chapt_08.pptdsfdfdfdsffsdffsdfsdfsdfsdfsdfsdfsdf

Assembly Language for Intel-Based
Assembly Language for Intel-Based
Computers, 5
Computers, 5th
th
Edition
Edition
Chapter 8: Advanced Procedures
(c) Pearson Education, 2006-2007. All rights reserved. You may modify and copy this slide show for your personal use,
or for use in the classroom, as long as this copyright statement, the author's name, and the title are not changed.
Slides prepared by Kip R. Irvine
Revision date: June 4, 2006
Kip R. Irvine

Irvine, Kip R. Assembly Language for Intel-Based Computers 5/e, 2007. 2
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Chapter Overview
Chapter Overview
• Stack Frames
• Recursion
• .MODEL Directive
• INVOKE, ADDR, PROC, and PROTO
• Creating Multimodule Programs

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Stack Frames
Stack Frames
• Stack Parameters
• Local Variables
• ENTER and LEAVE Instructions
• LOCAL Directive
• WriteStackFrame Procedure

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Stack Parameters
Stack Parameters
• More convenient than register parameters
• Two possible ways of calling DumpMem. Which is
easier?
pushad
mov esi,OFFSET array
mov ecx,LENGTHOF array
mov ebx,TYPE array
call DumpMem
popad
push TYPE array
push LENGTHOF array
push OFFSET array
call DumpMem

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Stack Frame
Stack Frame
• Also known as an activation record
• Area of the stack set aside for a procedure's return
address, passed parameters, saved registers, and
local variables
• Created by the following steps:
• Calling program pushes arguments on the stack and
calls the procedure.
• The called procedure pushes EBP on the stack, and
sets EBP to ESP.
• If local variables are needed, a constant is subtracted
from ESP to make room on the stack.

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Explicit Access to Stack Parameters
Explicit Access to Stack Parameters
• A procedure can explicitly access stack parameters
using constant offsets from EBP1
.
• Example: [ebp + 8]
• EBP is often called the base pointer or frame pointer
because it holds the base address of the stack frame.
• EBP does not change value during the procedure.
• EBP must be restored to its original value when a
procedure returns.
1
BP in Real-address mode

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RET Instruction
RET Instruction
• Return from subroutine
• Pops stack into the instruction pointer (EIP or IP).
Control transfers to the target address.
• Syntax:
• RET
• RET n
• Optional operand n causes n bytes to be added to
the stack pointer after EIP (or IP) is assigned a value.

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Stack Frame Example
Stack Frame Example (1 of 2)
(1 of 2)
.data
sum DWORD ?
.code
push 6 ; second argument
push 5 ; first argument
call AddTwo ; EAX = sum
mov sum,eax ; save the sum
AddTwo PROC
push ebp
mov ebp,esp
.
.

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AddTwo Procedure
AddTwo Procedure (1 of 2)
(1 of 2)
AddTwo PROC,
val1:DWORD, val2:DWORD
mov eax,val1
add eax,val2
ret
AddTwo ENDP
• Recall the AddTwo Procedure

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AddTwo Procedure
(2 of 2)
AddTwo PROC,
push ebp
mov ebp, esp
mov eax,val1
add eax,val2
leave
ret 8
AddTwo ENDP
• MASM generates the following code when we assemble
AddTwo (from the previous panel):
mov esp,ebp
pop ebp
The LEAVE instruction is
shorthand for:

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Passing Arguments by Reference
Passing Arguments by Reference (1 of 2)
(1 of 2)
• The ArrayFill procedure fills an array with 16-bit
random integers
• The calling program passes the address of the array,
along with a count of the number of array elements:
.data
count = 100
array WORD count DUP(?)
.code
push OFFSET array
push COUNT
call ArrayFill

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Passing Arguments by Reference
Passing Arguments by Reference (2 of 2)
(2 of 2)
ArrayFill PROC
push ebp
mov ebp,esp
pushad
mov esi,[ebp+12]
mov ecx,[ebp+8]
.
.
ESI points to the beginning of the array, so it's easy to use a
loop to access each array element. View the complete program.
ArrayFill can reference an array without knowing the array's
name:

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Local Variables
Local Variables
• To explicitly create local variables, subtract their total
size from ESP.
• The following example creates and initializes two 32-
bit local variables (we'll call them locA and locB):
MySub PROC
push ebp
mov ebp,esp
sub esp,8
mov [ebp-4],123456h ; locA
mov [ebp-8],0 ; locB
.
.

