has read the data. At the
end of the transaction, the processor returns its control outputs to their
default values, and the memory negates ready and removes the data from
the data bus. The processor continues with idle states until the next
transaction is required.
The timing for a bus write transaction is show in Figure7-5. Here also,
the transaction starts with the processor placing the address on the address
bus during a Ti state. After the leading edge of phi1 during the subsequent
T1 state, the processor negates fetch and asserts write. The read signal
remains false for the whole transaction. During the T1 state, the processor
also makes the data to be written available on the data bus. The memory


Ti T1 T2 Ti

phi1

phi2

valid address
a_bus

valid fetch
fetch

read

write

valid data in
d_bus

ready

Figure7-4. DP32 bus read transaction.

Ti T1 T2 Ti

phi1

phi2

valid address
a_bus

fetch

read

write

valid data out
d_bus

ready

Figure7-5. DP32 bus write transaction.


can accept this data during the T1 and subsequent T2 states. If it has
completed the write by the end of the T2 state, it asserts ready. The
processor then completes the transaction and continutes with Ti states, and
the memory removes the data from the data bus and negates ready. If the
memory has not had time to complete the write by the end of the T2 state, it
leaves ready false. The processor will then repeat T2 states until it detects
ready true.

7.3. Types and Entity
We start the description of the DP32 processor by defining a package
containing the data types to be used in the model, and some useful
operations on those types. The package declaration of dp32_types is listed in
Figure7-6.

package dp32_types is
constant unit_delay : Time := 1 ns;
type bool_to_bit_table is array (boolean) of bit;
constant bool_to_bit : bool_to_bit_table;
subtype bit_32 is bit_vector(31 downto 0);
type bit_32_array is array (integer range <>) of bit_32;
function resolve_bit_32 (driver : in bit_32_array) return bit_32;
subtype bus_bit_32 is resolve_bit_32 bit_32;
subtype bit_8 is bit_vector(7 downto 0);
subtype CC_bits is bit_vector(2 downto 0);
subtype cm_bits is bit_vector(3 downto 0);
constant op_add : bit_8 := X"00";
constant op_sub : bit_8 := X"01";
constant op_mul : bit_8 := X"02";
constant op_div : bit_8 := X"03";
constant op_addq : bit_8 := X"10";
constant op_subq : bit_8 := X"11";
constant op_mulq : bit_8 := X"12";
constant op_divq : bit_8 := X"13";
constant op_land : bit_8 := X"04";
constant op_lor : bit_8 := X"05";
constant op_lxor : bit_8 := X"06";
constant op_lmask : bit_8 := X"07";
constant op_ld : bit_8 := X"20";
constant op_st : bit_8 := X"21";
constant op_ldq : bit_8 := X"30";
constant op_stq : bit_8 := X"31";
constant op_br : bit_8 := X"40";
constant op_brq : bit_8 := X"50";
constant op_bi : bit_8 := X"41";
constant op_biq : bit_8 := X"51";
function bits_to_int (bits : in bit_vector) return integer;
function bits_to_natural (bits : in bit_vector) return natural;
procedure int_to_bits (int : in integer; bits : out bit_vector);
end dp32_types;

Figure7-6. Package declaration for dp32_types.


The constant unit_delay is used as the default delay time through-out the
DP32 description. This approach is common when writing models to
describe the function of a digital system, before developing a detailed timing
model.
The constant bool_to_bit is a lookup table for converting between boolean
conditions and the type bit. Examples of its use will be seen later. Note that
it is a deferred constant, so its value will be given in the package body.
The next declarations define the basic 32-bit word used in the DP32
model. The function resolve_bit_32 is a resolution function used to
determine the value on a 32-bit bus with multiple drivers. Such a bus is
declared with the subtype bus_bit_32, a resolved type.
The subtype bit_8 is part of a 32-bit word used as an op-code or register
address. CC_bits is the type for condition codes, and cm_bits is the type for
the condition mask in a branch op-code.
The next set of constant declarations define the op-code bit patterns for
valid op-codes. These symbolic names are used as a matter of good coding
style, enabling the op-code values to be changed without having to modify
the model code in numerous places.
Finally, a collection of conversion functions between bit-vector values
and numeric values is defined. The bodies for these subprograms are
hidden in the package body.
The body of the dp32_types package is listed in Figure7-7. Firstly the
value for the deferred constant bool_to_bit is given: false translates to '0' and
true translates to '1'. An example of the use of this table is:
flag_bit <= bool_to_bit(flag_condition);
Next, the body of the resolution function for 32-bit buses is defined. The
function takes as its parameter an unconstrained array of bit_32 values,
and produces as a result the bit-wide logical-or of the values. Note that the
function cannot assume that the length of the array will be greater than
one. If no drivers are active on the bus, an empty array will be passed to the
resolution function. In this case, the default value of all '0' bits (float_value)
is used as the result.

package body dp32_types is
constant bool_to_bit : bool_to_bit_table :=
(false => '0', true => '1');
function resolve_bit_32 (driver : in bit_32_array) return bit_32 is
constant float_value : bit_32 := X"0000_0000";
variable result : bit_32 := float_value;
begin
for i in driver'range loop
result := result or driver(i);
end loop;
return result;
end resolve_bit_32;

Figure7-7. Package body for dp32_types.


The function bits_to_int converts a bit vector representing a twos-
compliment signed integer into an integer type value. The local variable
temp is declared to be a bit vector of the same size and index range as the
parameter bits. The variable result is initialised to zero when the function
is invoked, and subsequently used to accumulate the weighted bit values in

function bits_to_int (bits : in bit_vector) return integer is
variable temp : bit_vector(bits'range);
variable result : integer := 0;
begin
if bits(bits'left) = '1' then -- negative number
temp := not bits;
else
temp := bits;
end if;
for index in bits'range loop -- sign bit of temp = '0'
result := result * 2 + bit'pos(temp(index));
end loop;
if bits(bits'left) = '1' then
result := (-result) - 1;
end if;
return result;
end bits_to_int;
function bits_to_natural (bits : in bit_vector) return natural is
variable result : natural := 0;
begin
for index in bits'range loop
result := result * 2 + bit'pos(bits(index));
end loop;
return result;
end bits_to_natural;
procedure int_to_bits (int : in integer; bits : out bit_vector) is
variable temp : integer;
variable result : bit_vector(bits'range);
begin
if int < 0 then
temp := -(int+1);
else
temp := int;
end if;
for index in bits'reverse_range loop
result(index) := bit'val(temp rem 2);
temp := temp / 2;
end loop;
if int < 0 then
result := not result;
result(bits'left) := '1';
end if;
bits := result;
end int_to_bits;
end dp32_types;



use work.dp32_types.all;
entity dp32 is
generic (Tpd : Time := unit_delay);
port (d_bus : inout bus_bit_32 bus;
a_bus : out bit_32;
read, write : out bit;
fetch : out bit;
ready : in bit;
phi1, phi2 : in bit;
reset : in bit);
end dp32;

Figure7-8. Entity declaration for dp32.

the for loop. The function bits_to_natural performs a similar function to
bits_to_int, but does not need to do any special processing for negative
numbers. Finally, the function int_to_bits performs the inverse of bits_to_int.
The entity declaration of the DP32 processor is shown in Figure7-8. The
library unit is preceded by a use clause referencing all the items in the
package dp32_types. The entity has a generic constant Tpd used to specify
the propagation delays between input events and output signal changes.
The default value is the unit delay specified in the dp32_types package.
There are a number of ports corresponding to those shown in Figure7-2.
The reset, clocks, and bus control signals are represented by values of type
bit. The address bus output is a simple bit-vector type, as the processor is
the only module driving that bus. On the other hand, the data bus is a
resolved bit-vector type, as it may be driven by both the processor and a
memory module. The word bus in the port declaration indicates that all
drivers for the data bus may be disconnected at the same time (ie, none of
them is driving the bus).

7.4. Behavioural Description
In this section a behavioural model of the DP32 processor will be
presented. This model can be used to run test programs in the DP32
instruction set by connecting it to a simulated memory model. The
architecture body for the behavioural description is listed in Figure7-9.
The declaration section for the architecture body contains the
declaration for the DP32 register file type, and array of 32-bit words, indexed
by a natural number constrained to be in the range 0 to 255.
The architecture body contains only one concurrent statement, namely
an anonymous process which implements the behaviour as a sequential
algorithm. This process declares a number of variables which represent
the internal state of the processor: the register file (reg), the program
counter (PC), and the current instruction register (current_instr). A number
of working variables and aliases are also declared.
The procedure memory_read implements the behavioural model of a
memory read transaction. The parameters are the memory address to read
from, a flag indicating whether the read is an instruction fetch, and a
result parameter returning the data read. The procedure refers to the


entity ports, which are visible because they are declared in the parent of the
procedure.
The memory_read model firstly drives the address and fetch bit ports, and
then waits until the next leading edge of phi1, indicating the start of the next
clock cycle. (The wait statement is sensitive to a change from '0' to '1' on
phi1.) When that event occurs, the model checks the state of the reset input
port, and if it is set, immediately returns without further action. If reset is
clear, the model starts a T1 state by asserting the read bit port a propagation
delay time after the clock edge. It then waits again until the next phi1
leading edge, indicating the start of the next clock cycle. Again, it checks
reset and discontinues if reset is set. The model then starts a loop executing
T2 states. It waits until phi2 changes from '1' to '0' (at the end of the cycle),
and then checks reset again, returning if it is set. Otherwise it checks the
ready bit input port, and if set, accepts the data from the data bus port and
exits the loop. If ready is not set, the loop repeats, adding another T2 state to
the transaction. After the loop, the model waits for the next clock edge
indicating the start of the Ti state at the end of the transaction. After
checking reset again, the model clears ready to complete the transaction,
and returns to the parent process.
The procedure memory_write is similar, implementing the model for a
memory write transaction. The parameters are simply the memory
address to write to, and the data to write. The model similarly has reset
checks after each wait point. One difference is that at the end of the
transaction, there is a null signal assignment to the data bus port. This
models the bahaviour of the processor disconnecting from the data bus, that
is, at this point it stops driving the port.

