Assembly Language for x86 Processors 6th Edition Chapter 8: Advanced Procedures (c) Pearson Education, 2010. All rights reserved. You may modify and copy

Assembly Language for x86 Processors 6th Edition

Chapter 8: Advanced Procedures

(c) Pearson Education, 2010. 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 the author.

Revision date: 2/15/2010

Kip R. Irvine

2

Parameter Passing We currently have two ways to pass parameters to a

procedure By using registers By using global variables

However these mechanisms to pass parameters are not suited if we want To use a variable number of parameters

[Limited # of registers] To permit a procedure to call itself (for using recursion)

[Global variables are static]

In these circumstances we can pass parameters via the stack This is the mechanism of parameter passing used by high

level languages

Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 3

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 (i.e. parameters)

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.


Passing Arguments to a Procedure1. Push arguments on stack

• Arguments pushed on the stack are called stack parameters

• (Use only 32-bit values in protected mode to keep the stack aligned)

• To pass by value: push argument’s value

• To pass by reference: push argument’s offset

2. Call the called-procedure

3. Accept a return value in EAX, if any

4. Remove arguments from the stack if the called-procedure did not remove them

5

Stack Parameters Suppose that we have a procedure, called IMUL2, who’s task is to

multiply two signed numbers and return the result into EAX.

The caller calls IMUL2 with parameters varA and varB like this:push varA ;push a dword variablepush varB ;another dword variablecall IMUL2 ;result in EAX, stack unchangedadd esp,8 ; clean the stack (i.e. restore ESP)

We have assumed that IMUL2 did not changed the stack: ESP just after returning from IMUL2 is pointing to the same place as it

was just before calling IMUL2. But, since 8 bytes of parameters were pushed on the stack, we need

to increase ESP by 8 after returning from IMUL2 Otherwise, ESP would be decreased by 8 at each IMUL2 usage and,

consequently, the stack could overflow if the 3 first statements were inside a loop

We say that the stack has been restored by the caller This is the method used by C/C++ compilers

6

Stack Parameters (cont.) Given that IMUL2 is called that

way, we can write it like this:

IMUL2 PROC push ebp mov ebp,esp mov eax,[ebp+12] imul eax,[ebp+8] pop ebp retIMUL2 ENDP

We use EBP to access the stack parameters (not ESP)

Compilers are using this method. But, more simply, we could have used ESP instead... [avoid using ESP, however]

These are called stack frames (or activation records)

varA

varB

ret addr.

orig. ebp

after mov ebp,esp

ebpesp

varA

varB esp

after ret


Accessing Stack Parameters (C/C++)

• C and C++ functions access stack parameters using constant offsets from EBP1.• Example: [ebp + 8]

• EBP is called the base pointer or frame pointer because it holds the base address of the stack frame.

• EBP does not change value during the function.• EBP must be restored to its original value when a

function returns.

1 BP in Real-address mode


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.


Who removes parameters from the stack?

Caller (C) ...... or ...... Called-procedure (STDCALL):

AddTwo PROCpush val2 push ebppush val1 mov ebp,espcall AddTwo mov eax,[ebp+12]add esp,8 add eax,[ebp+8]

pop ebp ret 8

The MODEL directive specifies calling conventions• See line: MODEL flat, STDCALL, in file Irvine.asm.• The Irvine32 library uses STDCALL calling convention, and hence,

your procedures should clean the stack by using ret n.

10

Stack Parameters (cont.) The other method is to let the

called procedure the responsibility of restoring the stack

This is the method used by Pascal compilers

The caller would simply do

push varA push varB call IMUL2 ; do not increm. ESP

But the procedure would now use ret n to return This performs a RET instruction

and then increments ESP further by n

The called procedure would now be:

IMUL2 PROC push ebp mov ebp,esp mov eax,[ebp+12] imul eax,[ebp+8] pop ebp ret 8 ; clean stackIMUL2 ENDP

