Department of Computer Science National Tsing Hua University Advanced Procedures Department of Computer Science National Tsing Hua University

Assembly Language for Intel-Based Computers, 5th Edition Kip Irvine Chapter 8: Advanced Procedures Slides prepared by the author Revision date: June 4, 2006 (c) Pearson Education, 2006-2007. 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.

Chapter Overview Stack Frames Recursion .MODEL Directive INVOKE, ADDR, PROC, and PROTO Creating Multimodule Programs

Two Ways to Pass Parameters Register vs. stack parameters: Register parameters require dedicating a register to each parameter. Stack parameters are passed through run-time stack Registers are faster than stack Ex: two ways of calling DumpMem; clearly the second is easier to write pushad mov esi,OFFSET array mov ecx,LENGTHOF array mov ebx,TYPE array call DumpMem popad push OFFSET array push LENGTHOF array push TYPE array call DumpMem

How to Pass Stack Parameters? Use stack frame (activation record) Area of run-time stack that stores everything a procedure needs in order to run, including return address, passed parameters, saved registers, and local variables  state of the procedure Each invocation to a procedure will push the corresponding stack frame into the run-time stack  different invocations (even to the same procedure) will occupy different areas of the stack Procedure return will pop entire stack frame Analog: mobile home

Contents of Stack Frame What data are needed for the following function to run? int AddTwo(int x, y) { int sum; sum = x + y; return sum; } Parameters: x, y Local variable: sum Return address Saved registers x y return addr sum “state” Size can be determined at assembly time

Accessing Stack Frame When procedure AddTwo is called, its corresponding stack frame is pushed onto the run-time stack How to reference the variables in stack, i.e., x, y, sum? ? ESP? ESP AddTwo PROC mov eax,x add eax,y mov sum,eax ret AddTwo ENDP x y return addr sum

Accessing Stack Frame Idea: use addresses relative to a base address of the stack frame in the run-time stack, which does not change during the procedure  base pointer or frame pointer Dedicate a register to hold this pointer  EBP A procedure can explicitly access stack parameters using constant offsets from EBP, e.g. [EBP + 8] EBP is restored to its original value when procedure returns [EBP+12] x y return addr sum [EBP+8] [EBP+4] EBP* * Exact location of frame pointer will be explained shortly

Preparing Stack Frame Must be done explicitly in the ASM program: Caller procedure pushes arguments on the stack and calls callee; the CALL instruction will push the return address on the stack Callee procedure pushes EBP (currently holding frame point of the caller procedure) on the stack To save the EBP of the caller procedure Callee then sets EBP to ESP To let EBP point to base of its own stack frame If local variables are needed, a constant is subtracted from ESP to make room on the stack Net results: set aside a region of memory in stack to hold all data needed for this procedure

Return from Procedure RET: pops top of stack into the instruction pointer (EIP or IP); control transfers to the target address What about stack parameters? Syntax: RET RET n Optional operand n causes n bytes to be added to the stack pointer after EIP (or IP) receives the popped stack

Stack Frame Example (1 of 2) .data sum DWORD ? .code push 6 ; second argument push 5 ; first argument call AddTwo ; EAX = sum mov sum,eax ; save the sum Return value in eax AddTwo PROC push ebp mov ebp,esp . old EBP

Stack Frame Example (2 of 2) old EBP AddTwo PROC push ebp mov ebp,esp ; base of stack frame mov eax,[ebp + 12]; second argument (6) add eax,[ebp + 8] ; first argument (5) pop ebp ret 8 ; clean up the stack AddTwo ENDP ; EAX contains the sum

Passing Arguments by Reference (1/2) Consider the ArrayFill procedure, which fills an array with 16-bit random integers Calling procedure passes address of the array, along with a count of number of array elements: .data count = 100 array WORD count DUP(?) .code push OFFSET array push COUNT call ArrayFill ...

Passing Arguments by Reference (2/2) ArrayFill can reference an array without knowing the array's name: ESI points to the beginning of the array, so it's easy to use a loop to access each array element ArrayFill PROC push ebp mov ebp,esp pushad mov esi,[ebp+12] mov ecx,[ebp+8] ...

Local Variables To explicitly create local variables, subtract their total size from ESP to leave space Still part of the stack frame Ex.: creates and initializes two 32-bit local variables (locA and locB): MySub PROC push ebp mov ebp,esp sub esp,8 ; make space mov DWORD PTR [ebp-4],123456h ; locA mov DWORD PTR [ebp-8],0 ; locB ...

Clean Local Variables on Return Before return, resets ESP by assigning it the value of EBP  release local variables from the stack MySub PROC push ebp mov ebp,esp sub esp,8 mov DWORD PTR [ebp-4],123456h ; locA mov DWORD PTR [ebp-8],0 ; locB mov esp,ebp pop ebp ret MySub ENDP

ENTER and LEAVE ENTER directive creates stack frame for a called procedure ENTER numbytes, nestinglevel pushes EBP on the stack sets EBP to the base of the stack frame reserves space for local variables LEAVE: reverse operations MySub PROC push ebp mov ebp,esp sub esp,8 MySub PROC enter 8,0

LOCAL Directive A local variable is created, used, and destroyed within a single procedure LOCAL directive declares a list of local variables LOCAL varlist immediately follows the PROC directive each variable is assigned a type MySub PROC LOCAL var1:BYTE, var2:WORD, var3:SDWORD

Using LOCAL Examples: myProc PROC, ; procedure LOCAL t1:BYTE, ; local variables LOCAL flagVals[20]:BYTE ; array of bytes LOCAL pArray:PTR WORD ; pointer to an array