architecture behaviour of dp32 is
subtype reg_addr is natural range 0 to 255;
type reg_array is array (reg_addr) of bit_32;
begin -- behaviour of dp32
process
variable reg : reg_array;
variable PC : bit_32;
variable current_instr : bit_32;
variable op: bit_8;
variable r3, r1, r2 : reg_addr;
variable i8 : integer;
alias cm_i : bit is current_instr(19);
alias cm_V : bit is current_instr(18);
alias cm_N : bit is current_instr(17);
alias cm_Z : bit is current_instr(16);
variable cc_V, cc_N, cc_Z : bit;
variable temp_V, temp_N, temp_Z : bit;
variable displacement, effective_addr : bit_32;

Figure7-9. Behavioural architecture body for dp32.


procedure memory_read (addr : in bit_32;
fetch_cycle : in boolean;
result : out bit_32) is
begin
-- start bus cycle with address output
a_bus <= addr after Tpd;
fetch <= bool_to_bit(fetch_cycle) after Tpd;
wait until phi1 = '1';
if reset = '1' then
return;
end if;
--
-- T1 phase
--
read <= '1' after Tpd;
if reset = '1' then
return;
end if;
--
-- T2 phase
--
loop
if reset = '1' then
return;
end if;
-- end of T2
if ready = '1' then
result := d_bus;
exit;
end if;
end loop;
if reset = '1' then
return;
end if;
--
-- Ti phase at end of cycle
--
end memory_read;



procedure memory_write (addr : in bit_32;
data : in bit_32) is
begin
-- start bus cycle with address output
a_bus <= addr after Tpd;
fetch <= '0' after Tpd;
if reset = '1' then
return;
end if;
--
-- T1 phase
--
write <= '1' after Tpd;
d_bus <= data after Tpd;
if reset = '1' then
return;
end if;
--
-- T2 phase
--
loop
if reset = '1' then
return;
end if;
-- end of T2
exit when ready = '1';
end loop;
if reset = '1' then
return;
end if;
--
-- Ti phase at end of cycle
--
d_bus <= null after Tpd;
end memory_write;



The next four procedures, add, subtract, multiply and divide, implement the
arithmetic operations on 32-bit words representing twos-complement
signed integers. They each take two integer operands, and produce a 32-bit
word result and the three condition code flags V (overflow), N (negative)
and Z (zero). The result parameter is of mode inout because the test for
negative and zero results read its value after it has been written. Each
procedure is carefully coded to avoid causing an integer overflow on the
host machine executing the model (assuming that machine uses 32-bit
integers). The add and subtract procedures wrap around if overflow occurs,
and multiply and divide return the largest or smallest integer.
Following these procedures is the body of the process which implements
the DP32 behavioural model. This process is activated during the
initialisation phase of a simulation. It consists of three sections which are
repeated sequentially: reset processing, instruction fetch, and instruction
execution.

procedure add (result : inout bit_32;
op1, op2 : in integer;
V, N, Z : out bit) is
begin
if op2 > 0 and op1 > integer'high-op2 then -- positive overflow
int_to_bits(((integer'low+op1)+op2)-integer'high-1, result);
V := '1';
elsif op2 < 0 and op1 < integer'low-op2 then -- negative overflow
int_to_bits(((integer'high+op1)+op2)-integer'low+1, result);
V := '1';
else
int_to_bits(op1 + op2, result);
V := '0';
end if;
N := result(31);
Z := bool_to_bit(result = X"0000_0000");
end add;
procedure subtract (result : inout bit_32;
begin
if op2 < 0 and op1 > integer'high+op2 then -- positive overflow
int_to_bits(((integer'low+op1)-op2)-integer'high-1, result);
V := '1';
elsif op2 > 0 and op1 < integer'low+op2 then -- negative overflow
int_to_bits(((integer'high+op1)-op2)-integer'low+1, result);
V := '1';
else
int_to_bits(op1 - op2, result);
V := '0';
end if;
N := result(31);
end subtract;



When the reset input is asserted, all of the control ports are returned to
their initial states, the data bus driver is disconnected, and the PC register
is cleared. The model then waits until reset is negated before proceeding.
Throughout the rest of the model, the reset input is checked after each bus
transaction. If the transaction was aborted by reset being asserted, no
further action is taken in fetching or executing an instruction, and control
falls through to the reset handling code.
The instruction fetch part is simply a call to the memory read
procedure. The PC register is used to provide the address, the fetch flag is
true, and the result is returned into the current instruction register. The
PC register is then incremented by one using the arithmetic procedure
previously defined.
The fetched instruction is next decoded into its component parts: the op-
code, the source and destination register addresses and an immediate
constant field. The op-code is then used as the selector for a case statement

procedure multiply (result : inout bit_32;
begin
if ((op1>0 and op2>0) or (op1<0 and op2<0)) -- result positive
and (abs op1 > integer'high / abs op2) then -- positive overflow
int_to_bits(integer'high, result);
V := '1';
elsif ((op1>0 and op2<0) or (op1<0 and op2>0)) -- result negative
and ((- abs op1) < integer'low / abs op2) then -- negative overflow
int_to_bits(integer'low, result);
V := '1';
else
int_to_bits(op1 * op2, result);
V := '0';
end if;
N := result(31);
end multiply;
procedure divide (result : inout bit_32;
begin
if op2=0 then
if op1>=0 then -- positive overflow
int_to_bits(integer'high, result);
else
int_to_bits(integer'low, result);
end if;
V := '1';
else
int_to_bits(op1 / op2, result);
V := '0';
end if;
N := result(31);
end divide;



which codes the instruction execution. For the arithmetic instructions
(including the quick forms), the arithmetic procedures previously defined
are invoked. For the logical instructions, the register bit-vector values are
used in VHDL logical expressions to determine the bit-vector result. The
condition code Z flag is set if the result is a bit-vector of all '0' bits.
The model executes a load instruction by firstly reading the
displacement from memory and incrementing the PC register. The
displacement is added to the value of the index register to form the effective
address. This is then used in a memory read to load the data into the result
register. A quick load is executed similarly, except that no memory read is
needed to fetch the displacement; the variable i8 decoded from the
instruction is used. The store and quick store instructions parallel the load
instructions, with the memory data read being replaced by a memory data
write.
Execution of a branch instruction starts with a memory read to fetch the
displacement, and an add to increment the PC register by one. The
displacement is added to the value of the PC register to form the effective
address. Next, the condition expression is evaluated, comparing the
condition code bits with the condition mask in the instruction, to determine
whether the branch is taken. If it is, the PC register takes on the effective
address value. The branch indexed instruction is similar, with the index
register value replacing the PC value to form the effective address. The
quick branch forms are also similar, with the immediate constant being
used for the displacement instead of a value fetched from memory.

begin
--
-- check for reset active
--
if reset = '1' then
PC := X"0000_0000";
wait until reset = '0';
end if;
--
-- fetch next instruction
--
memory_read(PC, true, current_instr);
if reset /= '1' then
add(PC, bits_to_int(PC), 1, temp_V, temp_N, temp_Z);
--
-- decode & execute
--
op := current_instr(31 downto 24);
r3 := bits_to_natural(current_instr(23 downto 16));
i8 := bits_to_int(current_instr(7 downto 0));



case op is
when op_add =>
add(reg(r3), bits_to_int(reg(r1)), bits_to_int(reg(r2)),
cc_V, cc_N, cc_Z);
when op_addq =>
add(reg(r3), bits_to_int(reg(r1)), i8, cc_V, cc_N, cc_Z);
when op_sub =>
subtract(reg(r3), bits_to_int(reg(r1)), bits_to_int(reg(r2)),
cc_V, cc_N, cc_Z);
when op_subq =>
subtract(reg(r3), bits_to_int(reg(r1)), i8, cc_V, cc_N, cc_Z);
when op_mul =>
multiply(reg(r3), bits_to_int(reg(r1)), bits_to_int(reg(r2)),
cc_V, cc_N, cc_Z);
when op_mulq =>
multiply(reg(r3), bits_to_int(reg(r1)), i8, cc_V, cc_N, cc_Z);
when op_div =>
divide(reg(r3), bits_to_int(reg(r1)), bits_to_int(reg(r2)),
cc_V, cc_N, cc_Z);
when op_divq =>
divide(reg(r3), bits_to_int(reg(r1)), i8, cc_V, cc_N, cc_Z);
when op_land =>
reg(r3) := reg(r1) and reg(r2);
cc_Z := bool_to_bit(reg(r3) = X"0000_0000");
when op_lor =>
reg(r3) := reg(r1) or reg(r2);
when op_lxor =>
reg(r3) := reg(r1) xor reg(r2);
when op_lmask =>
reg(r3) := reg(r1) and not reg(r2);
when op_ld =>
memory_read(PC, true, displacement);
add(effective_addr,
bits_to_int(reg(r1)), bits_to_int(displacement),
temp_V, temp_N, temp_Z);
memory_read(effective_addr, false, reg(r3));
end if;
when op_ldq =>
add(effective_addr,
bits_to_int(reg(r1)), i8,
memory_read(effective_addr, false, reg(r3));
when op_st =>
add(effective_addr,
memory_write(effective_addr, reg(r3));
end if;



when op_stq =>
add(effective_addr,
memory_write(effective_addr, reg(r3));
when op_br =>
add(effective_addr,
bits_to_int(PC), bits_to_int(displacement),
if ((cm_V and cc_V) or (cm_N and cc_N) or (cm_Z and cc_Z))
= cm_i then
PC := effective_addr;
end if;
end if;
when op_bi =>
add(effective_addr,
= cm_i then
end if;
end if;
when op_brq =>
add(effective_addr,
bits_to_int(PC), i8,
= cm_i then
end if;
when op_biq =>
add(effective_addr,
= cm_i then
end if;
when others =>
assert false report "illegal instruction" severity warning;
end case;
end if; -- reset /= '1'
end process;
end behaviour;



