Assembly Language for x 86 Processors 6 th

Assembly Language for x 86 Processors 6 th Edition Kip R. Irvine Chapter 8: Advanced Procedures Slides prepared by the author. Revision date: 2/15/2010 (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.

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 2

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. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 3

Passing Arguments to a Procedure 1. 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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 4

Stack Parameters § Suppose that we have a procedure, called IMUL 2, who’s task is to multiply two signed numbers and return the result into EAX. § The caller calls IMUL 2 with parameters var. A and var. B like this: push var. A ; push a dword variable push var. B ; another dword variable call IMUL 2 ; result in EAX, stack unchanged add esp, 8 ; clean the stack (i. e. restore ESP) § We have assumed that IMUL 2 did not changed the stack: § ESP just after returning from IMUL 2 is pointing to the same place as it was just before calling IMUL 2. § But, since 8 bytes of parameters were pushed on the stack, we need to increase ESP by 8 after returning from IMUL 2 § Otherwise, ESP would be decreased by 8 at each IMUL 2 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 5

Stack Parameters (cont. ) § Given that IMUL 2 is called that way, we can write it like this: IMUL 2 PROC push ebp mov ebp, esp mov eax, [ebp+12] imul eax, [ebp+8] pop ebp ret IMUL 2 ENDP § We use EBP to access the stack parameters (not ESP) 6 § 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) var. A var. B ret addr. orig. ebp esp after mov ebp, esp var. A var. B after ret esp

Accessing Stack Parameters (C/C++) • C and C++ functions access stack parameters using constant offsets from EBP 1. • 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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 7

RET Instruction • Return from subroutine • Pops stack into the instruction pointer (EIP or IP). Control transfers to the target address. • Syntax: • RET n • Optional operand n causes n bytes to be added to the stack pointer after EIP (or IP) is assigned a value. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 8

Who removes parameters from the stack? Caller (C) push val 2 push val 1 call Add. Two add esp, 8 . . . or. . . Called-procedure (STDCALL): Add. Two PROC push ebp mov ebp, esp mov eax, [ebp+12] add eax, [ebp+8] pop ret ebp 8 The MODEL directive specifies calling conventions • See line: MODEL flat, STDCALL, in file Irvine. asm. • The Irvine 32 library uses STDCALL calling convention, and hence, your procedures should clean the stack by using ret n. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 9

Stack Parameters (cont. ) § The other method is to let the called procedure the responsibility § The called procedure would of restoring the stack now be: § This is the method used by Pascal compilers § The caller would simply do push call ; do var. A var. B IMUL 2 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 10 IMUL 2 PROC push ebp mov ebp, esp mov eax, [ebp+12] imul eax, [ebp+8] pop ebp ret 8 ; clean stack IMUL 2 ENDP § Since 8 bytes of parameters have been pushed onto the stack

Passing a Variable Number of Parameters § To pass a variable number of arguments by the stack just push, § The called procedure: as the last parameter, the number of arguments Add. Some PROC § By popping this parameter, the procedure knows how much arguments were passed § The caller: push 35 push – 63 push 23 push 3 ; # of args call Add. Some add esp, 16 11 push ebp push ecx mov ebp, esp mov ecx, [ebp+12] ; # of args xor eax, eax ; hold sum add ebp, 16 ; last arg L 1: add eax, [ebp] add ebp, 4 ; point to next loop L 1 pop ecx pop ebp ret Add. Some ENDP

Passing an Array by Reference (1 of 2) • The Array. Fill 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 Array. Fill Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 12

Passing an Array by Reference (2 of 2) Array. Fill can reference an array without knowing the array's name: Array. Fill 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. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 13

Your turn. . . • Create a procedure named Difference that subtracts the first argument from the second one. Following is a sample call: push 14 push 30 call Difference ; first argument ; second argument ; EAX = 16 Difference PROC push ebp mov ebp, esp mov eax, [ebp + 8] sub eax, [ebp + 12] pop ebp ret 8 Difference ENDP ; second argument ; first argument Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 14

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: . data char. Val BYTE 'x'. code movzx eax, char. Val push eax call Uppercase Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 15

Passing Multiword Arguments • Push high-order values on the stack first; work backward in memory • Results in little-endian ordering of data • Example: . data long. Val QWORD 1234567800 ABCDEFh. code push DWORD PTR long. Val + 4 ; high doubleword push DWORD PTR long. Val ; low doubleword call Write. Hex 64 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 16