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LEA Instruction
LEA Instruction
• The LEA instruction returns offsets of both direct and
indirect operands.
• OFFSET operator can only return constant offsets.
• LEA is required when obtaining the offset of a stack
parameter or local variable. For example:
CopyString PROC,
count:DWORD
LOCAL temp[20]:BYTE
mov edi,OFFSET count ; invalid operand
mov esi,OFFSET temp ; invalid operand
lea edi,count ; ok
lea esi,temp ; ok

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Your turn . . .
Your turn . . .
• Create a procedure named Difference that subtracts
the first argument from the second one. Following is a
sample call:
push 14 ; first argument
push 30 ; second
argument
call Difference ; EAX = 16
Difference PROC
push ebp
mov ebp,esp
mov eax,[ebp + 8] ; second argument
sub eax,[ebp + 12] ; first argument
pop ebp
ret 8
Difference ENDP

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Parameter Classifications
Parameter Classifications
• An input parameter is data passed by a calling program to a
procedure.
• The called procedure is not expected to modify the
corresponding parameter variable, and even if it does, the
modification is confined to the procedure itself.
• An input-output parameter is a pointer to a variable containing input
that will be both used and modified by the procedure.
• The variable passed by the calling program is modified.
• An output parameter is created by passing a pointer to a variable
when a procedure is called.
• The procedure does not use any existing data from the variable,
but it fills in a new value before it returns.

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Example: Exchanging Two Integers
Example: Exchanging Two Integers
Swap PROC USES eax esi edi,
pValX:PTR DWORD, ; pointer to first
integer
pValY:PTR DWORD ; pointer to second
integer
mov esi,pValX ; get pointers
mov edi,pValY
mov eax,[esi] ; get first integer
xchg eax,[edi] ; exchange with second
mov [esi],eax ; replace first integer
ret
Swap ENDP
The Swap procedure exchanges the values of two 32-bit
integers. pValX and pValY do not change values, but the
integers they point to are modified.

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ENTER and LEAVE
ENTER and LEAVE
• ENTER instruction creates stack frame for a called
procedure
• pushes EBP on the stack
• sets EBP to the base of the stack frame
• reserves space for local variables
• Example:
MySub PROC
enter 8,0
• Equivalent to:
MySub PROC
push ebp
mov ebp,esp
sub esp,8

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LOCAL Directive
LOCAL Directive
• A local variable is created, used, and destroyed
within a single procedure
• The LOCAL directive declares a list of local
variables
• immediately follows the PROC directive
• each variable is assigned a type
• Syntax:
LOCAL varlist
Example:
MySub PROC
LOCAL var1:BYTE, var2:WORD, var3:SDWORD

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Using LOCAL
Using LOCAL
LOCAL flagVals[20]:BYTE ; array of bytes
LOCAL pArray:PTR WORD ; pointer to an array
myProc PROC, ; procedure
LOCAL t1:BYTE, ; local variables
Examples:

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LOCAL Example
LOCAL Example (1 of 2)
(1 of 2)
BubbleSort PROC
LOCAL temp:DWORD, SwapFlag:BYTE
. . .
ret
BubbleSort ENDP
BubbleSort PROC
push ebp
mov ebp,esp
add esp,0FFFFFFF8h ; add -8 to ESP
. . .
mov esp,ebp
pop ebp
ret
BubbleSort ENDP
MASM generates the following code:

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LOCAL Example
LOCAL Example (2 of 2)
(2 of 2)
Diagram of the stack frame for the BubbleSort
procedure:

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Non-Doubleword Local Variables
Non-Doubleword Local Variables
• Local variables can be different sizes
• How created in the stack by LOCAL directive:
• 8-bit: assigned to next available byte
• 16-bit: assigned to next even (word) boundary
• 32-bit: assigned to next doubleword boundary

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Local Byte Variable
Local Byte Variable
Example1 PROC
LOCAL var1:BYTE
mov al,var1 ; [EBP - 1]
ret
Example1 ENDP

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WriteStackFrame Procedure
WriteStackFrame Procedure
• Displays contents of current stack frame
• Prototype:
WriteStackFrame PROTO,
numParam:DWORD, ; number of passed
parameters
numLocalVal: DWORD, ; number of DWordLocal
variables
numSavedReg: DWORD ; number of saved
registers