CLOCK_GEN DP32 MEMORY
PHI1 PHI1 FETCH FETCH
PHI2 PHI2 READ READ
RESET RESET WRITE WRITE

READY A_BUS A_BUS READY

D_BUS D_BUS

Figure7-10. Test bench circuit for DP32.

entity clock_gen is
generic (Tpw : Time; -- clock pulse width
Tps : Time); -- pulse separation between phases
port (phi1, phi2 : out bit;
reset : out bit);
end clock_gen;
architecture behaviour of clock_gen is
constant clock_period : Time := 2*(Tpw+Tps);
begin
reset_driver :
reset <= '1', '0' after 2*clock_period+Tpw;
clock_driver : process
begin
phi1 <= '1', '0' after Tpw;
phi2 <= '1' after Tpw+Tps, '0' after Tpw+Tps+Tpw;
wait for clock_period;
end process clock_driver;
end behaviour;

Figure7-11. Description of clock_gen driver.

7.5. Test Bench
One way of testing the behavioural model of the DP32 processor is to
connect it in a test bench circuit, shown in Figure7-10. The clock_gen
component generates the two-phase clock and the reset signal to drive the
processor. The memory stores a test program and data. We write
behavioural models for these two components, and connect them in a
structural description of the test bench.
Figure7-11 lists the entity declaration and behavioural architecture of
the clock generator. The clock_gen entity has two formal generic constants.
Tpw is the pulse width for each of phi1 and phi2, that is, the time for which
each clock is '1'. Tps is the pulse separation, that is, the time between one
clock signal changing to '0' and the other clock signal changing to '1'.


Based on these values, the clock period is twice the sum of the pulse width
and the separation.
The architecture of the clock generator consists of two concurrent
statements, one to drive the reset signal and the other to drive the clock
signals. The reset driver schedules a '1' value on reset when it is activated
at simulation initialisation, followed by a '0' a little after two clock periods
later. This concurrent statement is never subsequently reactivated, since
its waveform list does not refer to any signals. The clock driver process,
when activated, schedules a pulse on phi1 immediately, followed by a pulse
on phi2, and then suspends for a clock period. When it resumes, it repeats,
scheduling the next clock cycle.
The entity declaration and behavioural architecture of the memory
module are shown in Figure7-12. The architecture body consists of one
process to implement the behaviour. The process contains an array
variable to represent the storage of the memory. When the process is
activated, it places the output ports in an initial state: the data bus
disconnected and the ready bit negated. It then waits for either a read or
write command. When one of these occurs, the address is sampled and
converted from a bit-vector to a number. If it is within the address bounds
of the memory, the command is acted upon.
For a write command, the ready bit is asserted after a delay representing
the write access time of the memory, and then the model waits until the end
of the write cycle. At that time, the value on the data bus from a
propagation delay beforehand is sampled and written into the memory
array. The use of this delayed value models the fact that memory devices
actually store the data that was valid a setup-time before the triggering edge
of the command bit.
For a read command, the data from the memory array is accessed and
placed on the data bus after a delay. This delay represents the read access
time of the memory. The ready bit is also asserted after the delay, indicating
that the processor may continue. The memory then waits until the end of
the read cycle.
At the end of a memory cycle, the process repeats, setting the data bus
and ready bit drivers to their initial state, and waiting for the next
command.
Figure7-13 shows the entity declaration and structural architecture of
the test bench circuit. The entity contains no ports, since there are no
external connections to the test bench. The architecture body contains
component declarations for the clock driver, the memory and the processor.
The ports in these component declarations correspond exactly to those of the
entity declarations. There are no formal generic constants, so the actuals
for the generics in the entity declarations will be specified in a
configuration. The architecture body next declares the signals which are
used to connect the components together. These signals may be traced by a
simulation monitor when the simulation is run. The concurrent
statements of the architecture body consist of the three component
instances.


entity memory is
a_bus : in bit_32;
read, write : in bit;
ready : out bit);
end memory;
architecture behaviour of memory is
begin
process
constant low_address : integer := 0;
constant high_address : integer := 65535;
type memory_array is
array (integer range low_address to high_address) of bit_32;
variable mem : memory_array;
variable address : integer;
begin
--
-- put d_bus and reply into initial state
--
ready <= '0' after Tpd;
--
-- wait for a command
--
wait until (read = '1') or (write = '1');
--
-- dispatch read or write cycle
--
address := bits_to_int(a_bus);
if address >= low_address and address <= high_address then
-- address match for this memory
if write = '1' then
wait until write = '0'; -- wait until end of write cycle
mem(address) := d_bus'delayed(Tpd); -- sample data from Tpd ago
else -- read = '1'
d_bus <= mem(address) after Tpd; -- fetch data
wait until read = '0'; -- hold for read cycle
end if;
end if;
end process;
end behaviour;

Figure7-12. Description of memory module.


entity dp32_test is
end dp32_test;
architecture structure of dp32_test is
component clock_gen
port (phi1, phi2 : out bit;
reset : out bit);
end component;
component dp32
a_bus : out bit_32;
read, write : out bit;
fetch : out bit;
ready : in bit;
phi1, phi2 : in bit;
reset : in bit);
end component;
component memory
a_bus : in bit_32;
read, write : in bit;
ready : out bit);
end component;
signal d_bus : bus_bit_32 bus;
signal a_bus : bit_32;
signal read, write : bit;
signal fetch : bit;
signal ready : bit;
signal phi1, phi2 : bit;
signal reset : bit;
begin
cg : clock_gen
port map (phi1 => phi1, phi2 => phi2, reset => reset);
proc : dp32
port map (d_bus => d_bus, a_bus => a_bus,
read => read, write => write, fetch => fetch,
ready => ready,
phi1 => phi1, phi2 => phi2, reset => reset);
mem : memory
port map (d_bus => d_bus, a_bus => a_bus,
read => read, write => write, ready => ready);
end structure;

Figure7-13. Description of test bench circuit.


configuration dp32_behaviour_test of dp32_test is
for structure
for cg : clock_gen
use entity work.clock_gen(behaviour)
generic map (Tpw => 8 ns, Tps => 2 ns);
end for;
for mem : memory
use entity work.memory(behaviour);
end for;
for proc : dp32
use entity work.dp32(behaviour);
end for;
end for;
end dp32_behaviour_test;

Figure7-14. Configuration of test bench using behaviour of DP32.

Lastly, a configuration for the test bench, using the behavioural
description of the DP32 processor, is listed in Figure7-14. The
configuration specifies that each of the components in the structure
architecture of the test bench should use the behaviour architecture of the
corresponding entity. Actual generic constants are specified for the clock
generator, giving a clock period of 20ns. The default values for the generic
constants of the other entities are used.
In order to run the test bench model, a simulation monitor is invoked
and a test program loaded into the array variable in the memory model.
The author used the Zycad System VHDL™ simulation system for this
purpose. Figure7-15 is an extract from the listing produced by an
assembler created for the DP32 processor. The test program initializes R0
to zero (the assembler macro initr0 generates an lmask instruction), and
then loops incrementing a counter in memory. The values in parentheses
are the instruction addresses, and the hexadecimal values in square
brackets are the assembled instructions.

™ Zycad System VHDL is a trademark of Zycad Corporation.


1. include dp32.inc $
2.
3. !!! conventions:
4. !!! r0 = 0
5. !!! r1 scratch
6.
7. begin
8. ( 0) [07000000 ] initr0
9. start:
10. ( 1) [10020000 ] addq(r2, r0, 0) ! r2 := 0
11. loop:
12. ( 2) [21020000 00000008] sta(r2, counter) ! counter := r2
13. ( 4) [10020201 ] addq(r2, r2, 1) ! increment r2
14. ( 5) [1101020A ] subq(r1, r2, 10) ! if r2 = 10 then
15. ( 6) [500900FA ] brzq(start) ! restart
16. ( 7) [500000FA ] braq(loop) ! else next loop
17.
18. counter:
19. ( 8) [00000000 ] data(0)
20. end

Figure7-15. Assembler listing of a test program.


