Basic structure of computers

Chapter 1. Basic
Structure of Computers

Functional Units
Arithmetic
and
logic

Input
Memory
Output

Control

I/O

Processor

Figure 1.1. Basic functional units of a computer.

Information Handled by a
Computer
 Instructions/machine




instructions

Govern the transfer of information within a computer as
well as between the computer and its I/O devices
Specify the arithmetic and logic operations to be
performed
Program

 Data



Used as operands by the instructions
Source program

 Encoded

in binary code – 0 and 1

Memory Unit
 Store

programs and data
 Two classes of storage


Primary storage





Fast
Programs must be stored in memory while they are being executed
Large number of semiconductor storage cells
Processed in words
Address
RAM and memory access time
Memory hierarchy – cache, main memory



Secondary storage – larger and cheaper







Arithmetic and Logic Unit
(ALU)
 Most

computer operations are executed in
ALU of the processor.
 Load the operands into memory – bring them
to the processor – perform operation in ALU –
store the result back to memory or retain in
the processor.
 Registers
 Fast control of ALU

Control Unit









All computer operations are controlled by the control
unit.
The timing signals that govern the I/O transfers are
also generated by the control unit.
Control unit is usually distributed throughout the
machine instead of standing alone.
Operations of a computer:
Accept information in the form of programs and data through an
input unit and store it in the memory
Fetch the information stored in the memory, under program control,
into an ALU, where the information is processed
Output the processed information through an output unit
Control all activities inside the machine through a control unit

The processor : Data Path and
Control

Two types of functional units:
elements that operate on data values (combinational)
 elements that contain state (state elements)

Five Execution Steps
Step name

Action for R-type
instructions

Action for Memoryreference Instructions

Action for
branches

Instruction fetch

IR = MEM[PC]
PC = PC + 4

Instruction decode/ register
fetch

Action for
jumps

A = Reg[IR[25-21]]
B = Reg[IR[20-16]]
ALUOut = PC + (sign extend (IR[15-0])<<2)

Execution, address
computation, branch/jump
completion

ALUOut = A op B

ALUOut = A+sign
extend(IR[15-0])

Memory access or R-type
completion

Reg[IR[15-11]] =
ALUOut

Load:MDR =Mem[ALUOut]
or
Store:Mem[ALUOut] = B

Memory read completion

Load: Reg[IR[20-16]] =
MDR

IF(A==B) Then
PC=ALUOut

PC=PC[3128]||(IR[250]<<2)

Review







Activity in a computer is governed by instructions.
To perform a task, an appropriate program
consisting of a list of instructions is stored in the
memory.
Individual instructions are brought from the memory
into the processor, which executes the specified
operations.
Data to be used as operands are also stored in the
memory.

A Typical Instruction







Add LOCA, R0
Add the operand at memory location LOCA to the
operand in a register R0 in the processor.
Place the sum into register R0.
The original contents of LOCA are preserved.
The original contents of R0 is overwritten.
Instruction is fetched from the memory into the
processor – the operand at LOCA is fetched and
added to the contents of R0 – the resulting sum is
stored in register R0.

Separate Memory Access and
ALU Operation
 Load

LOCA, R1
 Add R1, R0
 Whose contents will be overwritten?

Connection Between the
Processor and the Memory
Memory

MAR

MDR
Control

PC

R0
R1

Processor

IR
ALU
Rn

1

n general purpose
registers

Figure 1.2. Connections between the processor and the memory.