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WriteStackFrame Example
WriteStackFrame Example
main PROC
mov eax, 0EAEAEAEAh
mov ebx, 0EBEBEBEBh
INVOKE aProc, 1111h, 2222h
exit
main ENDP
aProc PROC USES eax ebx,
x: DWORD, y: DWORD
LOCAL a:DWORD, b:DWORD
PARAMS = 2
LOCALS = 2
SAVED_REGS = 2
mov a,0AAAAh
mov b,0BBBBh
INVOKE WriteStackFrame, PARAMS, LOCALS, SAVED_REGS

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Review
Review
1. (True/False): A subroutine’s stack frame always contains the
caller’s return address and the subroutine’s local variables.
2. (True/False): Arrays are passed by reference to avoid copying
them onto the stack.
3. (True/False): A procedure’s prologue code always pushes EBP
on the stack.
4. (True/False): Local variables are created by adding an integer
to the stack pointer.
5. (True/False): In 32-bit protected mode, the last argument to be
pushed on the stack in a procedure call is stored at location
ebp+8.
6. (True/False): Passing by reference requires popping a
parameter’s offset from the stack inside the called procedure.
7. What are two common types of stack parameters?

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What's Next
What's Next
• Stack Frames
• Recursion

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Recursion
Recursion
• What is recursion?
• Recursively Calculating a Sum
• Calculating a Factorial

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What is Recursion?
What is Recursion?
• The process created when . . .
• A procedure calls itself
• Procedure A calls procedure B, which in turn calls
procedure A
• Using a graph in which each node is a procedure
and each edge is a procedure call, recursion forms
a cycle:

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Recursively Calculating a Sum
Recursively Calculating a Sum
CalcSum PROC
cmp ecx,0 ; check counter value
jz L2 ; quit if zero
add eax,ecx ; otherwise, add to
sum
dec ecx ; decrement counter
call CalcSum ; recursive call
L2: ret
CalcSum ENDP
The CalcSum procedure recursively calculates the sum of an
array of integers. Receives: ECX = count. Returns: EAX = sum
Stack frame:
View the
complete program

Web site Examples
Calculating a Factorial
Calculating a Factorial (1 of 3)
(1 of 3)
int function factorial(int n)
{
if(n == 0)
return 1;
else
return n * factorial(n-1);
}
5! = 5 * 4!
4! = 4 * 3!
3! = 3 * 2!
2! = 2 * 1!
1! = 1 * 0!
0! = 1
(base case)
1 * 1 = 1
2 * 1 = 2
3 * 2 = 6
4 * 6 = 24
5 * 24 = 120
1 = 1
recursive calls backing up
This function calculates the factorial of integer n. A new value
of n is saved in each stack frame:
As each call instance returns, the
product it returns is multiplied by the
previous value of n.

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(2 of 3)
Factorial PROC
push ebp
mov ebp,esp
mov eax,[ebp+8] ; get n
cmp eax,0 ; n < 0?
ja L1 ; yes: continue
mov eax,1 ; no: return 1
jmp L2
L1: dec eax
push eax ; Factorial(n-1)
call Factorial
; Instructions from this point on execute when each
; recursive call returns.
ReturnFact:
mov ebx,[ebp+8] ; get n
mul ebx ; eax = eax * ebx
L2: pop ebp ; return EAX
ret 4 ; clean up stack
Factorial ENDP
See the program listing

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(3 of 3)
Suppose we want to
calculate 12!
This diagram shows the
first few stack frames
created by recursive calls
to Factorial
Each recursive call uses
12 bytes of stack space.

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Review
Review
1. (True/False): Given the same task to accomplish, a recursive
subroutine usually uses less memory than a nonrecursive one.
2. In the Factorial function, what condition terminates the
recursion?
3. Which instructions in the assembly language Factorial
procedure execute after each recursive call has finished?
4. What will happen to the Factorial program’s output when trying
to calculate 13 factorial?
5. Challenge: In the Factorial program, how many bytes of stack
space are used by the Factorial procedure when calculating 12
factorial?
6. Challenge: Write the pseudocode for a recursive algorithm that
generates the first 20 integers of the Fibonacci series (1, 1, 2,
3, 5, 8, 13, 21, . . .).

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What's Next
What's Next
• Stack Frames
• Recursion

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.MODEL Directive
.MODEL Directive
• .MODEL directive specifies a program's memory
model and model options (language-specifier).
• Syntax:
.MODEL memorymodel [,modeloptions]
• memorymodel can be one of the following:
• tiny, small, medium, compact, large, huge, or flat
• modeloptions includes the language specifier:
• procedure naming scheme
• parameter passing conventions

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Memory Models
Memory Models
• A program's memory model determines the number
and sizes of code and data segments.
• Real-address mode supports tiny, small, medium,
compact, large, and huge models.
• Protected mode supports only the flat model.
Small model: code < 64 KB, data (including stack) < 64 KB.
All offsets are 16 bits.
Flat model: single segment for code and data, up to 4 GB.
All offsets are 32 bits.