7.6. Register Transfer Architecture
The previous descriptions of the DP32 specified its behaviour without
reference to the internal structure of the processor. Such a description is
invaluable, as it allows the computer architect to evaluate the instruction
set and compare it with alternatives before commiting expensive resources
to detailed design and implementation.
Once this abstract architecture has been settled on, the next level of
architecture can be designed. Figure7-16 is a block diagram of a simple
architecture to implement the DP32 instrcuction set. (Most control signals
are not shown.) It consists mainly of a collection of registers and an
arithmetic and logic unit (ALU), connected by a number of buses. There
are also buffers for interfacing to the processor-memory bus, and a control
unit for sequencing operation of the processor.
The software addressable registers are implemented using a three-port
register file. Ports1 and2 supply source operands onto the op1 and op2
buses respectively. The address for port2 is normally taken from the r2
field of the current instruction, but a multiplexor is included to allow the r3
field to be used when a store instruction is executed. The op1 and op2 buses

A1 A2 A3

CC CC
comp
A1 A2 A3
Register
File
PC Q1 Q2 D3

Op1 Bus
Res
Op2 Bus

R Bus

op r3 r1 r2 Addr Disp

A2
D Bus
A1
A3
A Bus
Bus Command
Control Bus Reply

Figure7-16. DP32 data paths block diagram.


are connected to the ALU inputs, and the ALU output drives the result bus.
The result can be latched for writing back to the register file using port3.
The program counter (PC) register also supplies the op1 bus, and can be
loaded from the result bus. The ALU condition flags are latched into the
condition code (CC) register, and from there can be compared with the
condition mask from the current instruction. The memory bus interface
includes an address latch to drive the address bus, a data output buffer
driven from the op2 bus, a data input buffer driving the result bus, and a
displacement latch driving the op2 bus. An instruction fetched from
memory is stored in current instruction register. The r1, r2 and r3 fields
are used as register file addresses. The r2 field is also used as an
immediate constant and may be sign extended onto the op2 bus. Four bits
from the r3 field are used as the condition mask, and the opcode field is
used by the control unit.
In this section, descriptions will be given for each of the sub-modules in
this architecture, and then they will be used in a structural architecture
body of the DP32 entity.
7.6.1. Multiplexor
An entity declaration and architecture body for a 2-input multiplexor is
listed in Figure7-17. The entity has a select input bit, two bit-vector inputs
i0 and i1, and a bit-vector output y. The size of the bit-vector ports is
determined by the generic constant width, which must be specified when the
entity is used in a structural description. The architecture body contains a
concurrent selected signal assignment, which uses the value of the select
input to determine which of the two bit-vector inputs is passed through to
the output. The assignment is sensitive to all of the input signals, so when
any of them changes, the assignment will be resumed.
7.6.2. Transparent Latch
An entity declaration and architecture body for a latch is listed in
Figure7-18. The entity has an enable input bit, a bit-vector input d, and a
bit-vector output q. The size of the bit-vector ports is determined by the
generic constant width, which must be specified when the entity is used in a
structural description. The architecture body contains a process which is

entity mux2 is
generic (width : positive;
Tpd : Time := unit_delay);
port (i0, i1 : in bit_vector(width-1 downto 0);
y : out bit_vector(width-1 downto 0);
sel : in bit);
end mux2;
architecture behaviour of mux2 is
begin
with sel select
y <= i0 after Tpd when '0',
i1 after Tpd when '1';
end behaviour;

Figure7-17. Description of 2-input multiplexor.


entity latch is
generic (width : positive;
Tpd : Time := unit_delay);
port (d : in bit_vector(width-1 downto 0);
q : out bit_vector(width-1 downto 0);
en : in bit);
end latch;
architecture behaviour of latch is
begin
process (d, en)
begin
if en = '1' then
q <= d after Tpd;
end if;
end process;
end behaviour;

Figure7-18. Description of a transparent latch.

sensitive to the d and en inputs. The behaviour of the latch is such that
when en is '1', changes on d are transmitted through to q. However, when
en changes to '0', any new value on d is ignored, and the current value on q
is maintained. In the model shown in Figure7-18, the latch storage is
provided by the output port, in that if no new value is assigned to it, the
current value does not change.
7.6.3. Buffer
An entity declaration and architecture body for a buffer is listed in
Figure7-19. The entity has an enable input bit en, a bit-vector input a, and a
resolved bit-vector bus output b. It is not possible to make this entity generic
with respect to input and output port width, because of a limitation imposed
by the VHDL language semantics. The output port needs to be a resolved
signal, so a bus resolution function is specified in the definition of the port
type. This function takes a parameter which is an unconstrained array.
In order to make the buffer port width generic, we would need to specify a
bus resolution function which took as a parameter an unconstrained array
of bit-vector elements whose length is not known. VHDL does not allow the
element type of an unconstrained array to be an unconstrained array, so
this approach is not possible. For this reason, we define a buffer entity with
fixed port widths of 32bits.
The behaviour of the buffer is implemented by a process sensitive to the
en and a inputs. If en is '1', the a input is transmitted through to the b
output. If en is '0', the driver for b is disconnected, and the value on a is
ignored.


entity buffer_32 is
port (a : in bit_32;
b : out bus_bit_32 bus;
en : in bit);
end buffer_32;
architecture behaviour of buffer_32 is
begin
b_driver: process (en, a)
begin
if en = '1' then
b <= a after Tpd;
else
b <= null after Tpd;
end if;
end process b_driver;
end behaviour;

Figure7-19. Description of a buffer.

entity signext_8_32 is
port (a : in bit_8;
en : in bit);
end signext_8_32;
architecture behaviour of signext_8_32 is
begin
b_driver: process (en, a)
begin
if en = '1' then
b(7 downto 0) <= a after Tpd;
if a(7) = '1' then
b(31 downto 8) <= X"FFFF_FF" after Tpd;
else
b(31 downto 8) <= X"0000_00" after Tpd;
end if;
else
b <= null after Tpd;
end if;
end process b_driver;
end behaviour;

Figure7-20. Description of the sign extending buffer.


7.6.4. Sign Extending Buffer
The sign-extending buffer shown in Figure7-20 is almost identical to the
plain buffer, except that it has an 8-bit input. This input is treated as a
twos-complement signed integer, and the output is the same integer, but
extended to 32bits. The extension is achieved by replicating the sign bit into
bits8 to31 of the output.
7.6.5. Latching Buffer
Figure7-21 lists an entity declaration an architecture body for a latching
buffer. This model is a combination of those for the plain latch and buffer.
When latch_en is '1', changes on d are stored in the latch, and may be
transmitted through to q. However, when latch_en changes to '0', any new
value on d is ignored, and the currently stored value is maintained. The
out_en input controls whether the stored value is tranmitted to the output.
Unlike the plain latch, explicit storage must be provided (in the form of the
variable latched_value), since the output driver may be disconnected when a
new value is to be stored.
7.6.6. Program Counter Register
The entity declaration and architecture body of the PC register are listed
in Figure7-22. The PC register is a master/slave type register, which can
be reset to all zeros by asserting the reset input. When reset is negated, the
latch operates normally. With latch_en at '1', the value of the d input is
stored in the variable master_PC, but the output (if enabled) is driven from
the previously stored value in slave_PC. Then when latch_en changes from

entity latch_buffer_32 is
port (d : in bit_32;
q : out bus_bit_32 bus;
latch_en : in bit;
out_en : in bit);
end latch_buffer_32;
architecture behaviour of latch_buffer_32 is
begin
process (d, latch_en, out_en)
variable latched_value : bit_32;
begin
if latch_en = '1' then
latched_value := d;
end if;
if out_en = '1' then
q <= latched_value after Tpd;
else
q <= null after Tpd;
end if;
end process;
end behaviour;

Figure7-21. Description of a latching buffer.


entity PC_reg is
latch_en : in bit;
out_en : in bit;
reset : in bit);
end PC_reg;
architecture behaviour of PC_reg is
begin
process (d, latch_en, out_en, reset)
variable master_PC, slave_PC : bit_32;
begin
if reset = '1' then
slave_PC := X"0000_0000";
elsif latch_en = '1' then
master_PC := d;
else
slave_PC := master_PC;
end if;
if out_en = '1' then
q <= slave_PC after Tpd;
else
q <= null after Tpd;
end if;
end process;
end behaviour;

Figure7-22. Description of the PC register.

'1' to '0', the slave value is update from the master value, and any
subsequent changes in the d input are ignored. This behaviour means that
the PC register output can be used to derive a new value, and the new value
written back at the same time. If an ordinary transparent latch were used,
a race condition would be created, since the new value would be transmitted
through to the output in place of the old value, affecting the calculation of
the new value.
7.6.7. Register File
Figure7-23 lists the description of the 3-port register file, with two read
ports and one write port. Each port has an address input ( a1, a2 and a3)
and an enable input (en1, en2 and en3). The read ports have data bus
outputs (q1 and q2), and the write port has a data input (d3). The number
bits in the port addresses is determined by the generic constant depth. The
behaviour of the entity is implemented by the process reg_file. It declares a
numeric type used to index the register file, and an array for the register
file storage. When any of the inputs change, firstly the write port enable is
checked, and if asserted, the addressed register is updated. Then each of
the read port enables is checked. If asserted, the addressed data is fetched
and driven onto the corresponding data output bus. If the port is disabled,
the data output bus driver is disconnected.


entity reg_file_32_rrw is
generic (depth : positive; -- number of address bits
Tpd : Time := unit_delay;
Tac : Time := unit_delay);
port (a1 : in bit_vector(depth-1 downto 0);
q1 : out bus_bit_32 bus;
en1 : in bit;
a2 : in bit_vector(depth-1 downto 0);
en2 : in bit;
d3 : in bit_32;
en3 : in bit);
end reg_file_32_rrw;
architecture behaviour of reg_file_32_rrw is
begin
reg_file: process (a1, en1, a2, en2, a3, d3, en3)
subtype reg_addr is natural range 0 to depth-1;
type register_array is array (reg_addr) of bit_32;
variable registers : register_array;
begin
if en3 = '1' then
registers(bits_to_natural(a3)) := d3;
end if;
if en1 = '1' then
q1 <= registers(bits_to_natural(a1)) after Tac;
else
q1 <= null after Tpd;
end if;
if en2 = '1' then
q2 <= registers(bits_to_natural(a2)) after Tac;
else
q2 <= null after Tpd;
end if;
end process reg_file;
end behaviour;

Figure7-23. Description of the 3-port register file.