Since 8 bytes of parameters have been pushed onto the stack

11

Passing a Variable Number of Parameters To pass a variable number of

arguments by the stack just push, as the last parameter, the number of arguments

By popping this parameter, the procedure knows how much arguments were passed

The caller:

push 35push –63push 23push 3 ;# of argscall AddSomeadd esp,16

AddSome PROC push ebp push ecx mov ebp,esp

mov ecx,[ebp+12] ;# of args xor eax,eax ;hold sum add ebp,16 ;last arg L1: add eax,[ebp] add ebp,4 ;point to next loop L1

pop ecx pop ebp retAddSome ENDP

The called procedure:


Passing an Array by Reference (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:

.datacount = 100array WORD count DUP(?).code

push OFFSET arraypush COUNTcall ArrayFill


Passing an Array by Reference (2 of 2)

ArrayFill PROCpush ebpmov ebp,esppushadmov 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:


Your turn . . .

• Create a procedure named Difference that subtracts the first argument from the second one. Following is a sample call:

push 14 ; first argumentpush 30 ; second argumentcall Difference ; EAX = 16

Difference PROCpush ebpmov ebp,espmov eax,[ebp + 8] ; second argumentsub eax,[ebp + 12] ; first argumentpop ebpret 8Difference ENDP


Passing 8-bit and 16-bit Arguments• Cannot push 8-bit values on stack

• Pushing 16-bit operand may cause page fault or ESP alignment problem• incompatible with Windows API functions

• Expand smaller arguments into 32-bit values, using MOVZX or MOVSX:

.datacharVal BYTE 'x'.code

movzx eax,charValpush eaxcall Uppercase


Passing Multiword Arguments

• Push high-order values on the stack first; work backward in memory

• Results in little-endian ordering of data• Example:

.data longVal QWORD 1234567800ABCDEFh.code

push DWORD PTR longVal + 4 ; high doublewordpush DWORD PTR longVal ; low doublewordcall WriteHex64


Saving and Restoring Registers

• Push registers on stack just after assigning ESP to EBP• local registers are modified inside the procedure

MySub PROCpush ebpmov ebp,esppush ecx ; save local registerspush edx


Stack Affected by USES Operator

MySub1 PROC USES ecx edxret

MySub1 ENDP

• USES operator generates code to save and restore registers:

MySub1 PROCpush ecxpush edx

pop edxpop ecx

ret


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Local Variables

• Only statements within subroutine can view or modify local variables

• Storage used by local variables is released when subroutine ends

• local variable name can have the same name as a local variable in another function without creating a name clash

• Essential when writing recursive procedures, as well as procedures executed by multiple execution threads


Creating LOCAL Variables

Example - create two DWORD local variables:Say: int x=10, y=20;

ret addresssaved ebp EBP 10 (x) [ebp-4]

MySub PROC 20 (y) [ebp-8]push ebpmov ebp,espsub esp,8 ;create 2 DWORD variables

mov DWORD PTR [ebp-4],10 ; initialize x=10mov DWORD PTR [ebp-8],20 ; initialize y=20


LEA Instruction

• LEA returns offsets of direct and indirect operands• OFFSET operator only returns constant offsets

• LEA required when obtaining offsets of stack parameters & local variables

• Example

CopyString PROC,count:DWORDLOCAL temp[20]:BYTE

mov edi,OFFSET count ; invalid operandmov esi,OFFSET temp ; invalid operandlea edi,count ; oklea esi,temp ; ok


LEA Example

Suppose you have a Local variable at [ebp-8]

And you need the address of that local variable in ESI

You cannot use this: mov esi, OFFSET [ebp-8] ; error

Use this instead:lea esi,[ebp-8]


ENTER Instruction

• 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 ebpmov ebp,espsub esp,8


LEAVE Instruction

Terminates the stack frame for a procedure.

MySub PROCenter 8,0.........leaveret

MySub ENDP

push ebpmov ebp,espsub esp,8 ; 2 local DWORDs

mov esp,ebp ; free local spacepop ebp

Equivalent operations


LOCAL Directive

• 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 PROCLOCAL var1:BYTE, var2:WORD, var3:SDWORD


Using LOCAL

LOCAL flagVals[20]:BYTE ; array of bytes

LOCAL pArray:PTR WORD ; pointer to an array

myProc PROC, ; procedureLOCAL t1:BYTE, ; local variables

Examples:


LOCAL Example (1 of 2)