Saving and Restoring Registers • Push registers on stack just after assigning ESP to EBP • local registers are modified inside the procedure My. Sub PROC push ebp mov ebp, esp push ecx push edx Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. ; save local registers 17

Stack Affected by USES Operator My. Sub 1 PROC USES ecx edx ret My. Sub 1 ENDP • USES operator generates code to save and restore registers: My. Sub 1 PROC push ecx push edx pop ecx ret Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 18

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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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 20

Creating LOCAL Variables Example - create two DWORD local variables: Say: int x=10, y=20; ret address saved ebp EBP 10 (x) [ebp-4] My. Sub PROC 20 (y) [ebp-8] push mov sub ebp, esp, 8 mov DWORD PTR [ebp-4], 10 ; initialize x=10 DWORD PTR [ebp-8], 20 ; initialize y=20 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. ; create 2 DWORD variables 21

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 Copy. String PROC, count: DWORD LOCAL temp[20]: BYTE mov lea edi, OFFSET esi, OFFSET edi, count ; esi, temp ; count ; invalid operand temp ; invalid operand ok ok Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 22

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] Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 23

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: My. Sub PROC enter 8, 0 • Equivalent to: My. Sub PROC push ebp mov ebp, esp sub esp, 8 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 24

LEAVE Instruction Terminates the stack frame for a procedure. Equivalent operations My. Sub PROC enter 8, 0. . leave ret My. Sub ENDP push ebp mov ebp, esp sub esp, 8 ; 2 local DWORDs mov pop Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. esp, ebp ; free local space ebp 25

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: My. Sub PROC LOCAL var 1: BYTE, var 2: WORD, var 3: SDWORD Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 26

Using LOCAL Examples: LOCAL flag. Vals[20]: BYTE ; array of bytes LOCAL p. Array: PTR WORD ; pointer to an array my. Proc PROC, ; procedure LOCAL t 1: BYTE, ; local variables Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 27

LOCAL Example (1 of 2) Bubble. Sort PROC LOCAL temp: DWORD, Swap. Flag: BYTE. . . ret Bubble. Sort ENDP MASM generates the following code: Bubble. Sort PROC push ebp mov ebp, esp add esp, 0 FFFFFFF 8 h ; add -8 to ESP. . . mov esp, ebp pop ebp ret Bubble. Sort ENDP Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 28

LOCAL Example (2 of 2) Diagram of the stack frame for the Bubble. Sort procedure: Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 29

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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 30

Local Byte Variable Example 1 PROC LOCAL var 1: BYTE mov al, var 1 ret Example 1 ENDP Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. ; [EBP - 1] 31

Write. Stack. Frame Procedure • Displays contents of current stack frame • Prototype: Write. Stack. Frame PROTO, num. Param: DWORD, ; number of passed parameters num. Local. Val: DWORD, ; number of DWord. Local variables num. Saved. Reg: DWORD ; number of saved registers Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 32

Write. Stack. Frame Example main PROC mov eax, 0 EAEAh mov ebx, 0 EBEBh INVOKE a. Proc, 1111 h, 2222 h exit main ENDP a. Proc PROC USES eax ebx, x: DWORD, y: DWORD LOCAL a: DWORD, b: DWORD PARAMS = 2 LOCALS = 2 SAVED_REGS = 2 mov a, 0 AAAAh mov b, 0 BBBBh INVOKE Write. Stack. Frame, PARAMS, LOCALS, SAVED_REGS Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 33

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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 8 call factorial ; result in EAX = 40320 add esp, 4 ; restore the stack 35

Recursively Calculating Sum 1 + … + n The Calc. Sum procedure recursively calculates the sum 1+2+…+n. Receives: ECX = count = n. Returns: EAX = sum Calc. Sum PROC cmp ecx, 0 ; check counter value jz L 2 ; quit if zero add eax, ecx ; otherwise, add to sum dec ecx ; decrement counter call Calc. Sum ; recursive call L 2: ret Calc. Sum ENDP Stack frame: Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. View the complete program 36

Calculating a Factorial (1 of 3) This function calculates the factorial of integer n. A new value of n is saved in each stack frame: int function factorial(int n) { if(n == 0) return 1; else return n * factorial(n 1); } As each call instance returns, the product it returns is multiplied by the previous value of n. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 37