7.6.8. Arithmetic & Logic Unit
The description of the ALU is listed in Figure7-24. The package
ALU_32_types defines an enumerated type for specifying the ALU function.
This must be placed in a package, since it is required for both the ALU
description and for entities that make use of the ALU. There is no
corresponding package body, since the type is fully defined in the package
specification.
The ALU entity declaration uses the ALU_32_types package as well as the
general dp32_types package. It has two operand input ports, a result output
and condition code output ports, and a command input port. This last port
is an example of a port which is of an enumerated type, since at this stage


of design, no encoding is known or specified for the ALU function
command.
The ALU behaviour is implemented by the process ALU_function, sensitive
to changes on the operand and command input ports. If the command to be
performed is an arithmetic operation, the model firstly converts the
operands to integers. This is followed by a case statement dispatching on
the command. For the disable command, no operation is performed, and for
the pass1 command, the result is operand1 unchanged. The result for logic
commands is derived by applying the corresponding VHDL logical
operations to the bit-vector operands. For arithmetic commands the result
is computed the same was as it was in the behavioural model of the DP32
presented in Section7.4. Also, the overflow condition code bit (cc_V), which
is only defined for arithmetic operations, is assigned here. Finally, the
result and remaining condition code bits are assigned. The result output is
only driven if the command is not disable, otherwise it is disconnected.

package ALU_32_types is
type ALU_command is (disable, pass1, incr1,
add, subtract, multiply, divide,
log_and, log_or, log_xor, log_mask);
end ALU_32_types;

use work.dp32_types.all, work.ALU_32_types.all;
entity ALU_32 is
port (operand1 : in bit_32;
operand2 : in bit_32;
result : out bus_bit_32 bus;
cond_code : out CC_bits;
command : in ALU_command);
end ALU_32;

Figure7-24. Description of the Arithmetic and Logic Unit.


architecture behaviour of ALU_32 is
alias cc_V : bit is cond_code(2);
alias cc_N : bit is cond_code(1);
alias cc_Z : bit is cond_code(0);
begin
ALU_function: process (operand1, operand2, command)
variable a, b : integer;
variable temp_result : bit_32;
begin
case command is
when add | subtract | multiply | divide =>
a := bits_to_int(operand1);
b := bits_to_int(operand2);
when incr1 =>
a := bits_to_int(operand1);
b := 1;
when others =>
null;
end case;
case command is
when disable =>
null;
when pass1 =>
temp_result := operand1;
when log_and =>
temp_result := operand1 and operand2;
when log_or =>
temp_result := operand1 or operand2;
when log_xor =>
temp_result := operand1 xor operand2;
when log_mask =>
temp_result := operand1 and not operand2;
when add | incr1 =>
if b > 0 and a > integer'high-b then -- positive overflow
int_to_bits(((integer'low+a)+b)-integer'high-1, temp_result);
cc_V <= '1' after Tpd;
elsif b < 0 and a < integer'low-b then -- negative overflow
int_to_bits(((integer'high+a)+b)-integer'low+1, temp_result);
else
int_to_bits(a + b, temp_result);
end if;
when subtract =>
if b < 0 and a > integer'high+b then -- positive overflow
int_to_bits(((integer'low+a)-b)-integer'high-1, temp_result);
elsif b > 0 and a < integer'low+b then -- negative overflow
int_to_bits(((integer'high+a)-b)-integer'low+1, temp_result);
else
int_to_bits(a - b, temp_result);
end if;



when multiply =>
if ((a>0 and b>0) or (a<0 and b<0)) -- result positive
and (abs a > integer'high / abs b) then
-- positive overflow
int_to_bits(integer'high, temp_result);
elsif ((a>0 and b<0) or (a<0 and b>0)) -- result negative
and ((- abs a) < integer'low / abs b) then
-- negative overflow
int_to_bits(integer'low, temp_result);
else
int_to_bits(a * b, temp_result);
end if;
when divide =>
if b=0 then
if a>=0 then -- positive overflow
int_to_bits(integer'high, temp_result);
else
int_to_bits(integer'low, temp_result);
end if;
else
int_to_bits(a / b, temp_result);
end if;
end case;
if command /= disable then
result <= temp_result after Tpd;
else
result <= null after Tpd;
end if;
cc_Z <= bool_to_bit(temp_result = X"00000000") after Tpd;
cc_N <= bool_to_bit(temp_result(31) = '1') after Tpd;
end process ALU_function;

end behaviour;



entity cond_code_comparator is
port (cc : in CC_bits;
cm : in cm_bits;
result : out bit);
end cond_code_comparator;
architecture behaviour of cond_code_comparator is
alias cc_V : bit is cc(2);
alias cc_N : bit is cc(1);
alias cc_Z : bit is cc(0);
alias cm_i : bit is cm(3);
alias cm_V : bit is cm(2);
alias cm_N : bit is cm(1);
alias cm_Z : bit is cm(0);
begin
result <= bool_to_bit(((cm_V and cc_V)
or (cm_N and cc_N)
or (cm_Z and cc_Z)) = cm_i) after Tpd;
end behaviour;

Figure7-25. Description of the condition code comparator.

7.6.9. Condition Code Comparator
The description of the condition code comparator is listed in Figure7-25.
The cc input port contains the three condition code bits V, N and Z, and the
cm input contains the four condition mask bits derived from a DP32
instruction. Aliases for each of these bits are declared in the architecture
body. The behaviour is implemented by a single concurrent signal
assignment statement, which is sensitive to all of the input bits. Whenever
any of the bits changes value, the assignment will be resumed and a new
result bit computed.
7.6.10. Structural Architecture of the DP32
In this section, a structural architecture body for the DP32 processor,
corresponding to Figure7-16, will be described. See Figure7-26 for a listing
of the architecture body.


use work.dp32_types.all, work.ALU_32_types.all;
architecture RTL of dp32 is
component reg_file_32_rrw
generic (depth : positive);
port (a1 : in bit_vector(depth-1 downto 0);
en1 : in bit;
en2 : in bit;
d3 : in bit_32;
en3 : in bit);
end component;
component mux2
port (i0, i1 : in bit_vector(width-1 downto 0);
y : out bit_vector(width-1 downto 0);
sel : in bit);
end component;
component PC_reg
latch_en : in bit;
out_en : in bit;
reset : in bit);
end component;
component ALU_32
port (operand1 : in bit_32;
operand2 : in bit_32;
result : out bus_bit_32 bus;
cond_code : out CC_bits;
command : in ALU_command);
end component;
component cond_code_comparator
port (cc : in CC_bits;
cm : in cm_bits;
result : out bit);
end component;
component buffer_32
port (a : in bit_32;
en : in bit);
end component;
component latch
port (d : in bit_vector(width-1 downto 0);
q : out bit_vector(width-1 downto 0);
en : in bit);
end component;

Figure7-26. Structural description of the DP32 processor.


component latch_buffer_32
latch_en : in bit;
out_en : in bit);
end component;
component signext_8_32
port (a : in bit_8;
en : in bit);
end component;
signal op1_bus : bus_bit_32;
signal op2_bus : bus_bit_32;
signal r_bus : bus_bit_32;
signal ALU_CC : CC_bits;
signal CC : CC_bits;
signal current_instr : bit_32;
alias instr_a1 : bit_8 is current_instr(15 downto 8);
alias instr_op : bit_8 is current_instr(31 downto 24);
alias instr_cm : cm_bits is current_instr(19 downto 16);
signal reg_a2 : bit_8;
signal reg_result : bit_32;
signal addr_latch_en : bit;
signal disp_latch_en : bit;
signal disp_out_en : bit;
signal d2_en : bit;
signal dr_en : bit;
signal instr_latch_en : bit;
signal immed_signext_en : bit;
signal ALU_op : ALU_command;
signal CC_latch_en : bit;
signal CC_comp_result : bit;
signal PC_latch_en : bit;
signal PC_out_en : bit;
signal reg_port1_en : bit;
signal reg_port2_mux_sel : bit;
signal reg_res_latch_en : bit;
begin -- architecture RTL of dp32
reg_file : reg_file_32_RRW
generic map (depth => 8)
port map (a1 => instr_a1, q1 => op1_bus, en1 => reg_port1_en,
a2 => reg_a2, q2 => op2_bus, en2 => reg_port2_en,
a3 => instr_a3, d3 => reg_result, en3 => reg_port3_en);
reg_port2_mux : mux2
port map (i0 => instr_a2, i1 => instr_a3, y => reg_a2,
sel => reg_port2_mux_sel);



