Assembly Language for Intel

Assembly Language for Intel

Assembly Language for Intel - - Based Based Computers, 4

Computers, 4

Edition Edition

Chapter 8:Advanced Procedures

Kip R. Irvine

Chapter Overview Chapter Overview

• Local Variables

• Stack Parameters

• Stack Frames

• Recursion

• Creating Multimodule Programs

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

MASM MASM - - Generated Code Generated Code

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

MASM generates the following code:

MASM MASM - - Generated Code Generated Code

Diagram of the stack frame for the BubbleSort procedure:

return address

EBP EBP

[EBP - 4]

ESP

temp

SwapFlag [EBP - 8]

Stack Parameters Stack Parameters

• Register vs. Stack Parameters

• INVOKE Directive

• PROC Directive

• PROTO Directive

• Passing by Value or by Reference

• Parameter Classifications

• Example: Exchanging Two Integers

• Trouble-Shooting Tips

Register vs. Stack Parameters Register vs. Stack Parameters

• Register parameters require dedicating a register to each parameter. Stack parameters are more

convenient

• Imagine two possible ways of calling the DumpMem procedure. Clearly the second is easier:

pushad

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

call DumpMem popad

push OFFSET array push LENGTHOF array push TYPE array

call DumpMem

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

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]

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:

PROC Directive PROC Directive

• 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.

PROC Examples

PROC Examples

AddTwo PROC,

val1:DWORD, val2:DWORD mov eax,val1

add eax,val2 ret

AddTwo ENDP

• The AddTwo procedure receives two integers and returns their sum in EAX.

PROC Examples

PROC Examples

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 fills value that will be copied to each element of the array, and the size of the array.

PROC Examples

PROC Examples

ReadFile PROC,

pBuffer:PTR BYTE

LOCAL fileHandle:DWORD . . .

ReadFile ENDP Swap PROC,

pValX:PTR DWORD, pValY:PTR DWORD . . .

Swap ENDP

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.

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 .

.

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

Passing by Value 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. Example:

.data

myData WORD 1000h .code

main PROC

INVOKE Sub1, myData

MASM generates the following code:

Passing by Reference Passing by Reference

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

.data

myData WORD 1000h .code

main PROC

INVOKE Sub1, ADDR myData

push OFFSET myData call Sub1

MASM generates the following code:

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 represents a value passed as input to a procedure, which the procedure may modify.

• 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 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.

Trouble

Trouble - - Shooting Tips 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 PTR 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-

Stack Frames Stack Frames

• Memory Models

• Language Specifiers

• Explicit Access to Stack Parameters

• Passing Arguments by Reference

• Creating Local Variables

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

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.

.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 (a single segment, used by .com programs), small (one code segment and one data segment), medium (multiple code segments and a single data segment), compact (one code segment and multiple data segments), large (multiple code and data segments), huge (same as the large model, except that individual data item may be larger than a single segment), or flat (protected mode. Uses 32-bit offsets for

Language

Language Specifiers Specifiers

• modeloptions includes the language specifier:

• procedure naming scheme

• parameter passing conventions

• stdcall

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

• called procedure cleans up the stack

push 6 ; second argument

push 5 ; first argument

INVOKE AddTwo,5,6

Explicit Access to Stack Parameters Explicit Access to Stack Parameters

• A procedure can explicitly access stack parameters using constant offsets from EBP.

• 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.

Stack Frame Example

Stack Frame Example

.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 .

.

00000006 00000005 return address

EBP, ESP [EBP + 4]

[EBP + 8]

[EBP + 12]

EBP

Stack Frame Example

Stack Frame Example

AddTwo PROC push ebp

mov ebp,esp ; base of stack frame mov eax,[ebp + 12] ; second argument (6) add eax,[ebp + 8] ; first argument (5) pop ebp

ret 8 ; clean up the stack

00000006 00000005 return address

EBP, ESP [EBP + 4]

[EBP + 8]

[EBP + 12]

EBP

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

Passing Arguments by Reference

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:

.data

count = 100

array WORD count DUP(?) .code

push OFFSET array push COUNT

call ArrayFill

Passing Arguments by Reference

Passing Arguments by Reference

ArrayFill PROC push ebp

mov ebp,esp pushad

mov esi,[ebp+12]

mov ecx,[ebp+8]

. .

offset(array) count

EBP [EBP + 8]

[EBP + 12]

return address EBP

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:

LEA Instruction LEA Instruction

• The LEA instruction returns offsets of indirect operands.

• OFFSET operator can only return constant assembly time 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

Creating Local Variables Creating 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

. .

ENTER and LEAVE Instructions ENTER and LEAVE Instructions

• ENTER instruction automatically creates a stack frame for a called procedure.

• Push EBP on the stack

• Set EBP to the base of the stack (mov ebp,esp)

• Reserve space for local variables (sub esp,numbytes)

• ENTER localbytes,nestinglevel

• LEAVE instruction terminates the stack frame for a procedure.

MySub PROC enter 8,0

. .

MySub PROC push ebp mov ebp,esp sub esp,8

.

Recursion Recursion

• What is recursion?

• Recursively Calculating a Sum

• Calculating a Factorial

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:

A

B E

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

View the complete program

Calculating a Factorial

Calculating a Factorial

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! 1 * 1 = 1 2 * 1 = 2 3 * 2 = 6 4 * 6 = 24 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:

As each call instance returns, the product it returns is multiplied by the

Calculating a Factorial

Calculating a Factorial

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 ; ax = ax * bx

L2: pop ebp ; return EAX

Calculating a Factorial

Calculating a Factorial

12 n

n-1 ReturnMain

ebp₀ 11 ReturnFact

ebp₁ 10 ReturnFact

ebp₂ 9 ReturnFact

ebp₃

n-2

n-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.

Multimodule

Multimodule Programs 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.

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

Creating a

Creating a Multimodule Multimodule Program 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

Example: ArraySum Program Example: ArraySum Program

• Let's review the ArraySum program from Chapter 5.

Summation Program (main)

Clrscr PromptForIntegers ArraySum DisplaySum

WriteString

WriteString ReadInt WriteIntWriteInt

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

INCLUDE File INCLUDE File

INCLUDE Irvine32.inc

PromptForIntegers PROTO,

ptrPrompt:PTR BYTE, ; prompt string

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,

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