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Computer Organization &

Computer Organization &

Assembly Languages Assembly Languages

Pu-Jen Cheng

Advanced Procedure

Adapted from the slides prepared by Kip Irvine for the book, Assembly Language for Intel-Based Computers, 5th Ed.


Chapter Overview


Stack Frames




.MODEL Directive




Creating Multimodule Programs


Stack Frames


Stack Parameters


Local Variables


ENTER and LEAVE Instructions


LOCAL Directive


Stack Parameters


More convenient than register parameters


Two possible ways of calling DumpMem.

Which is easier?

pushad push TYPE array


mov esi,OFFSET array mov ecx,LENGTHOF array mov ebx,TYPE array

call DumpMem popad

push LENGTHOF array push OFFSET array call DumpMem

Register-based Method Stack-based Method


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:


Created by the following steps:

¾ Calling program pushes arguments on the stack and calls the procedure.

¾ The called procedure pushes EBP on the stack, and sets EBP to ESP.

¾ If local variables are needed, a constant is

subtracted from ESP to make room on the stack.


Explicit Access to Stack Parameters


A procedure can explicitly access stack

parameters using constant offsets from EBP.

¾ 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.


RET Instruction


Return from subroutine


Pops stack into the instruction pointer (EIP or IP).

Control transfers to the target address.





¾ RET n


Optional operand n causes n bytes to be added to the stack pointer after EIP (or IP) is assigned a



Stack Frame Example


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 .


00000006 00000005 return address

EBP, ESP [EBP + 4]

[EBP + 8]

[EBP + 12]



Passing Arguments by Reference


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:



count = 100

array WORD count DUP(?) .code

push OFFSET array push COUNT

call ArrayFill


Passing Arguments by Reference (cont.)

ArrayFill PROC push ebp

mov ebp,esp


count [EBP + 8]

[EBP + 12]

ArrayFill can reference an array without knowing the array's name:

p p pushad

mov esi,[ebp+12]

mov ecx,[ebp+8]

. .

EBP return address


ESI points to the beginning of the array, so it's easy to use a loop to access each array element.


Variable Number of Parameters


For most procedures, the number of parameters is fixed

¾ Every time the procedure is called, the same number of parameter values are passed


In procedures that can have variable number of parameters

¾ With each procedure call, the number of parameter values passed can be different

„ C supports procedures with variable number of parameters such as printf

¾ Easy to support variable number of parameters using the stack method


Variable Number of Parameters (cont.)


To implement

variable number of parameter passing:

¾ Parameter count should be one of the should be one of the parameters passed

¾ This count should be the last parameter pushed onto the stack


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



MySub PROC push ebp

mov ebp,esp sub esp,8

mov [ebp-4],123456h ; locA

mov [ebp-8],0 ; locB

. .


Local Variables (cont.)


To clear local variables, set ESP to be EBP

MySub PROC push ebp

mov ebp,esp sub esp 8 sub esp,8

mov [ebp-4],123456h ; locA

mov [ebp-8],0 ; locB

. .

mov esp, ebp pop ebp



LEA Instruction


The LEA instruction returns offsets of both direct and indirect operands.

¾ OFFSET operator can only return constant offsets.


LEA is required when obtaining the offset of a stack parameter or local variable For example:

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




ENTER instruction creates stack frame for a called procedure

¾ pushes EBP on the stack

¾ sets EBP to the base of the stack frame

¾ reserves space for local variables

¾ Example:

„ MySub PROC

„ enter 8,0

¾ Equivalent to:

„ MySub PROC

„ push ebp

„ mov ebp,esp

„ sub esp,8



MySub PROC push ebp

mov ebp, esp sub esp, 8

The LEAVE instruction

mov eax,val1 add eax,val2 leave

ret 8 AddTwo ENDP

mov esp,ebp pop ebp

The LEAVE instruction is shorthand for:


LOCAL Directive


A local variable is created, used, and destroyed within a single procedure


The LOCAL directive declares a list of local variables

¾ immediately follows the PROC directive

¾ each variable is assigned a type



„ LOCAL varlist



LOCAL var1:BYTE, var2:WORD, var3:SDWORD



LOCAL flagVals[20]:BYTE ; array of bytes

LOCAL pArray:PTR WORD ; pointer to an array


myProc PROC, ; procedure

LOCAL t1:BYTE, ; local variables t2:WORD,



LOCAL Example

BubbleSort PROC

LOCAL temp:DWORD, SwapFlag:BYTE . . .