The architecture refers to the items declared in the packages dp32_types
and ALU_32_types, so a use clause for these packages is included. The
declaration section of the architecture contains a number of component
declarations, corresponding to the entity declarations listed in Sections7.6.1
to7.6.9. Instances of these components are subsequently used to construct
the processor architecture.
Next, a number of signals are declared, corresponding to the buses
illustrated in Figure7-16. These are followed by further signal declarations
for control signals not shown in the figure. The control signals are used to
connect the data path component instances with the control unit
implemented in the block called controller.

reg_res_latch : latch
port map (d => r_bus, q => reg_result, en => reg_res_latch_en);
PC : PC_reg
port map (d => r_bus, q => op1_bus,
latch_en => PC_latch_en, out_en => PC_out_en,
reset => reset);
ALU : ALU_32
port map (operand1 => op1_bus, operand2 => op2_bus,
result => r_bus, cond_code => ALU_CC,
command => ALU_op);
CC_reg : latch
port map (d => ALU_CC, q => CC, en => CC_latch_en);
CC_comp : cond_code_comparator
port map (cc => CC, cm => instr_cm, result => CC_comp_result);
dr_buffer : buffer_32
port map (a => d_bus, b => r_bus, en => dr_en);
d2_buffer : buffer_32
port map (a => op2_bus, b => d_bus, en => d2_en);
disp_latch : latch_buffer_32
port map (d => d_bus, q => op2_bus,
latch_en => disp_latch_en, out_en => disp_out_en);
addr_latch : latch
port map (d => r_bus, q => a_bus, en => addr_latch_en);
instr_latch : latch
port map (d => r_bus, q => current_instr, en => instr_latch_en);
immed_signext : signext_8_32
port map (a => instr_a2, b => op2_bus, en => immed_signext_en);



controller : block
port (phi1, phi2 : in bit;
reset : in bit;
opcode : in bit_8;
read, write, fetch : out bit;
ready : in bit;
addr_latch_en : out bit;
disp_latch_en : out bit;
disp_out_en : out bit;
d2_en : out bit;
dr_en : out bit;
instr_latch_en : out bit;
immed_signext_en : out bit;
ALU_op : out ALU_command;
CC_latch_en : out bit;
CC_comp_result : in bit;
PC_latch_en : out bit;
PC_out_en : out bit;
reg_port1_en : out bit;
reg_port2_mux_sel : out bit;
reg_res_latch_en : out bit);
port map (phi1 => phi1, phi2 => phi2,
reset => reset,
opcode => instr_op,
read => read, write => write, fetch => fetch,
ready => ready,
addr_latch_en => addr_latch_en,
disp_latch_en => disp_latch_en,
disp_out_en => disp_out_en,
d2_en => d2_en,
dr_en => dr_en,
instr_latch_en => instr_latch_en,
immed_signext_en => immed_signext_en,
ALU_op => ALU_op,
CC_latch_en => CC_latch_en,
CC_comp_result => CC_comp_result,
PC_latch_en => PC_latch_en, PC_out_en => PC_out_en,
reg_port1_en => reg_port1_en,
reg_port2_mux_sel => reg_port2_mux_sel,
reg_res_latch_en => reg_res_latch_en);



The control unit is a state machine, whose behaviour is described by a
single process called state_machine. The controller sequences through the
states listed in the declaration of the type controller_state to fetch, decode and
execute instructions. The variable state holds the controller state for the
current clock cycle, and next_state is set to determine the state for the next
clock cycle. Write_back_pending is a flag used to schedule a register write
operation for the next clock cycle. The constant ALU_op_select is a lookup
table used to determine the ALU function from the instruction op-code.


b e g i n -- block controller
state_machine: process
type controller_state is
(resetting, fetch_0, fetch_1, fetch_2, decode,
disp_fetch_0, disp_fetch_1, disp_fetch_2,
execute_0, execute_1, execute_2);
variable state, next_state : controller_state;
variable write_back_pending : boolean;
type ALU_op_select_table is
array (natural range 0 to 255) of ALU_command;
constant ALU_op_select : ALU_op_select_table :=
(16#00# => add,
16#01# => subtract,
16#02# => multiply,
16#03# => divide,
16#10# => add,
16#11# => subtract,
16#12# => multiply,
16#13# => divide,
16#04# => log_and,
16#05# => log_or,
16#06# => log_xor,
16#07# => log_mask,
others => disable);


The body of the state machine process starts by waiting for the leading
edge of the phi1 clock, indicating the start of a clock cycle. When this
occurs, the reset signal is checked, and if it is asserted the controller state is
set to resetting and all control outputs are negated. On the other hand, if
reset is negated, the controller state is updated to the previously computed
next state.

b e g i n -- process state_machine
--
-- start of clock cycle
--
--
-- check for reset
--
if reset = '1' then
state := resetting;
--
-- reset external bus signals
--
--
-- reset dp32 internal control signals
--
addr_latch_en <= '0' after Tpd;
disp_latch_en <= '0' after Tpd;
disp_out_en <= '0' after Tpd;
d2_en <= '0' after Tpd;
dr_en <= '0' after Tpd;
instr_latch_en <= '0' after Tpd;
immed_signext_en <= '0' after Tpd;
ALU_op <= disable after Tpd;
CC_latch_en <= '0' after Tpd;
PC_latch_en <= '0' after Tpd;
PC_out_en <= '0' after Tpd;
reg_port1_en <= '0' after Tpd;
reg_port2_mux_sel <= '0' after Tpd;
reg_res_latch_en <= '0' after Tpd;
--
-- clear write-back flag
--
write_back_pending := false;
--
else -- reset = '0'
state := next_state;
end if;



The remainder of the state machine body is a case statement using the
current state to determine the action to be performed for this clock cycle. If
the processor is being reset (in the resetting state), it waits until the trailing
edge of phi2 at the end of the clock cycle, and checks the reset signal again.
If reset has been negated, the processor can start fetching instructions, so
the next state is set to fetch_0, otherwise it is is set to resetting again.

--
-- dispatch action for current state
--
case state is
when resetting =>
--
-- check for reset going inactive at end of clock cycle
--
if reset = '0' then
next_state := fetch_0;
else
next_state := resetting;
end if;
--
when fetch_0 =>
--
-- clean up after previous execute cycles
--
dr_en <= '0' after Tpd;
d2_en <= '0' after Tpd;
--
-- handle pending register write-back
--
if write_back_pending then
end if;
--
-- enable PC via ALU to address latch
--
PC_out_en <= '1' after Tpd; -- enable PC onto op1_bus
ALU_op <= pass1 after Tpd; -- pass PC to r_bus
--
addr_latch_en <= '1' after Tpd; -- latch instr address
--
--



The processor fetches an instruction from memory by sequencing
through the states fetch_0, fetch_1 and fetch_2 on successive clock cycles.
Figure7-27 shows the timing of control signals for an instruction fetch.
The fetch_0 processor cycle corresponds to a Ti cycle on the memory bus.
During this cycle, the PC register output is enabled onto the op1 bus, and
the ALU function set to pass1. The ALU passes the PC value through to the
result bus, and it is latched into the memory address register during the
second half of the cycle. The PC value is thus set up on the memory address
bus. The fetch_1 cycle corresponds to a memory bus T1 cycle. The controller
starts the memory transaction by asserting fetch and read. At the same
time, it changes the ALU function code to incr1, causing the ALU to place

when fetch_1 =>
--
-- clear pending register write-back
--
if write_back_pending then
write_back_pending := false;
end if;
--
-- increment PC & start bus read
--
ALU_op <= incr1 after Tpd; -- increment PC onto r_bus
--
PC_latch_en <= '1' after Tpd; -- latch incremented PC
--
--
when fetch_2 =>
--
-- cleanup after previous fetch_1
--
PC_out_en <= '0' after Tpd; -- disable PC from op1_bus
ALU_op <= disable after Tpd; -- disable ALU from r_bus
--
-- latch current instruction
--
dr_en <= '1' after Tpd; -- enable fetched instr onto r_bus
--
instr_latch_en <= '1' after Tpd; -- latch fetched instr from r_bus
instr_latch_en <= '0' after Tpd;
--
if ready = '1' then
next_state := decode;
else
next_state := fetch_2; -- extend bus read
end if;



fetch_0 fetch_1 fetch_2 decode

phi1

phi2

PC_out_en

PC_latch_en

addr_latch_en

pass1 incr1 disable
ALU_op

dr_en

instr_latch_en

valid address
a_bus

fetch

read

valid data in
d_bus

ready

Figure7-27. Timing for DP32 instruction fetch.