BubbleSort PROCLOCAL temp:DWORD, SwapFlag:BYTE. . .ret

BubbleSort ENDP

BubbleSort PROCpush ebpmov ebp,espadd esp,0FFFFFFF8h ; add -8 to ESP. . .mov esp,ebppop ebpret

BubbleSort ENDP

MASM generates the following code:


LOCAL Example (2 of 2)

Diagram of the stack frame for the BubbleSort procedure:

return address

EBP EBP

[EBP - 4]

ESP

temp

SwapFlag [EBP - 8]


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


Local Byte Variable

Example1 PROC LOCAL var1:BYTE mov al,var1 ; [EBP - 1] retExample1 ENDP


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


WriteStackFrame Example

main PROCmov eax, 0EAEAEAEAhmov ebx, 0EBEBEBEBhINVOKE aProc, 1111h, 2222hexit

main ENDP

aProc PROC USES eax ebx,x: DWORD, y: DWORDLOCAL a:DWORD, b:DWORDPARAMS = 2LOCALS = 2SAVED_REGS = 2mov a,0AAAAhmov b,0BBBBhINVOKE WriteStackFrame, PARAMS, LOCALS, SAVED_REGS


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35

Recursion A recursive procedure is one that calls itself

Recursive procedures can easily be implemented in ASM when parameter passing is done via the stack

Ex: a C implementation of factorial:

int factorial(int n){if (n<=1) { return 1; }else { return n*factorial(n-1); }

}

An ASM caller needs to push the argument into the stack:

push 8call factorial ;result in EAX = 40320add esp,4 ;restore the stack


Recursively Calculating Sum 1 + … + n

CalcSum PROCcmp ecx,0 ; check counter valuejz L2 ; quit if zeroadd eax,ecx ; otherwise, add to sumdec ecx ; decrement countercall CalcSum ; recursive callL2: retCalcSum ENDP

The CalcSum procedure recursively calculates the sum 1+2+…+n. Receives: ECX = count = n. Returns: EAX = sum

Stack frame:View the complete program


Calculating a Factorial (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.



Factorial PROCpush ebpmov ebp,espmov eax,[ebp+8] ; get ncmp eax,0 ; n < 0?ja L1 ; yes: continuemov eax,1 ; no: return 1jmp L2

L1: dec eaxpush eax ; Factorial(n-1)call Factorial

; Instructions from this point on execute when each; recursive call returns.

ReturnFact:mov ebx,[ebp+8] ; get nmul ebx ; eax = eax * ebx

L2: pop ebp ; return EAXret 4 ; clean up stack

Factorial ENDP

See the program listing



12 n

n-1

ReturnMain

ebp0

11

ReturnFact

ebp1

10

ReturnFact

ebp2

9

ReturnFact

ebp3

n-2

n-3

(etc...)

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.

40

Exercises Ex1: Rewrite the factorial procedure when stack cleaning

is done by the caller (ie: in the Java/C/C++way) Ex2: Write a procedure who’s task is to fill with value 0

the first k bytes of a byte array. All parameters must be passed by the stack and stack cleaning must be done by the caller. Give an example of how this procedure would be called.

Ex3: Rewrite the AddSome procedure when stack cleaning is done by the called procedure (ie: in the Pascal way)

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, . . .).

41

Modular Programming Large projects need to be broken into small modules

with clean interfaces between modules

The way to program a module should only depend on the interfaces provided by other modules – not their implementation

One possibility would be to place groups of related procedures into different files and then include them with the include directive

The include directive instructs the assembler to include the file (at assembly time) at the place of the directive

We must then ensure that the code will be placed in the .code segment and the data will be placed in the .data segment

42

Modular Programming (cont.) Hence, in each file, we should always put .code before the code

and .dada before the data. Ex:

File my_prog.asmINCLUDE Irvine32.incINCLUDE Macros.inc.data msg1 BYTE "In main",0

.codemain PROC

mWriteString msg1call procAcall procB

exitmain ENDPinclude procA.asminclude procB.asmEND main

File procA.asm.code procA PROC mWriteString msg2 ret procA ENDP.datamsg2 BYTE "In procA",0File procB.asm.code procB PROC mWriteString msg3 ret procB ENDP.datamsg3 BYTE "In procB",0