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Language Specifiers
Language Specifiers
• C:
• procedure arguments pushed on stack in reverse order
(right to left)
• calling program cleans up the stack
• STDCALL
• procedure arguments pushed on stack in reverse order
(right to left)
• called procedure cleans up the stack

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Review Questions
Review Questions
1. Describe the small memory model.
2. Describe the flat memory model.
3. How is the C language option (of the .MODEL
directive) different from that of STDCALL in regard to
removing arguments from the stack?

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What's Next
What's Next
• Stack Frames
• Recursion

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INVOKE, ADDR, PROC, and PROTO
INVOKE, ADDR, PROC, and PROTO
• INVOKE Directive
• ADDR Operator
• PROC Directive
• PROTO Directive
• Parameter Classifications
• Example: Exchaning Two Integers
• Debugging Tips

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INVOKE Directive
INVOKE Directive
• The INVOKE directive is a powerful replacement for
Intel’s CALL instruction that lets you pass multiple
arguments
• Syntax:
INVOKE procedureName [, argumentList]
• ArgumentList is an optional comma-delimited list of
procedure arguments
• Arguments can be:
• immediate values and integer expressions
• variable names
• address and ADDR expressions
• register names

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INVOKE Examples
INVOKE Examples
.data
byteVal BYTE 10
wordVal WORD 1000h
.code
; direct operands:
INVOKE Sub1,byteVal,wordVal
; address of variable:
INVOKE Sub2,ADDR byteVal
; register name, integer expression:
INVOKE Sub3,eax,(10 * 20)
; address expression (indirect operand):
INVOKE Sub4,[ebx]

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ADDR Operator
ADDR Operator
.data
myWord WORD ?
.code
INVOKE mySub,ADDR myWord
• Returns a near or far pointer to a variable, depending on
which memory model your program uses:
• Small model: returns 16-bit offset
• Large model: returns 32-bit segment/offset
• Flat model: returns 32-bit offset
• Simple example:

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PROC Directive
PROC Directive (1 of 2)
(1 of 2)
• The PROC directive declares a procedure with an
optional list of named parameters.
• Syntax:
label PROC paramList
• paramList is a list of parameters separated by
commas. Each parameter has the following syntax:
paramName : type
type must either be one of the standard ASM types
(BYTE, SBYTE, WORD, etc.), or it can be a pointer to
one of these types.

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PROC Directive
PROC Directive (2 of 2)
(2 of 2)
• Alternate format permits parameter list to be on one or
more separate lines:
label PROC,
paramList
• The parameters can be on the same line . . .
param-1:type-1, param-2:type-2, . . ., param-n:type-n
• Or they can be on separate lines:
param-1:type-1,
param-2:type-2,
. . .,
param-n:type-n
comma required

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AddTwo Procedure
(1 of 2)
AddTwo PROC,
mov eax,val1
add eax,val2
ret
AddTwo ENDP
• The AddTwo procedure receives two integers and returns
their sum in EAX.

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PROC Examples
PROC Examples (2 of 3)
(2 of 3)
FillArray PROC,
pArray:PTR BYTE, fillVal:BYTE
arraySize:DWORD
mov ecx,arraySize
mov esi,pArray
mov al,fillVal
L1: mov [esi],al
inc esi
loop L1
ret
FillArray ENDP
FillArray receives a pointer to an array of bytes, a single byte fill
value that will be copied to each element of the array, and the
size of the array.

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PROC Examples
PROC Examples (3 of 3)
(3 of 3)
ReadFile PROC,
pBuffer:PTR BYTE
LOCAL fileHandle:DWORD
. . .
ReadFile ENDP
Swap PROC,
pValX:PTR DWORD,
pValY:PTR DWORD
. . .
Swap ENDP

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PROTO Directive
PROTO Directive
• Creates a procedure prototype
• Syntax:
• label PROTO paramList
• Every procedure called by the INVOKE directive must
have a prototype
• A complete procedure definition can also serve as its
own prototype

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PROTO Directive
PROTO Directive
• Standard configuration: PROTO appears at top of the program
listing, INVOKE appears in the code segment, and the procedure
implementation occurs later in the program:
MySub PROTO ; procedure prototype
.code
INVOKE MySub ; procedure call
MySub PROC ; procedure implementation
.
.
MySub ENDP