MASM-Generated Code (1/2) 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-Generated Code (2/2) Diagram of the stack frame for BubbleSort:

Non-Doubleword Local Variables Local variables can have different sizes How are their memory space allocated 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] ret Example1 ENDP

LEA Instruction Return offsets of direct and indirect operands OFFSET can only return constant offsets LEA required when obtaining the offset of a stack parameter or local variable. For example: void CopyString(int *count) { char temp[20]; ... } CopyString PROC, count:DWORD ; parameter LOCAL temp[20]:BYTE ; local variable mov edi,OFFSET count ; invalid operand mov esi,OFFSET temp ; invalid operand lea edi,count ; ok lea esi,temp ; ok

Review (True/False): A procedure’s stack frame always contains caller’s return address and procedure’s local variables. (True/False): Arrays are passed by reference to avoid copying them onto the stack. (True/False): A procedure’s prologue code always pushes EBP on the stack. (True/False): Local variables are created by adding an integer to the stack pointer. (True/False): In 32-bit protected mode, the last parameter to be pushed on the stack in a procedure call is stored at location EBP+8. (True/False): Passing by reference requires popping a parameter’s offset from the stack in the called procedure. What are two common types of parameters?

What's Next Stack Frames Recursion .MODEL Directive INVOKE, ADDR, PROC, and PROTO Creating Multimodule Programs

What is Recursion? The process created when . . . A procedure calls itself Procedure A calls procedure B, which in turn calls procedure A Using a graph in which each node is a procedure and each edge is a procedure call, recursion forms a cycle:

Recursively Calculating a Sum Recursively calculates sum of array of integers. Receives: ECX = count. Returns: EAX = sum CalcSum PROC cmp ecx,0 ; check counter value jz L2 ; quit if zero add eax,ecx ; otherwise, add to sum dec ecx ; decrement counter call CalcSum ; recursive call L2: ret CalcSum ENDP Stack frame:

Calculating a Factorial (1/3) This function calculates the factorial of integer n. A new value of n is saved in each stack frame: int factorial(int n) { if(n == 0) return 1; else return n*factorial(n-1); }

Calculating a Factorial (2/3) 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

Calculating a Factorial (3/3) 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.

What's Next Stack Frames Recursion .MODEL Directive INVOKE, ADDR, PROC, and PROTO Creating Multimodule Programs

INVOKE Directive The INVOKE directive is a powerful replacement for Intel’s CALL instruction that lets you pass multiple arguments Syntax: INVOKE procedureName [, argumentList] Arguments can be: immediate values and integer expressions variable names address and ADDR expressions register names

INVOKE Examples .data byteVal BYTE 10 wordVal WORD 1000h .code ; direct operands: INVOKE Sub1,byteVal,wordVal ; address of variable: INVOKE Sub2,ADDR byteVal ; register name, integer expression: INVOKE Sub3,eax,(10 * 20) ; address expression (indirect operand): INVOKE Sub4,[ebx]

ADDR Operator .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: .data myWord WORD ? .code INVOKE mySub,ADDR myWord

PROC Directive (1/2) The PROC directive declares a procedure with an optional list of 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 (2/2) 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, ... Or they can be on separate lines: param-1:type-1, param-2:type-2, . . ., param-n:type-n comma required

Sanity Check #1 Remember that PROC and INVOKE are directives, not instructions. (i.e., CPU does not understand it.)

PROC Examples (1/2) AddTwo PROC, val1:DWORD, val2:DWORD mov eax,val1 The AddTwo procedure receives two integers and returns their sum in EAX C++ programs typically return 32-bit integers from functions in EAX AddTwo PROC, val1:DWORD, val2:DWORD mov eax,val1 add eax,val2 ret AddTwo ENDP

PROC Examples (2/2) 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. 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

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 Example: Prototype for the ArraySum procedure, showing its parameter list ArraySum PROTO, ptrArray:PTR DWORD, szArray:DWORD

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

Sanity Check #2 So, PROC and INVOKE are directives, not instruction. How does MASM handle the parameters?

Passing by Value When a procedure argument is passed by value, a copy of a 16-bit or 32-bit integer is pushed on the stack. Example: MASM generates the following code: .data myData WORD 1000h .code main PROC INVOKE Sub1, myData push myData call Sub1

Passing by Reference When an argument is passed by reference, its address is pushed on the stack. Example: MASM generates the following code: .data myData WORD 1000h .code main PROC INVOKE Sub1, ADDR myData push OFFSET myData call Sub1

Example: Exchanging Two Integers Swap procedure exchanges the values of two 32-bit integers. pValX and pValY do not change values, but the integers they point to are modified. Swap PROC USES eax esi edi, pValX:PTR DWORD, ; pointer to 1st int pValY:PTR DWORD ; pointer to 2nd int mov esi,pValX ; get pointers mov edi,pValY mov eax,[esi] ; get first integer xchg eax,[edi] ; exchange with second mov [esi],eax ; replace first integer ret Swap ENDP

Example: Exchanging Two Integers Will the following work? Swap PROC USES eax esi edi, X:DWORD, Y:DWORD mov eax, X ; get first integer xchg eax, Y ; exchange with second mov X,eax ; replace first integer ret Swap ENDP A DWORD 10 B DWROD 20 INVOKE Swap A, B INVOKE Swap ADDR A, ADDR B ;for previous slide

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.

Summary Stack parameters Local variables 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 MASM procedure-related directives INVOKE, PROC, PROTO