Calculating a Factorial (2 of 3) Factorial PROC push ebp mov ebp, esp mov eax, [ebp+8] ; get n cmp eax, 0 ; n < 0? ja L 1 ; yes: continue mov eax, 1 ; no: return 1 jmp L 2 L 1: dec eax push eax call Factorial ; Factorial(n-1) ; Instructions from this point on execute when each ; recursive call returns. Return. Fact: mov ebx, [ebp+8] mul ebx L 2: pop ebp ret 4 Factorial ENDP ; get n ; eax = eax * ; return EAX ; clean up stack See the program listing Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 38

Calculating a Factorial (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. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 39

Exercises § Ex 1: Rewrite the factorial procedure when stack cleaning is done by the caller (ie: in the Java/C/C++way) § Ex 2: 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. § Ex 3: Rewrite the Add. Some 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, . . . ). 40

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 41 § 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

Modular Programming (cont. ) § Hence, in each file, we should always put. code before the code and. dada before the data. Ex: File my_prog. asm INCLUDE Irvine 32. inc INCLUDE Macros. inc. data msg 1 BYTE "In main", 0. code main PROC m. Write. String msg 1 call proc. A call proc. B exit main ENDP include proc. A. asm include proc. B. asm END main 42 File proc. A. asm. code proc. A PROC m. Write. String msg 2 ret proc. A ENDP. data msg 2 BYTE "In proc. A", 0 File proc. B. asm. code proc. B PROC m. Write. String msg 3 ret proc. B ENDP. data msg 3 BYTE "In proc. B", 0

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 Irvine 32. inc (or INCLUDE Macros. inc) and an END directive in each separate file. 43 § The file containing the main program must have END main as last line

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 proc. A, var. C, label. B § 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 names BYTE, WORD, DWORD. . . for variable names § Example: EXTERN proc. A@0: proc, var. A: dword, var. B: word § Place the directives extern and public just after INCLUDE directives 44

Exampl e File my_prog. asm INCLUDE Irvine 32. inc INCLUDE Macros. inc EXTERN proc. A@0: proc, proc. B@0: proc. data msg 1 BYTE "In main", 0. code main PROC m. Write. String msg 1 call proc. A call proc. B exit main ENDP END main 45 File proc. A. asm INCLUDE Irvine 32. inc INCLUDE Macros. inc public proc. A. code proc. A PROC m. Write. String msg 1 ret proc. A ENDP. data msg 1 BYTE "In proc. A", 0 END File proc. B. asm INCLUDE Irvine 32. inc INCLUDE Macros. inc public proc. B. code proc. B PROC m. Write. String msg 1 ret proc. B ENDP. data msg 1 BYTE "In proc. B", 0 END

Example (cont. ) § To assemble each file separately and link them do: ML –c proc. A. asm ML –c proc. B. asm ML my_prog. asm proc. A. obj proc. B. 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 msg 1. These refer to different memory locations since the assembler and linker will produce a different memory address for each variable msg 1. 46

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 47

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 48

Global Variable Example File mp. asm File proc. A. asm INCLUDE Irvine 32. inc PUBLIC var. A EXTERN proc. A@0: proc PUBLIC proc. A EXTERN var. A: dword . data var. A DWORD ? . code proc. A PROC mov eax, var. A call Write. Dec ret proc. A ENDP END . code main PROC mov var. A, 333 call proc. A exit main ENDP END main To assemble and link, you can do: ML mp. asm proc. A. asm 49

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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 50

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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 51

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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 52

Example: Array. Sum Program • Let's review the Array. Sum program from Chapter 5. Each of the four white rectangles will become a module. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 53

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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 54

INCLUDE File The sum. inc file contains prototypes for external functions that are not in the Irvine 32 library: INCLUDE Irvine 32. inc Prompt. For. Integers PROTO, ptr. Prompt: PTR BYTE, ptr. Array: PTR DWORD, array. Size: DWORD ; prompt string ; points to the ; size of the array Array. Sum PROTO, ptr. Array: PTR DWORD, ; points to the array count: DWORD ; size of the array Display. Sum PROTO, ptr. Prompt: PTR BYTE, ; prompt string the. Sum: DWORD ; sum of the array Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 55

Inspect Individual Modules • • Main Prompt. For. Integers Array. Sum Display. Sum Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 56