the incremented PC value on the result bus. This is then latched back into
the PC register during the second half of the cycle. The fetch_2 processor
cycle corresponds to the memory bus T2 cycle, during which data is
returned to the processor from the memory. The controller disables the PC
from the op1 bus and the ALU from the result bus, and enables the data
input buffer to accept memory data onto the result bus. This data is latched
into the current instruction register during the second half of the cycle. If
ready is false, the processor repeats the F2 cycle, otherwise it completes the
bus transaction and moves to the decode state, corresponding to a bus Ti
cycle.
Returning to the VHDL description, we see that the fetch_0 branch of the
case statement implements the first cycle of an instruction fetch. Firstly,
any signals left asserted from previous cycle are negated again. Next, any
register write scheduled from the previously executed instruction is


handled. (This will be described fully below.) Then the PC register output
is enabled and the ALU function set, as described above. The process then
waits until the leading edge of phi2, by which time the PC should be valid on
the result bus. It pulses the address latch enable signal by asserting it,
waiting until the trailing edge of phi2, then negating the signal. Finally,
the next state variable is set to fetch_1, so that when the process resumes in
the next cycle, it will move to this state.
When the process is in state fetch_1, it starts the cycle by terminating any
register write back that may have been pending. It then changes the ALU
function code to increment the PC value, and starts the bus transaction. In
the second half of the cycle, when phi2 is asserted, the PC latch enable is
asserted to store the incremented PC value. The next state is then set to

when decode =>
--
-- terminate bus read from previous fetch_2
--
dr_en <= '0' after Tpd; -- disable fetched instr from r_bus
--
-- delay to allow decode logic to settle
--
--
-- next state based on opcode of currect instruction
--
case opcode is
when op_add | op_sub | op_mul | op_div
| op_addq | op_subq | op_mulq | op_divq
| op_land | op_lor | op_lxor | op_lmask
| op_ldq | op_stq =>
next_state := execute_0;
when op_ld | op_st =>
next_state := disp_fetch_0; -- fetch offset
when op_br | op_bi =>
if CC_comp_result = '1' then -- if branch taken
next_state := disp_fetch_0; -- fetch displacement
else -- else
next_state := execute_0; -- increment PC
-- past displacement
end if;
when op_brq | op_biq =>
if CC_comp_result = '1' then -- if branch taken
next_state := execute_0; -- add immed
-- displacement to PC
else -- else
next_state := fetch_0; -- no action needed
end if;
when others =>
assert false report "illegal instruction" severity warning;
next_state := fetch_0; -- ignore and carry on
end case; -- op
--



fetch_2.
The last cycle of the instruction fetch is state fetch_2. The controller
disables the PC register and ALU outputs, and enables the buffer between
the memory data bus and the result bus. During the second half of the
cycle, it asserts the instruction register latch enable. At the end of the
cycle, when phi2 has returned to '0', the ready input is checked. If it is
asserted, the state machine can continue to the decode state in the next
cycle, otherwise the fetch_2 state must be repeated.
In the decode state, the controller terminates the previous bus
transaction and disables the bus input buffer. It then delays for the rest of
the cycle, modeling the time required for decode logic to analyse the current
instruction and for the condition code comparator to stabilize. The op-code
part of the instruction is then examined to determine the next state. For
arithmetic, logical and quick load/store instructions, the next state is
execute_0, in which the instruction is interpreted. For load/store
instructions with a long displacement, a bus transaction must be
performed to read the displacement, so the next state is disp_fetch_0. For
branch instructions with a long displacement, the fetch is only required if
the branch is to be taken, in which case the next state is disp_fetch_0.
Otherwise the next state is execute_0, in which the PC will be incremented
past the displacement stored in memory. For branch quick instructions,
the displacement is encoded in the instruction. If the branch is taken, the
next state is execute_0 to update the PC. Otherwise no further action is
needed to interpret the instruction, so the next state is fetch_0. If any other
op-code is detected, an assertion is used to report the illegal instruction.
The instruction is ignored and execution continues with the next
instruction, so the next state is fetch_0.


when disp_fetch_0 =>
--
-- enable PC via ALU to address latch
--
PC_out_en <= '1' after Tpd; -- enable PC onto op1_bus
ALU_op <= pass1 after Tpd; -- pass PC to r_bus
--
addr_latch_en <= '1' after Tpd; -- latch displacement address
--
next_state := disp_fetch_1;
--
--
-- increment PC & start bus read
--
ALU_op <= incr1 after Tpd; -- increment PC onto r_bus
--
--
next_state := disp_fetch_2;
--
--
-- cleanup after previous disp_fetch_1
--
PC_out_en <= '0' after Tpd; -- disable PC from op1_bus
ALU_op <= disable after Tpd; -- disable ALU from r_bus
--
-- latch displacement
--
disp_latch_en <= '1' after Tpd; -- latch fetched disp from r_bus
disp_latch_en <= '0' after Tpd;
--
if ready = '1' then
else
next_state := disp_fetch_2; -- extend bus read
end if;



disp_ disp_ disp_
fetch_0 fetch_1 fetch_2 execute_0

phi1

phi2

PC_out_en

PC_latch_en

addr_latch_en

pass1 incr1 disable
ALU_op

disp_latch_en

disp address
a_bus

fetch

read

valid data in
d_bus

ready

Figure7-28. Timing for DP32 displacement fetch.

The sequence for fetching a displacement from memory is similar to
that for fetching the instruction word. The only difference is that instead of
the read word being enabled onto the result bus and latched into the
instruction register, the word is simply latched from the memory data bus
into the displacement latch. The timing for a displacement fetch is shown
in Figure7-28. The sequence consists of the processor states disp_fetch_0,
disp_fetch_1 and one or more repetitions of disp_fetch_2, corresponding to bus
states Ti, T1 and T2 respectively. This sequence is always followed by the
first execute state, corresponding to the bus Ti state at the end of the bus
transaction. In the VHDL description, the case branches for disp_fetch_0,
disp_fetch_1 and disp_fetch_2 implement this behaviour.


when execute_0 =>
--
-- terminate bus read from previous disp_fetch_2
--
--
case opcode is
when op_add | op_sub | op_mul | op_div
| op_addq | op_subq | op_mulq | op_divq
| op_land | op_lor | op_lxor | op_lmask =>
-- enable r1 onto op1_bus
if opcode = op_addq or opcode = op_subq
or opcode = op_mulq or opcode = op_divq then
-- enable i8 onto op2_bus
else
-- select a2 as port2 address
-- enable r2 onto op2_bus
end if;
-- select ALU operation
ALU_op <= ALU_op_select(bits_to_int(opcode)) after Tpd;
--
-- latch cond codes from ALU
-- latch result for reg write
--
next_state := fetch_0; -- execution complete
write_back_pending := true; -- register write_back required
--
when op_ld | op_st | op_ldq | op_stq =>
-- enable r1 to op1_bus
if opcode = op_ld or opcode = op_st then
-- enable displacement to op2_bus
else
-- enable i8 to op2_bus
end if;
ALU_op <= add after Tpd; -- effective address to r_bus
--
addr_latch_en <= '1' after Tpd; -- latch effective address
--
--



execute_0 fetch_0

phi1

phi2

reg_port1_en

reg_port2_
mux_sel

reg_port2_en

op
ALU_op

CC_latch_en

reg_res_
latch_en

reg_port3_en

Figure7-29. Execution of register/register operations.

Execution of instructions starts in state execute_0. The first action is to
negate the bus control signals that may have been active from a previous
displacement fetch sequence. Subsequent action depends on the instruction
being executed, so a nested case statement is used, with the op-code as the
selection expression.
Arithmetic and logic instructions only require one cycle to exectute. The
processor timing for the case where both operands are in registers is shown
in Figure7-29. The address for register port1 is derived from the r1 field of
the current instruction, and this port output is enabled onto the op1 bus.
The multiplexor for the address for register port2 is set to select field r2 of
the current instruction, and this port output is enabled onto the op2 bus.
The ALU function code is set according to the op-code of the current
instruction, and the ALU output is placed on the result bus. During the
second half of the cycle, when the ALU result and condition codes are
stable, the register result latch and condition code latch are enabled,
capturing the results of the operation. In the next cycle, the register read
ports and the latches are are disabled, and the register write port is enabled
to write the result back into the destination register. This write back
operation overlaps the first cycle of the next instruction fetch. The result
register address, derived from the r3 field of the current instruction, is not
overwritten until the end of the next instruction fetch, so the write back is
performed to the correct register.


The timing for arithmetic and logical instructions where the second
operand is an immediate constant is shown in Figure7-30. The difference
is that register port2 is not enabled; instead, the sign extension buffer is
enabled. This converts the 8-bit signed i8 field of the current instruction to a
32-bit signed integer on the op2 bus.
Looking again at the exectute_0 branch of the state machine, the nested
case statement contains a branch for arithmetic and logical instructions.
It firstly enables port1 of the register file, and then enables either port2 or
the sign extension buffer, depending on the op-code. The lookup table
ALU_op_select is indexed by the op-code to determine the ALU function code.
The process then waits until the leading edge of phi2, and asserts the
register result and condition code latch enables while phi2 is '1'. At the end
of the cycle, the next state is set to fetch_0, and the write back pending flag is
set. During the subsequent instruction fetch, this flag is checked (in the
fetch_0 branch of the outer case statement). The register port3 write enable
control signal is asserted during the fetch_0 state, and then at the beginning
of the fetch_1 state it is negated and the flag cleared.

when op_br | op_bi | op_brq | op_biq =>
if CC_comp_result = '1' then
if opcode = op_br then
elsif opcode = op_bi then
elsif opcode = op_brq then
else -- opcode = op_biq
end if;
ALU_op <= add after Tpd;
else
assert opcode = op_br or opcode = op_bi
report "reached state execute_0 "
& "when brq or biq not taken"
severity error;
ALU_op <= incr1 after Tpd;
end if;
--
--
--
when others =>
null;
end case; -- op
--



execute_0 fetch_0

phi1

phi2

reg_port1_en

immed_
signext_en

op
ALU_op

CC_latch_en

reg_res_
latch_en

reg_port3_en

Figure7-30. Execution of register/immed operations.

