Credits and Disclaimers

Size: px

Start display at page:

Download "Credits and Disclaimers"

Albert McDaniel
5 years ago
Views:

1 Credits and Disclaimers 1 The eamples and discussion in the following slides have been adapted from a variet of sources, including: Chapter 3 of Computer Sstems 3 nd Edition b Brant and O'Hallaron 86 Assembl/GAS Snta on WikiBooks ( Using Assembl Language in Linu b Phillip ( The C code was compiled to assembl with gcc version on CentOS 7. Unless noted otherwise, the assembl code was generated using the following command line: gcc S m64 -fno-asnchronous-unwind-tables mno-red-zone O0 file.c AT&T assembl snta is used, rather than Intel snta, since that is what the gcc tools use.

2 Stack Operations: push 2 The push instruction: - decrements the stack pointer b 8, making room on the stack - copies the value of its operand to the top of the stack main: pushq % current stack not in the stack value in before after

3 Stack Operations: pop 3 The pop instruction: - copies the item at the top of the stack into its operand - increments the stack pointer b 8, removing the old top item main: popq % value of before after

4 C Code static int ma(int A, int B); 4 int main() { // caller int = 7; int = 12; } int retval = ma(, ); return 0; int ma(int A, int B) { //called procedure } int Bigger = A; if ( A < B ) Bigger = B; return Bigger;

5 Logical Steps in a Procedure Call 5 Calling a procedure (function) in C would seem to involve four steps: 1 set up the parameters that will be passed into the called procedure 2 cause eecution to jump to the first instruction within the procedure 3 when the procedure is done, cause eecution to jump back to the net instruction in the caller 4 access the value returned b the called procedure, if an // caller int, ; int retval = ma(, ); // called int ma(int A, int B) { int Bigger; 2 return Bigger; }

6 Preparing Parameters The caller's stack frame (prior to the call) will logicall reflect the current state of the caller's eecution. 6 // caller int, ; int retval = ma(, ); old frame ptr caller's frame before parameters are set end caller frame

7 Preparing Parameters 7 The first 6 parameters are passed in registers, parameters 7 and up are passed on the stack. While there s are onl 2 parameters in this eample, note order of (unneeded) parameters on the stack: // caller int, ; int retval = ma(, ); rdi rsi Registers rd rc r8 r9 old frame ptr - 4 : 7-8 : passed value: argument n passed value: argument 7 caller's frame after parameters are set

8 Assembl View 8 main: # usual / contortions make room on # stack for locals and passed values subq movl movl movl movl movl movl $16, % $7, -4(%) $12, -8(%) -8(%), %ed -4(%), %ea %ed, %esi %ea, %edi rdi rsi Registers

9 Jumping In 9 We jump into the called procedure b using the assembl instruction call: main: # this follows the code # shown previousl movl %ed, %esi movl %ea, %edi call ma movl %ea, -12(%)) The call instruction has two effects: 1 push the address of the net instruction in the caller onto the stack 2 put the address represented b the smbol ma into the PC the Stack local stuff for caller + 8

10 Called Procedure Overview 10 ma:.l4: pushq % # stack setup movq %, % subq $24, % movl %edi, -20(%) # function bod movl %esi, -24(%) movl -20(%), %ea movl %ea, -4(%) movl -20(%), %ea cmpl -24(%), %ea jge.l4 movl -24(%), %ea movl %ea, -4(%) movl -4(%), %ea # set return value leave # jump back to caller ret

11 Local Stack Setup 11 ma: pushq % # save old frame ptr movq %, % # set to this frame subq $24, % # make room for locals movl %edi, -20(%) # move parameters movl %esi, -24(%) Registers + 8 local stuff for caller old frame ptr fame for main() rdi rsi local: Bigger... fame for ma() the Stack

12 The Computations 12 ma:.l4: movl -20(%), %ea movl %ea, -4(%) movl -20(%), %ea cmpl -24(%), %ea jge.l4 movl -24(%), %ea movl %ea, -4(% old frame ptr local: Bigger the Stack

13 Returning a Value 13 We use the ea (or %ra) register to hold the return value: ma: movl -4(%), %ea # set return value to Bigger

14 Preparing to Leave 14 The leave instruction resets the stack and frame pointers prior to returning: ma: leave must be reset point to the beginning of the stack frame of the caller must be reset point to where the top of the stack was when the call instruction was made local stuff for caller the Stack

15 Preparing to Leave older frame ptr before old frame ptr local: Bigger older frame ptr after old frame ptr movl %, % # 1 popl % # 2

16 Jumping Back We jump back b using the assembl instruction ret: 16 ma: ret popq %rip older frame ptr before older frame ptr after

17 Stack Summar old frame ptr Stack before eecution of the call instruction......

18 Stack Summar stack after eecution of the call instruction old frame ptr stack after eecution of stack setup code in ma() older frame ptr old frame pointer local: Bigger...

19 Stack Summar 19 older frame ptr stack after eecution of leave instruction in ma()

20 Stack Summar 20 older frame ptr stack after eecution of ret instruction in ma(), eecution now proceeds in main()

Credits and Disclaimers

Credits and Disclaimers 1 The examples and discussion in the following slides have been adapted from a variety of sources, including: Chapter 3 of Computer Systems 3 nd Edition by Bryant and O'Hallaron