BubbleSort ENDP

MASM generates the following code:

BubbleSort PROC push ebp

mov ebp,esp

add esp,0FFFFFFF8h ; add -8 to ESP . . .

mov esp,ebp pop ebp


BubbleSort ENDP


LOCAL Example (cont.)

Diagram of the stack frame for the BubbleSort procedure:

return address



[EBP - 4]



SwapFlag [EBP - 8]


Non-Doubleword Local Variables


Local variables can be different sizes


How created in the stack by LOCAL directive:

¾ 8-bit: assigned to next available byte

¾ 16-bit: assigned to next even (word) boundary

¾ 32 bit: assigned to next doubleword boundary

¾ 32-bit: assigned to next doubleword boundary


Local Byte Variable

Example1 PROC


mov al,var1 ; [EBP - 1]


Example1 ENDP Example1 ENDP




What is recursion?


Recursively Calculating a Sum


Calculating a Factorial


What is Recursion?


The process created when . . .

¾ A procedure calls itself

¾ Procedure A calls procedure B, which in turn calls procedure A


Using a graph in which each node is a g g p

procedure and each edge is a procedure call, recursion forms a cycle:






Recursively Calculating a Sum

CalcSum PROC

cmp ecx,0 ; check counter value

jz L2 ; quit if zero

add eax,ecx ; otherwise, add to sum

The CalcSum procedure recursively calculates the sum of an array of integers. Receives: ECX = count. Returns: EAX = sum

dec ecx ; decrement counter

call CalcSum ; recursive call L2: ret

CalcSum ENDP

Stack frame:


Calculating a Factorial

int function factorial(int n) {

if(n == 0) return 1

5! = 5 * 4! 5 * 24 = 120 recursive calls backing up

This function calculates the factorial of integer n. A new value of n is saved in each stack frame:

return 1;


return n * factorial(n-1);


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

1 = 1

As each call instance returns, the product it returns is multiplied by the previous value of n.


Calculating a Factorial (cont.)

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)

push eax ; Factorial(n 1)

call Factorial

; Instructions from this point on execute when each

; recursive call returns.


mov ebx,[ebp+8] ; get n

mul ebx ; eax = eax * ebx

L2: pop ebp ; return EAX

ret 4 ; clean up stack

Factorial ENDP


Calculating a Factorial (cont.)

12 n

n-1 ReturnMain

ebp0 11 ReturnFact


Suppose we want to calculate 12!

This diagram shows

the first few stack p1

10 ReturnFact

ebp2 9 ReturnFact





the first few stack frames created by recursive calls to Factorial

Each recursive call

uses 12 bytes of stack space.


Reserving Stack Space


.stack 4096


Sub1 calls Sub2, Sub2 calls Sub3


1[ ] b

LOCAL array1[50]:DWORD ; 200 bytes


LOCAL array2[80]:WORD ; 160 bytes


LOCAL array3[300]:WORD ; 300 bytes


What's Next


Stack Frames




.MODEL Directive




Creating Multimodule Programs


.MODEL Directive


.MODEL directive specifies a program's memory model and model options (language-specifier).



„ .MODEL memorymodel [,modeloptions]


memorymodel can be one of the following:

¾ tiny, small, medium, compact, large, huge, or flat


modeloptions includes the language specifier:

¾ procedure naming scheme

¾ parameter passing conventions


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


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.


Language Specifiers

„ C

¾ procedure arguments pushed on stack in reverse order (right to left)

¾ calling program cleans up the stack PASCAL


¾ procedure arguments pushed in forward order (left to right)

¾ called procedure cleans up the stack


¾ procedure arguments pushed on stack in reverse order (right to left)

¾ called procedure cleans up the stack


What's Next


Stack Frames




.MODEL Directive




Creating Multimodule Programs




INVOKE Directive


ADDR Operator


PROC Directive


PROTO Directive


Parameter Classifications


Debugging Tips


INVOKE Directive


The INVOKE directive is a powerful

replacement for Intel’s CALL instruction that lets you pass multiple arguments



„ INVOKE procedureName [, argumentList]


ArgumentList is an optional comma-delimited list of procedure arguments


Arguments can be:

¾ immediate values and integer expressions

¾ variable names

¾ address and ADDR expressions

¾ register names


INVOKE Examples


byteVal BYTE 10 wordVal WORD 1000h .code

; direct operands:

INVOKE Sub1,byteVal,wordVal i

; address of variable:

INVOKE Sub2,ADDR byteVal

; register name, integer expression:

INVOKE Sub3,eax,(10 * 20)

; address expression (indirect operand):

