Introduction to Assembly II Abed Asi Extended System Programming Laboratory (ESPL) CS BGU Fall 2014/2015 ; Data section begins section.data var1 dd 40 var2 dd 20 var3 dd 30 section.text global _start _start: mov ecx, [var1] cmp ecx, [var2] jg check_third_var mov ecx, [var2] check_third_var: cmp ecx, [var3] jg _exit mov ecx, [var3] _exit: mov ebx, ecx mov eax, 1 int 80h Abed Asi - ESPL 2 section.text global _start ;must be declared for linker (ld) section.data msg db 'Hello world!',0xa ;our dear string len equ $ - msg ;length of our dear string _start: ;tell linker entry point mov edx,len ;message length mov ecx,msg ;message to write mov ebx,1 ;file descriptor (stdout) mov eax,4 ;system call number (sys_write) int 0x80 ;call kernel mov eax,1 ;system call number (sys_exit) int 0x80 ;call kernel Abed Asi - ESPL 3 Functions and the Stack Pentium Implementation of the stack Uses of the stack Calling Procedures Abed Asi - ESPL 4 A stack is a last-in-first-out (LIFO) data structure The top-of-the-stack (TOS) is indicated by ESP register The key characteristics: Only words (16-bit) or doublewords (32-bit) are saved on the stack The stack grows toward lower memory address (downward) TOS always points to the last inserted data item TOS points to the lower byte of the last inserted word Abed Asi - ESPL 5 push source pop destination The operands can be a 16-bit or 32-bit general purpose registers, or a word or a doubleword in memory Abed Asi - ESPL 6 7 8 push 21ABH push 7FBD329AH pop EBX Abed Asi - ESPL 9 The stack is used for three main purposes Abed Asi - ESPL 10 Temporary Storage of Data Transfer of Control Parameter Passing Abed Asi - ESPL 11 The Pentium provides call and ret instructions After the call instruction, the EIP points to the next instruction to be executed The processor pushes the content of the EIP onto the stack call proc-name Abed Asi - ESPL 12 ESP = ESP 4 ESP = EIP EIP = EIP + d High Low The ret instruction is used to transfer control from the called procedure to the calling procedure ret Note: integral return value of procedures are stored in EAX 13 Abed Asi - ESPL High Low EIP = ESP ESP = ESP + 4 It is more complicated than that used in high-level languages The calling procedure first places all the parameters need by the called procedure in the stack Abed Asi - ESPL 14 For example, consider passing two 32-bit parameters to a SUM procedure pushnumber1 pushnumber2 call sum So, how do we retrieve the parameters now ? Since the stack is a sequence of memory location ESP+4 points to number2, and ESP+8 to number1 For instance, to read number2 we can invoke: Abed Asi - ESPL 15 movEBX, [ESP+4] Are we done ? What type of problems we would encounter? The stack pointer is updated by the push and pop instructions the relative offset changes A better alternative is to use the EBP register Abed Asi - ESPL 16 movEBP, ESP mov AX, [EBP+4] Done? push EBP movEBP, ESP mov AX, [EBP+4] Since every procedure uses the EBP register, it should be preserved Abed Asi - ESPL 17 push number1 push number2 call sum sum: push EBP mov EBP, ESP mov ESP, EBP pop EBP ret Abed Asi - ESPL 18 func: push EBP movEBP, ESP sub ESP, 8... section.DATA string db ESPL,0 section.CODE mov EAX, string ;EAX = string[0] pointer push EAX inc EAX push EAX ;EAX = string[1] pointer call swap swap: push EBP mov EBP, ESP push EBX;save EBX procedure uses EBX mov EBX, [EBP+12]; EBX = first character pointer xchg AL, [EBX]; swap between operands mov EBX, [EBP+8]; EBX = second character pointer xchg AL, [EBX] mov EBX, [EBP+12]; EBX = first character pointer xchg AL, [EBX] pop EBX mov ESP, EBP pop EBP ret Abed Asi - ESPL 19