Function calls in UNIX Write assembly code to implement the

Function calls in UNIX

Write assembly code to implement the following function calls, using C/UNIX calling conventions. Show the contents of the stack on entering the called function.

1. int myfun1(int,int,int,char); //prototype

...

myfun1(d,a,5,\'m\'); //function call

2.int myfunc2(int,char*); //prototype

...

char[] bob=”Bob\'s your uncle!”;

...

myfunc2(18, bob); //function call

3. printf(“CSU presidents get a car allowance of $%d per month\ ”, 1000);

Solution

2.Ans

3.Ans

section .bss
buf resb 8 ; define an array of 8 uninitialised bytes
section .text

global _start
_start:
mov rax, 3 ; system call number (sys_read)
xor rbx, rbx ; file descriptor 0 (stdin)
mov rcx, buf ; buffer to store data
mov rdx, 8 ; Lenght of buffer
int 0x80

xor rax,rax ; rax = 0
mov rbx, buf ; rbx = &buf
xor rcx, rcx

StrToDec:
mov cl, [rbx] ; cl = *rbx
cmp rcx, 0 ; check for nul THIS IS WRONG
cmp rcx, 10 ; we have to check for NL ascii code
je end ; if rcx == 0 goto end
imul rax, 10 ; rax *= 10
sub rcx, 48 ; rcx -= 48 (48 is acii for \'0\')
add rax, rcx ; rax += rcx
inc rbx ; rbx++
jmp StrToDec

end:
mov rbx, rax ; rbx = rax
mov rax, 1 ; rax = 1
int 0x80