1. 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. 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

3. 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.

4. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 4 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

5. 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 variable push varB ;another dword variable call IMUL2 ;result in EAX, stack unchanged add 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. 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 ret IMUL2 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 ebp esp varA varBesp after ret

7. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 7 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

8. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 8 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.

9. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 9 Who removes parameters from the stack? Caller (C)...... or...... Called-procedure (STDCALL): AddTwo PROC push val2 push ebp push val1 mov ebp,esp call 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. 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 stack IMUL2 ENDP  Since 8 bytes of parameters have been pushed onto the stack

11. 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 35 push –63 push 23 push 3 ;# of args call AddSome add 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 ret AddSome ENDP  The called procedure:

12. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 12 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:.data count = 100 array WORD count DUP(?).code push OFFSET array push COUNT call ArrayFill

13. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 13 Passing an Array by Reference (2 of 2) ArrayFill PROC push ebp mov ebp,esp pushad mov esi,[ebp+12] mov ecx,[ebp+8]. ESI points to the beginning of the array, so it's easy to use a loop to access each array element. View the complete program.View the complete program ArrayFill can reference an array without knowing the array's name:

14. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 14 Your turn... Create a procedure named Difference that subtracts the first argument from the second one. Following is a sample call: push 14; first argument push 30; second argument call Difference; EAX = 16 Difference PROC push ebp mov ebp,esp mov eax,[ebp + 8]; second argument sub eax,[ebp + 12]; first argument pop ebp ret 8 Difference ENDP

15. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 15 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 charVal BYTE 'x'.code movzxeax,charVal pusheax callUppercase

16. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 16 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 pushDWORD PTR longVal + 4; high doubleword pushDWORD PTR longVal; low doubleword callWriteHex64

17. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 17 Saving and Restoring Registers Push registers on stack just after assigning ESP to EBP local registers are modified inside the procedure MySub PROC pushebp movebp,esp pushecx; save local registers pushedx

18. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 18 Stack Affected by USES Operator MySub1 PROC USES ecx edx ret MySub1 ENDP USES operator generates code to save and restore registers: MySub1 PROC pushecx pushedx popedx popecx ret

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20. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 20 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

21. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 21 Creating LOCAL Variables Example - create two DWORD local variables: Say: int x=10, y=20; ret address saved ebp EBP 10 (x) [ebp-4] MySub PROC 20 (y) [ebp-8] pushebp movebp,esp sub esp,8;create 2 DWORD variables movDWORD PTR [ebp-4],10 ; initialize x=10 movDWORD PTR [ebp-8],20 ; initialize y=20

22. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 22 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: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

23. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 23 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]

24. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 24 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 ebp mov ebp,esp sub esp,8

25. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 25 LEAVE Instruction Terminates the stack frame for a procedure. MySub PROC enter 8,0... leave ret MySub ENDP pushebp movebp,esp subesp,8 ; 2 local DWORDs movesp,ebp ; free local space popebp Equivalent operations

26. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 26 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 PROC LOCAL var1:BYTE, var2:WORD, var3:SDWORD

27. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 27 Using LOCAL LOCAL flagVals[20]:BYTE; array of bytes LOCAL pArray:PTR WORD; pointer to an array myProc PROC,; procedure LOCAL t1:BYTE,; local variables Examples:

28. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 28 LOCAL Example (1 of 2) BubbleSort PROC LOCAL temp:DWORD, SwapFlag:BYTE... ret BubbleSort ENDP BubbleSort PROC push ebp mov ebp,esp add esp,0FFFFFFF8h; add -8 to ESP... mov esp,ebp pop ebp ret BubbleSort ENDP MASM generates the following code:

29. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 29 LOCAL Example (2 of 2) Diagram of the stack frame for the BubbleSort procedure:

30. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 30 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

31. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 31 Local Byte Variable Example1 PROC LOCAL var1:BYTE mov al,var1 ; [EBP - 1] ret Example1 ENDP

32. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 32 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

33. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 33 WriteStackFrame Example main PROC mov eax, 0EAEAEAEAh mov ebx, 0EBEBEBEBh INVOKE aProc, 1111h, 2222h exit main ENDP aProc PROC USES eax ebx, x: DWORD, y: DWORD LOCAL a:DWORD, b:DWORD PARAMS = 2 LOCALS = 2 SAVED_REGS = 2 mov a,0AAAAh mov b,0BBBBh INVOKE WriteStackFrame, PARAMS, LOCALS, SAVED_REGS

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

36. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 36 Recursively Calculating Sum 1 + … + n 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 1+2+…+n. Receives: ECX = count = n. Returns: EAX = sum Stack frame: View the complete programcomplete program

37. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 37 Calculating a Factorial (1 of 3) int function factorial(int n) { if(n == 0) return 1; else return n * factorial(n-1); } 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.

38. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 38 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 L1; yes: continue mov eax,1; no: return 1 jmp L2 L1:dec eax push eax; Factorial(n-1) call Factorial ; Instructions from this point on execute when each ; recursive call returns. ReturnFact: mov ebx,[ebp+8] ; get n mul ebx ; eax = eax * ebx L2:pop ebp; return EAX ret 4; clean up stack Factorial ENDP See the program listingprogram listing

39. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 39 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.