INVOKE Sub4,[ebx]


INVOKE Example


val1 DWORD 12345h val2 DWORD 23456h .code

INVOKE AddTwo val1 val2 INVOKE AddTwo, val1, val2

push val1 push val2 call AddTwo


ADDR Operator

• 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


myWord WORD ? .code

INVOKE mySub,ADDR myWord

• Simple example:


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 30p ; second argument; g

„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


Passing by Value

„ When a procedure argument is passed by value, a copy of a 16-bit or 32-bit integer is pushed on the stack.



myData WORD 1000h .code

main PROC

INVOKE Sub1, myData

push myData call Sub1

MASM generates the following code:


Passing by Reference

„ When an argument is passed by reference, its address is pushed on the stack. Example:


myData WORD 1000h .code

main PROC

INVOKE Sub1, ADDR myData

push OFFSET myData call Sub1

MASM generates the following code:


PROC Directive


The PROC directive declares a procedure with an optional list of named parameters.



label PROC paramList


paramList is a list of parameters separated by


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.


PROC Directive (cont.)


Alternate format permits parameter list to be on one or more separate lines:

label PROC, paramList

Th t b th li

comma required


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, . . .,



PROC Examples

FillArray PROC,

pArray:PTR BYTE, fillVal:BYTE arraySize:DWORD

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.


mov ecx,arraySize mov esi,pArray mov al,fillVal L1: mov [esi],al

inc esi loop L1 ret

FillArray ENDP


PROC Examples (cont.)

Swap PROC,



ReadFile PROC,

pBuffer:PTR BYTE

LOCAL fileHandle:DWORD . . .

ReadFile ENDP


PROTO Directive


Creates a procedure prototype



¾ label PROTO paramList


Every procedure called by the INVOKE directive y p y must have a prototype


A complete procedure definition can also serve

as its own prototype


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 MySub PROTO ; procedure prototype .code

INVOKE MySub ; procedure call

MySub PROC ; procedure implementation .




PROTO Example


Prototype for the ArraySum procedure, showing its parameter list:

ArraySum PROTO,

ptrArray:PTR DWORD, ; points to the array szArray:DWORD ; array size

szArray:DWORD ; array size


WriteStackFrame Procedure


Displays contents of current stack frame

¾ Prototype:

WriteStackFrame PROTO,

numParam:DWORD, ; number of passed parameters numLocalVal: DWORD ; number of DWordLocal variables numLocalVal: DWORD, ; number of DWordLocal variables numSavedReg: DWORD ; number of saved registers


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


„ PARAMS = 2

„ LOCALS = 2


„ mov a,0AAAAh

„ mov b,0BBBBh



Parameter Classifications

„ An input parameter is data passed by a calling program to a procedure.

¾ 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.

A t t t i t d b i i t t

• 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.


Example: Exchanging Two Integers

Swap PROC USES eax esi edi,

pValX:PTR DWORD, ; pointer to first integer

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.

p , ; p g

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



Trouble-Shooting Tips

„ Save and restore registers when they are modified by a procedure.

¾ 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.


What's Next


Stack Frames




.MODEL Directive




Creating 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

separate OBJ file.


All OBJ files belonging to the same program are linked using the link utility into a single EXE file.

¾ This process is called static linking



„ 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


Creating a Multimodule Program


Here are some basic steps to follow when creating a multimodule program:

¾ Create the main module

¾ Create a separate source code module for each procedure or set of related procedures each procedure or set of related procedures

¾ Create an include file that contains procedure prototypes for external procedures (ones that are called between modules)

¾ Use the INCLUDE directive to make your procedure prototypes available to each module


Example: ArraySum Program

Summation Program (main)

Clrscr PromptForIntegers ArraySum DisplaySum


WriteString ReadInt WriteIntWriteInt

Each of the four white rectangles will become a module.


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




PromptForIntegers PROTO,

ptrPrompt:PTR BYTE, ; prompt string

The file contains prototypes for external functions that are not in the Irvine32 library:

ptrArray:PTR DWORD, ; points to the array arraySize:DWORD ; size of the array ArraySum PROTO,

ptrArray:PTR DWORD, ; points to the array count:DWORD ; size of the array DisplaySum PROTO,

ptrPrompt:PTR BYTE, ; prompt string theSum:DWORD ; sum of the array



TITLE Integer Summation Program INCLUDE


main PROC

call Clrscr

INVOKE PromptForIntegers, ADDR prompt1,

ADDR array, Count


call Crlf

INVOKE ExitProcess,0 main ENDP

END main





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