43

Modular Programming (cont.) Hence, by doing

ML my_prog.asm

The assembler will create a single object file my_prog.obj which will contain all the included code and data

The scope of each name used (in any included file) will be the object module in which they will be assembled. Here it is my_prog.obj Hence an error will be detected by the assembler if two different

included files use the same name Hence this method of included files should be avoided for large

projects

Instead, we should assemble each file separately to obtain a separate object module for each file and, thus, have a private namespace for each file Make sure, however, to have an INCLUDE Irvine32.inc (or INCLUDE

Macros.inc) and an END directive in each separate file. The file containing the main program must have END main as last

line

44

Separately Assembled Modules However any module that wants to be used need to provide at least

one name to be used by others

Use the directive PUBLIC to enable other modules to use names defined in the module where PUBLIC is. Ex:

PUBLIC procA, varC, labelB Note that the usage is the same for any kind of names (procedures,

variables, label...) Use the directive EXTERN to declare names that are defined in other

modules But now we need to provide the qualifiers:

PROC for procedure namesBYTE, WORD, DWORD... for variable names

Example:EXTERN procA@0:proc, varA:dword, varB:word

Place the directives extern and public just after INCLUDE directives

45

Example

File my_prog.asm

INCLUDE Irvine32.incINCLUDE Macros.incEXTERN procA@0:proc, procB@0:proc

.data msg1 BYTE "In main",0.code main PROC

mWriteString msg1call procAcall procB

exit main ENDPEND main

File procA.asm

INCLUDE Irvine32.incINCLUDE Macros.incpublic procA.code procA PROC

mWriteString msg1ret

procA ENDP.data msg1 BYTE "In procA",0END

File procB.asm

INCLUDE Irvine32.incINCLUDE Macros.incpublic procB.code procB PROC

mWriteString msg1ret

procB ENDP.data msg1 BYTE "In procB",0END

46

Example (cont.) To assemble each file separately and link them do:

ML –c procA.asmML –c procB.asmML my_prog.asm procA.obj procB.obj

The –c is the “compile only” option: it only produces an object file [no executable file is produced]

The last command will produce my_prog.obj and link all the .obj files to produce my_prog.exe

All .data segments will be concatenated into a single .data segment and all .code segments will be concatenated into a single .code segment

Each .asm file now provides a separate namespace since each file has been assembled separately Note that all three files are using the same name msg1. These refer to

different memory locations since the assembler and linker will produce a different memory address for each variable msg1.

47

The Program’s Entry Point An executable program must have only one entry point (the address

of the first instruction to execute).

This entry point must be in your main program, and is the very first instruction to be executed

The file containing the main program must end with the line “END main”

A program must have only one single entry point

Any file other than the one containing the main program should terminate with the line END

48

Using Global Variables A variable made public in one object module will be

accessible to every other object module that will be linked into the same .exe file

As long as the other object modules are declaring this variable to be extern

Such a variable, which is said to be global, can be used by procedures to pass a value across different modules.

This mechanism increases the complexity of the interfaces (since every module must be aware of all the global variables)

Hence the number of global variables should be limited

49

Global Variable Example

File procA.asm

INCLUDE Irvine32.inc

PUBLIC procAEXTERN varA:dword

.codeprocA PROC mov eax,varA call WriteDec retprocA ENDPEND

File mp.asm


PUBLIC varAEXTERN procA@0:proc

.data varA DWORD ?

.codemain PROC mov varA,333 call procA exitmain ENDPEND main

To assemble and link, you can do:ML mp.asm procA.asm


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


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


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


Example: ArraySum Program

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

SummationProgram (main)

Clrscr PromptForIntegers ArraySum DisplaySum

WriteStringWriteString 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


INCLUDE File


PromptForIntegers PROTO,ptrPrompt:PTR BYTE, ; prompt stringptrArray:PTR DWORD, ; points to the arrayarraySize:DWORD ; size of the array

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

DisplaySum PROTO,ptrPrompt:PTR BYTE, ; prompt stringtheSum:DWORD ; sum of the array