Registers
 Instruction

register (IR)
 Program counter (PC)
 General-purpose register (R – R )
0
n-1
 Memory

address register (MAR)
 Memory data register (MDR)

Typical Operating Steps
 Programs

reside in the memory through input

devices
 PC is set to point to the first instruction
 The contents of PC are transferred to MAR
 A Read signal is sent to the memory
 The first instruction is read out and loaded
into MDR
 The contents of MDR are transferred to IR
 Decode and execute the instruction

Typical Operating Steps
(Cont’)
 Get



operands for ALU

General-purpose register
Memory (address to MAR – Read – MDR to ALU)

 Perform

operation in ALU
 Store the result back



To general-purpose register
To memory (address to MAR, result to MDR – Write)

 During

the execution, PC is
incremented to the next instruction

Interrupt







Normal execution of programs may be preempted if
some device requires urgent servicing.
The normal execution of the current program must
be interrupted – the device raises an interrupt
signal.
Interrupt-service routine
Current system information backup and restore (PC,
general-purpose registers, control information,
specific information)

Bus Structures
 There

are many ways to connect different
parts inside a computer together.
 A group of lines that serves as a connecting
path for several devices is called a bus.
 Address/data/control

Bus Structure
 Single-bus

Input

Output

Memory

Figure 1.3. Singlebus structure.

Processor

Speed Issue
 Different

devices have different
transfer/operate speed.
 If the speed of bus is bounded by the slowest
device connected to it, the efficiency will be
very low.
 How to solve this?
 A common approach – use buffers.

Performance
 The

most important measure of a computer is
how quickly it can execute programs.
 Three factors affect performance:




Hardware design
Instruction set
Compiler

Performance


Processor time to execute a program depends on the hardware
involved in the execution of individual machine instructions.
Main
memory

Cache
memory

Processor

Bus

Figure 1.5.

The processor cache.

Performance
 The

processor and a relatively small cache
memory can be fabricated on a single
integrated circuit chip.
 Speed
 Cost
 Memory management

Processor Clock
 Clock,

clock cycle, and clock rate
 The execution of each instruction is divided
into several steps, each of which completes
in one clock cycle.
 Hertz – cycles per second

Basic Performance Equation






T – processor time required to execute a program that has been
prepared in high-level language
N – number of actual machine language instructions needed to
complete the execution (note: loop)
S – average number of basic steps needed to execute one
machine instruction. Each step completes in one clock cycle
R – clock rate
Note: these are not independent to each other

N×S
T=
R
How to improve T?

Pipeline and Superscalar
Operation







Instructions are not necessarily executed one after
another.
The value of S doesn’t have to be the number of
clock cycles to execute one instruction.
Pipelining – overlapping the execution of successive
instructions.
Add R1, R2, R3
Superscalar operation – multiple instruction
pipelines are implemented in the processor.
Goal – reduce S (could become <1!)

Clock Rate
 Increase



clock rate

Improve the integrated-circuit (IC) technology to make
the circuits faster
Reduce the amount of processing done in one basic step
(however, this may increase the number of basic steps
needed)

 Increases

in R that are entirely caused by
improvements in IC technology affect all
aspects of the processor’s operation equally
except the time to access the main memory.

CISC and RISC
 Tradeoff

between N and S
 A key consideration is the use of pipelining




S is close to 1 even though the number of basic steps
per instruction may be considerably larger
It is much easier to implement efficient pipelining in
processor with simple instruction sets

 Reduced

Instruction Set Computers (RISC)
 Complex Instruction Set Computers (CISC)

Compiler







A compiler translates a high-level language program
into a sequence of machine instructions.
To reduce N, we need a suitable machine instruction
set and a compiler that makes good use of it.
Goal – reduce N×S
A compiler may not be designed for a specific
processor; however, a high-quality compiler is
usually designed for, and with, a specific processor.

Performance Measurement







T is difficult to compute.
Measure computer performance using benchmark programs.
System Performance Evaluation Corporation (SPEC) selects and
publishes representative application programs for different application
domains, together with test results for many commercially available
computers.
Compile and run (no simulation)
Reference computer

Running time on the reference computer
SPEC rating =
Running time on the computer under test
n

SPEC rating = (∏SPECi )
i =1

1
n

Multiprocessors and
Multicomputers


Multiprocessor computer





Execute a number of different application tasks in parallel
Execute subtasks of a single large task in parallel
All processors have access to all of the memory – shared-memory
multiprocessor
Cost – processors, memory units, complex interconnection networks



Multicomputers



Each computer only have access to its own memory
Exchange message via a communication network – messagepassing multicomputers






Basic structure of computers

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