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 x 86 Processors 6/e, 2010. 57

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 m. Print. X Macro mov al, ’X’ call Write. Char ENDM m. Put. Char Macro cvar push eax mov al, cvar call Write. Char pop eax ENDM • Defined using MACRO and ENDM directives Macro. Name Macro parameter-1, parameter-2, … Statement-list ENDM Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 58

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 mov al, ’A’ mov ecx, 20 Iterate: m. Put. Char al ; macro call inc al loop Iterate mov al, ’A’ mov ecx, 20 Iterate: 1 push eax 1 mov al, cvar 1 call Write. Char 1 pop eax inc al loop Iterate • At compile time: the actual source code (on the left) is expanded by substituting all occurences of m. Put. Char 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 59 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010.

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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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 61

INVOKE Directive • The INVOKE directive is a powerful replacement for Intel’s CALL instruction that lets you pass multiple arguments • Syntax: INVOKE procedure. Name [, argument. List] • Argument. List 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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 62

INVOKE Examples. data byte. Val BYTE 10 word. Val WORD 1000 h. code ; direct operands: INVOKE Sub 1, byte. Val, word. Val ; address of variable: INVOKE Sub 2, ADDR byte. Val ; register name, integer expression: INVOKE Sub 3, eax, (10 * 20) ; address expression (indirect operand): INVOKE Sub 4, [ebx] Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 63

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 • Simple example: . data my. Word WORD ? . code INVOKE my. Sub, ADDR my. Word Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 64

PROC Directive (1 of 2) • The PROC directive declares a procedure with an optional list of named parameters. • Syntax: label PROC param. List • param. List is a list of parameters separated by commas. Each parameter has the following syntax: param. Name : 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. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 65

PROC Directive (2 of 2) • Alternate format permits parameter list to be on one or more separate lines: label PROC, comma required param. List • 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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 66

Add. Two Procedure (1 of 2) • The Add. Two procedure receives two integers and returns their sum in EAX. Add. Two PROC, val 1: DWORD, val 2: DWORD mov eax, val 1 add eax, val 2 ret Add. Two ENDP Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 67

PROC Examples (2 of 3) Fill. Array 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. Fill. Array PROC, p. Array: PTR BYTE, fill. Val: BYTE array. Size: DWORD mov ecx, array. Size mov esi, p. Array mov al, fill. Val L 1: mov [esi], al inc esi loop L 1 ret Fill. Array ENDP Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 68

PROC Examples (3 of 3) Swap PROC, p. Val. X: PTR DWORD, p. Val. Y: PTR DWORD. . . Swap ENDP Read. File PROC, p. Buffer: PTR BYTE LOCAL file. Handle: DWORD. . . Read. File ENDP Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 69

PROTO Directive • Creates a procedure prototype • Syntax: • label PROTO param. List • Every procedure called by the INVOKE directive must have a prototype • A complete procedure definition can also serve as its own prototype Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 70

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: My. Sub PROTO ; procedure prototype . code INVOKE My. Sub ; procedure call My. Sub PROC. . My. Sub ENDP ; procedure implementation Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 71

PROTO Example • Prototype for the Array. Sum procedure, showing its parameter list: Array. Sum PROTO, ptr. Array: PTR DWORD, ; points to the array sz. Array: DWORD ; array size Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 72

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 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. • 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. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 73

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 generalprotection fault. Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 74

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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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 76

Java Virual Machine (JVM) • Essential part of the Java Platform • Executes compiled bytecodes • machine language of compiled Java programs Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 77

Java Methods • Each method has its own stack frame • Areas of the stack frame: • local variables • operands • execution environment Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 78

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) Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 79

Primitive Data Types • Signed integers are in twos complement format, stored in big-endian order Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 80

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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 81

JVM Instruction Set • Conditional Branching • jump to label if st(0) <= 0 ifle label • Unconditional Branching • call subroutine jsr label Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 82

Java Disassembly Examples • Adding Two Integers int int sum A = B = sum = A 3; 2; = 0; + B; Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 83

Java Disassembly Examples • Adding Two Doubles double A = 3. 1; double B = 2; double sum = A + B; Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 84

Java Disassembly Examples • Conditional Branch double A = 3. 0; boolean result = false; if( A > 2. 0 ) result = false; else result = true; Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 85

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 Irvine, Kip R. Assembly Language for x 86 Processors 6/e, 2010. 86

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