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


Inspect Individual Modules

• Main• PromptForIntegers• ArraySum• DisplaySum


Macros• Read Chapter 10, Section 10.2

• Macro procedures are named block of ASM statements

• Can be invoked as many times in a program as you wish

• When invoking a macro, a copy of its code is inserted directly into the program at the location where it is being invoked• Automatic code insertion

• Book’s macro codes are defined in the Macro.inc file • Use INCLUDE Macro.inc when using macros from the book

Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.58

Defining Macros• Defined directly at the beginning of a source program, or,

placed in separate file and included using INCLUDE directive

• Example: Macros to display character ‘X’ or a char variable

• Defined using MACRO and ENDM directives

mPrintX Macromov al, ’X’call WriteChar

ENDM

MacroName Macro parameter-1, parameter-2, …

Statement-list

ENDM

mPutChar Macro cvarpush eaxmov al, cvarcall WriteCharpop eax

ENDM

Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.

59

Invoking Macros• Macros are called (invoked) by simply inserting their names,

possibly followed by their arguments

• Example: Display the first 20 letters of the alphabet

• At compile time: the actual source code (on the left) is expanded by substituting all occurences of mPutChar al with its actual macro code. The expanded code (on the right) is visible in the source listing file.

• Macros execute faster than PROCs but tend to yield larger programs

mov al, ’A’mov ecx, 20Iterate: mPutChar al ; macro call inc al loop Iterate

mov al, ’A’mov ecx, 20Iterate: 1 push eax 1 mov al, cvar 1 call WriteChar 1 pop eax inc al loop Iterate


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INVOKE, ADDR, PROC, and PROTO

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


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


INVOKE Examples

.databyteVal BYTE 10wordVal 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

.datamyWord WORD ?.codeINVOKE 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 (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.


PROC Directive (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


AddTwo Procedure (1 of 2)

AddTwo PROC,val1:DWORD, val2:DWORD

mov eax,val1add eax,val2

retAddTwo ENDP

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


PROC Examples (2 of 3)

FillArray PROC,pArray:PTR BYTE, fillVal:BYTEarraySize:DWORD

mov ecx,arraySizemov esi,pArraymov al,fillVal

L1: mov [esi],alinc esiloop L1ret

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.


PROC Examples (3 of 3)

ReadFile PROC,pBuffer:PTR BYTELOCAL fileHandle:DWORD. . .

ReadFile ENDP

Swap PROC,pValX:PTR DWORD,pValY:PTR DWORD. . .Swap ENDP


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

• 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

.codeINVOKE MySub ; procedure call

MySub PROC ; procedure implementation..

MySub ENDP


PROTO Example

• Prototype for the ArraySum procedure, showing its parameter list:

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


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.


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.


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Java Bytecodes

• Stack-oriented instruction format• operands are on the stack• instructions pop the operands, process, and push

result back on stack• Each operation is atomic• Might be be translated into native code by a just in

time compiler


Java Virual Machine (JVM)

• Essential part of the Java Platform• Executes compiled bytecodes

• machine language of compiled Java programs


Java Methods

• Each method has its own stack frame• Areas of the stack frame:

• local variables• operands• execution environment


Bytecode Instruction Format

• 1-byte opcode• iload, istore, imul, goto, etc.

• zero or more operands

• Disassembling Bytecodes• use javap.exe, in the Java Development Kit (JDK)


Primitive Data Types

• Signed integers are in twos complement format, stored in big-endian order


JVM Instruction Set

• Comparison Instructions pop two operands off the stack, compare them, and push the result of the comparison back on the stack

• Examples: fcmp and dcmp


JVM Instruction Set

• Conditional Branching • jump to label if st(0) <= 0

ifle label

• Unconditional Branching• call subroutine

jsr label


Java Disassembly Examples

• Adding Two Integers

int A = 3;int B = 2;int sum = 0;sum = A + B;



• Adding Two Doubles

double A = 3.1;double B = 2;double sum = A + B;



• Conditional Branchdouble A = 3.0;boolean result = false;if( A > 2.0 ) result = false;else result = true;


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• Java Bytecodes – another approch to programming


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Documents

Assembly Language for x86 Processors 6th Edition Chapter 8: Advanced Procedures (c) Pearson Education, 2010. All rights reserved. You may modify and copy