execute_0 execute_0

phi1 phi1

phi2 phi2

PC_out_en reg_port1_en

PC_latch_en PC_latch_en

immed_ immed_
signext_en signext_en

add add
ALU_op ALU_op

(a) (b)

Figure7-31. Execution of quick branch with branch taken.


when execute_1 =>
--
-- opcode is load or store instruction.
-- cleanup after previous execute_0
--
if opcode = op_ld or opcode = op_st then
-- disable displacement from op2_bus
else
-- disable i8 from op2_bus
end if;
ALU_op <= add after Tpd; -- disable ALU from r_bus
--
-- start bus cycle
--
if opcode = op_ld or opcode = op_ldq then
fetch <= '0' after Tpd; -- start bus read
else -- opcode = op_st or opcode = op_stq
reg_port2_mux_sel <= '1' after Tpd; -- address a3 to port2
reg_port2_en <= '1' after Tpd; -- reg port2 to op2_bus
d2_en <= '1' after Tpd; -- enable op2_bus to d_bus buffer
write <= '1' after Tpd; -- start bus write
end if;
--
--
when execute_2 =>
--
-- opcode is load or store instruction.
-- for load, enable read data onto r_bus
--
if opcode = op_ld or opcode = op_ldq then
dr_en <= '1' after Tpd; -- enable data to r_bus
-- latch data in reg result latch
write_back_pending := true; -- write-back pending
end if;
--
--
end case; -- state
end process state_machine;
end block controller;
end RTL;



execute_0

phi1

phi2

PC_out_en

PC_latch_en

incr1
ALU_op

Figure7-32. Execution of branch with branch not taken.

We now move on to the execution of branch instructions. We saw
previously that for quick branches, when the branch is not taken execution
completes after the decode state. When the branch is taken a single execute
cycle is required to update the PC with the effective address. The timing for
this case is shown in Figure7-31. Figure7-31(a) shows an ordinary quick
branch, in which the PC is enabled onto the op1 bus. Figure7-31(b) shows
an indexed quick branch, in which the index register, read from register
file port1 is enabled onto the op1 bus. The sign extension buffer is enabled
to place the immediate displacement on the op2 bus, and the ALU function
code is set to add the two values, forming the effective address of the branch
on the result bus. This is latched back into the PC register during the
second half of the execution cycle.
For branches with a long displacement, a single execution cycle is

execute_0 execute_0

phi1 phi1

phi2 phi2

PC_out_en reg_port1_en

PC_latch_en PC_latch_en

add add
ALU_op ALU_op

disp_out_en disp_out_en

(a) (b)

Figure7-33. Execution of branch with branch taken.


always required. If the branch is not taken, the PC must be incremented to
point to the instruction after the displacment. The timing for this is shown
in Figure7-32. The PC is enabled onto the op1 bus, and the ALU function is
set to incr1. This increments the value and places it on the result bus. Then
during the second half of the cycle, the new value is latched back into the
PC register.
For long displacement branches where the branch is taken, the PC must
be updated with the effective address. Figure7-33(a) shows the timing for
an ordinary branch, in which the PC is enabled onto the op1 bus.
Figure7-33(b) shows the timing for an indexed branch, in which the index
register is enabled from register port1 onto the op1 bus. The displacement
register output is enabled onto the op2 bus, and the ALU function is set to
add, to add the displacement to the base address, forming the effective
address on the result bus. This is latched back into the PC register during
the second half of the cycle.
The VHDL description implements the execution of a branch instruction
as part of the nested case statement for the execute_0 state. The process
checks the result bit from the condition code comparator. If it is set, the
branch is taken, so the base address and displacement are enabled
(depending on the type of branch), and the ALU function code set to add.
Otherwise, if the condition code comparator result is clear, the branch is
not taken. This should only be the case for long branches, since quick
branches should never get to the execute_0 state. An assertion statement is
used to verify this condition. For long branches which are not taken, the PC
is enabled onto the op1 bus and the ALU function code set to incr1 to
increment the value past the displacement in memory. The PC latch
enable signal is then pulsed when phi2 changes to '1'. Finally, the next
state is set to fetch_0, so the processor will continue with the next
instruction.
The remaining instructions to be considered are the load and store
instructions. These all take three cycles to execute, since a bus transaction
is required to transfer the data to or from the memory. For long
displacement loads and stores, the displacement has been previously
fetched into the displacement register. For the quick forms, the immediate
displacement in the instruction word is used.
Figure7-34 shows the timing for execution of load and quick load
instructions. The base address register is read from register file port1 and
enabled onto the op1 bus. For long displacement loads, the previously
fetched displacement is enabled onto the op2 bus, and for quick loads, the
sign extended immediate displacement is enabled onto the op2 bus. The
ALU function code is set to add, to form the effective address on the result
bus, and this is latched into the memory bus address register during the
second half of the first execute cycle. During the next two cycles the
controller performs a memory read transaction, with the fetch signal held
negated. The data from the data bus is enabled onto the result bus through
the connecting buffer, and latched into the register result latch. This value
is then written back to the register file during the first cycle of the
subsequent instruction fetch.


execute_0 execute_1 execute_2 fetch_0

phi1

phi2

reg_port1_en
disp_out_en
or immed_
signext_en

add disable
ALU_op

addr_latch_en

dr_en

reg_res_
latch_en

reg_port3_en

load address
a_bus

fetch

read

valid data in
d_bus

ready

Figure7-34. Execution of load instructions.


The timing for execution of store and quick store instructions is shown
in Figure7-35. As with load instructions, the base address and
displacement are added, and the effective address is latched in the memory
bus address register. During the next two cycles the controller performs a
bus write transaction. The multiplexor for the register file port2 address is
set to select the r3 field of the instruction, which specifies the register to be
stored, and the port2 output is enabled onto the op2 bus. The buffer between
the op2 bus and the memory data bus is enabled to transmit the data to the
memory. Execution of the instruction completes at the end of the bus
transaction.
Returning to the VHDL description, the first cycle of execution of load
and store instructions is included as a branch of the nested case in the
execute_0 state. The base address register output port is enabled, and either
the displacement latch output or the sign extension buffer is enabled,
depending on the instruction type. The ALU function code is set to add the
two to form the effective address. The process then waits until phi2 changes
to '1', indicating the second half of the cycle, and pulses the address latch
enable. The next state is then set to execute_1 to continue execution of the
instruction.
In state execute_1, the process firstly removes the base address,
displacement and effective address from the DP32 internal buses, then
starts a memory bus transaction. For load instructions, the fetch signal is
negated and the read signal is asserted. For store instructions, the source
register value is enabled onto the op2 bus, the memory data bus output
buffer is enabled, and the write signal is aserted. The next state variable is
then set to execute_2 for the next cycle.
In state execute_2, for load instructions, the memory data bus input
buffer is enabled to transmit the data onto the result bus. The process then
waits until phi2 is '1', in the second half of the cycle, and pulses the enable
for the register result latch. The write back pending flag is then set to
schedule the destination register write during the next instruction fetch
cycle. For both load and store instructions, the next state is fetch_0. All
control signals set during the execute_1 state will be returned to their
negated values in the fetch_0 state.
The test bench described in Section7.5 can be used to test the register
transfer architecture of the DP32. This is done using an alternate
configuration, replacing the behavioural architecture in the test bench with
the register transfer architecture. Figure7-36 shows such a configuration.
The entity bindings for the clock generator and memory are the same,
using the behavioural architectures, but the processor component instance
uses the rtl architecture of the dp32 entity. This binding indication is
followed by a configuration for that architecture, binding the entities
described in Sections7.6.1–7.6.9 to the component instances contained in
the architecture. The newly configured description can be simulated using
the same test programs as before, and the results compared to verify that
they implement the same behaviour.


execute_0 execute_1 execute_2

phi1

phi2

reg_port1_en
disp_out_en
or immed_
signext_en

add disable
ALU_op

addr_latch_en

reg_port2_
mux_sel

reg_port2_en

d2_en

store address
a_bus

fetch

read

write

valid data out
d_bus

ready

Figure7-35. Execution of store instructions.


configuration dp32_rtl_test of dp32_test is
for structure
for cg : clock_gen
use entity work.clock_gen(behaviour)
generic map (Tpw => 8 ns, Tps => 2 ns);
end for;
for mem : memory
use entity work.memory(behaviour);
end for;
for proc : dp32
use entity work.dp32(rtl);
for rtl
for all : reg_file_32_rrw
use entity work.reg_file_32_rrw(behaviour);
end for;
for all : mux2
use entity work.mux2(behaviour);
end for;
for all : latch
use entity work.latch(behaviour);
end for;
for all : PC_reg
use entity work.PC_reg(behaviour);
end for;
for all : ALU_32
use entity work.ALU_32(behaviour);
end for;
for all : cond_code_comparator
use entity work.cond_code_comparator(behaviour);
end for;
for all : buffer_32
use entity work.buffer_32(behaviour);
end for;
for all : latch_buffer_32
use entity work.latch_buffer_32(behaviour);
end for;
for all : signext_8_32
use entity work.signext_8_32(behaviour);
end for;
end for;
end for;
end for;
end dp32_rtl_test;

Figure7-36. Configuration using register transfer architecture of DP32.

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