40. 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. 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. 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.asm INCLUDE Irvine32.inc INCLUDE Macros.inc.data msg1 BYTE "In main",0.code main PROC mWriteString msg1 call procA call procB exit main ENDP include procA.asm include procB.asm END main File procA.asm.code procA PROC mWriteString msg2 ret procA ENDP.data msg2 BYTE "In procA",0 File procB.asm.code procB PROC mWriteString msg3 ret procB ENDP.data msg3 BYTE "In procB",0

43. 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. 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 names BYTE, 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. 45 Example File my_prog.asm INCLUDE Irvine32.inc INCLUDE Macros.inc EXTERN procA@0:proc, procB@0:proc.data msg1 BYTE "In main",0.code main PROC mWriteString msg1 call procA call procB exit main ENDP END main File procA.asm INCLUDE Irvine32.inc INCLUDE Macros.inc public procA.code procA PROC mWriteString msg1 ret procA ENDP.data msg1 BYTE "In procA",0 END File procB.asm INCLUDE Irvine32.inc INCLUDE Macros.inc public procB.code procB PROC mWriteString msg1 ret procB ENDP.data msg1 BYTE "In procB",0 END

46. 46 Example (cont.)  To assemble each file separately and link them do: ML –c procA.asm ML –c procB.asm ML 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. 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. 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. 49 Global Variable Example File procA.asm INCLUDE Irvine32.inc PUBLIC procA EXTERN varA:dword.code procA PROC mov eax,varA call WriteDec ret procA ENDP END File mp.asm INCLUDE Irvine32.inc PUBLIC varA EXTERN procA@0:proc.data varA DWORD ?.code main PROC mov varA,333 call procA exit main ENDP END main To assemble and link, you can do: ML mp.asm procA.asm

50. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 50 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

51. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 51 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

52. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 52 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

53. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 53 Example: ArraySum Program Let's review the ArraySum program from Chapter 5. Each of the four white rectangles will become a module.

54. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 54 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

55. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 55 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, ptrPrompt:PTR BYTE,; prompt string theSum:DWORD; sum of the array The sum.inc file contains prototypes for external functions that are not in the Irvine32 library:

56. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 56 Inspect Individual Modules Main PromptForIntegers ArraySum DisplaySum

57. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 57 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

58. 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 Macro mov al, ’X’ call WriteChar ENDM MacroName Macro parameter-1, parameter-2, … Statement-list ENDM mPutChar Macro cvar push eax mov al, cvar call WriteChar pop eax ENDM

59. 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, 20 Iterate: mPutChar al ; macro call inc al loop Iterate mov al, ’A’ mov ecx, 20 Iterate: 1 push eax 1 mov al, cvar 1 call WriteChar 1 pop eax inc al loop Iterate

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61. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 61 INVOKE, ADDR, PROC, and PROTO INVOKE Directive ADDR Operator PROC Directive PROTO Directive Parameter Classifications Example: Exchaning Two Integers Debugging Tips

62. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 62 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

63. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 63 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]

64. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 64 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:

65. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 65 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.

66. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 66 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

67. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 67 AddTwo Procedure (1 of 2) 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.

68. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 68 PROC Examples (2 of 3) FillArray PROC, pArray:PTR BYTE, fillVal:BYTE arraySize:DWORD mov ecx,arraySize mov esi,pArray mov al,fillVal L1:mov [esi],al inc esi loop L1 ret FillArray ENDP FillArray receives a pointer to an array of bytes, a single byte fill value that will be copied to each element of the array, and the size of the array.

69. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 69 PROC Examples (3 of 3) ReadFile PROC, pBuffer:PTR BYTE LOCAL fileHandle:DWORD... ReadFile ENDP Swap PROC, pValX:PTR DWORD, pValY:PTR DWORD... Swap ENDP

70. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 70 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

71. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 71 PROTO Directive Standard configuration: PROTO appears at top of the program listing, INVOKE appears in the code segment, and the procedure implementation occurs later in the program: MySub PROTO ; procedure prototype.code INVOKE MySub ; procedure call MySub PROC ; procedure implementation. MySub ENDP

72. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 72 PROTO Example Prototype for the ArraySum procedure, showing its parameter list: ArraySum PROTO, ptrArray:PTR DWORD,; points to the array szArray:DWORD; array size

73. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 73 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.

74. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 74 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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76. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 76 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

77. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 77 Java Virual Machine (JVM) Essential part of the Java Platform Executes compiled bytecodes machine language of compiled Java programs

78. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 78 Java Methods Each method has its own stack frame Areas of the stack frame: local variables operands execution environment

79. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 79 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)

80. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 80 Primitive Data Types Signed integers are in twos complement format, stored in big-endian order

81. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 81 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

82. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 82 JVM Instruction Set Conditional Branching jump to label if st(0) <= 0 ifle label Unconditional Branching call subroutine jsr label

83. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 83 Java Disassembly Examples Adding Two Integers int A = 3; int B = 2; int sum = 0; sum = A + B;

84. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 84 Java Disassembly Examples Adding Two Doubles double A = 3.1; double B = 2; double sum = A + B;

85. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 85 Java Disassembly Examples Conditional Branch double A = 3.0; boolean result = false; if( A > 2.0 ) result = false; else result = true;

86. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 86 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

87. Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010. 87 53 68 75 72 79 6F