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PROTO Example
PROTO Example
• Prototype for the ArraySum procedure, showing its
parameter list:
ArraySum PROTO,
ptrArray:PTR DWORD, ; points to the array
szArray:DWORD ; array size

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Trouble-Shooting Tips
Trouble-Shooting Tips
• Save and restore registers when they are modified by a
procedure.
• Except a register that returns a function result
• When using INVOKE, be careful to pass a pointer to the correct
data type.
• For example, MASM cannot distinguish between a DWORD
argument and a PTR BYTE argument.
• Do not pass an immediate value to a procedure that expects a
reference parameter.
• Dereferencing its address will likely cause a general-
protection fault.

Web site Examples
Review
Review
1. (True/False): The CALL instruction cannot include procedure arguments.
2. (True/False): The INVOKE directive can include up to a maximum of three
arguments.
3. (True/False): The INVOKE directive can only pass memory operands, but
not register values.
4. (True/False):The PROC directive can contain a USES operator, but the
PROTO directive cannot.
5. (True/False): When using the PROC directive, all parameters must be listed
on the same line.
6. (True/False): If you pass a variable containing the offset of an array of bytes
to a procedure that expects a pointer to an array of words, the assembler will
not catch your error.
7. (True/False): If you pass an immediate value to a procedure that expects a
reference parameter, you can generate a general-protection fault (in
protected mode).
8. Declare a procedure named MultArray that receives two pointers to arrays of
doublewords, and a third parameter indicating the number of array elements.

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What's Next
What's Next
• Stack Frames
• Recursion

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Multimodule Programs
Multimodule Programs
• A multimodule program is a program whose source
code has been divided up into separate ASM files.
• Each ASM file (module) is assembled into a separate
OBJ file.
• All OBJ files belonging to the same program are
linked using the link utility into a single EXE file.
• This process is called static linking

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Advantages
Advantages
• Large programs are easier to write, maintain, and
debug when divided into separate source code
modules.
• When changing a line of code, only its enclosing module
needs to be assembled again. Linking assembled
modules requires little time.
• A module can be a container for logically related
code and data (think object-oriented here...)
• encapsulation: procedures and variables are
automatically hidden in a module unless you declare
them public

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Creating a Multimodule Program
Creating a Multimodule Program
• Here are some basic steps to follow when
creating a multimodule program:
• Create the main module
• Create a separate source code module for each
procedure or set of related procedures
• Create an include file that contains procedure
prototypes for external procedures (ones that are
called between modules)
• Use the INCLUDE directive to make your
procedure prototypes available to each module

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Example: ArraySum Program
Example: ArraySum Program
• Let's review the ArraySum program from Chapter 5.
Each of the four white rectangles will become a module.

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Sample Program output
Sample Program output
Enter a signed integer: -25
Enter a signed integer: 36
Enter a signed integer: 42
The sum of the integers is: +53

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INCLUDE File
INCLUDE File
INCLUDE Irvine32.inc
PromptForIntegers PROTO,
ptrPrompt:PTR BYTE, ; prompt string
arraySize:DWORD ; size of the array
ArraySum PROTO,
count:DWORD ; size of the array
DisplaySum PROTO,
ptrPrompt:PTR BYTE, ; prompt string
theSum:DWORD ; sum of the array
The sum.inc file contains prototypes for external functions that
are not in the Irvine32 library:

Web site Examples
Inspect Individual Modules
Inspect Individual Modules
• Main
• PromptForIntegers
• ArraySum
• DisplaySum
Custom batch file for assembling and linking.

Web site Examples
Review Questions
Review Questions
1. (True/False): Linking OBJ modules is much faster
than assembling ASM source files.
2. (True/False): Separating a large program into short
modules makes a program more difficult to maintain.
3. (True/False): In a multimodule program, an END
statement with a label occurs only once, in the
startup module.
4. (True/False): PROTO directives use up memory, so
you must be careful not to include a PROTO
directive for a procedure unless the procedure is
actually called.

Web site Examples
Summary
Summary
• Stack parameters
• more convenient than register parameters
• passed by value or reference
• ENTER and LEAVE instructions
• Local variables
• created on the stack below stack pointer
• LOCAL directive
• Recursive procedure calls itself
• Calling conventions (C, stdcall)
• MASM procedure-related directives
• INVOKE, PROC, PROTO

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The End
The End

chapt_08.pptdsfdfdfdsffsdffsdfsdfsdfsdfsdfsdfsdf

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