Assembly! I can't get the loop to exit? - linux

Question

I can't figue out why my program goes into an infinite loop when i want it to exit after the value of ecx is equal to 0? Please help?
section .data
;get external functions
extern printf
global main
main:
;set up stack frame
push rbp
mov rbp, rsp
;if(x<y)
;print x is less
;else
;print y is larger than x
;mov values into register to compare them
mov rax,[x]
mov rbx,[y]
cmp rax,rbx ;cmp x,y
jg .x_is_greater
lea rdi,[y_less]
xor eax,eax ;must clear eax when using printf
call printf
jmp .done
.x_is_greater:
;print "X is greater to the screen"
;mov r11,[count]
;lea rdi,[x_greater]
;xor eax,eax
;call printf
;mov r12,[zero]
;cmp r11,r12
;jg .myloop ;jump to myloop if greater than zero
;jmp .done ;return if equal to 0
mov ecx, 3; [count]
;mov r12, [zero]
jmp .myloop
.myloop:
;;dec r11
;dec rcx
lea rdi,[fmt]
lea rsi,[ecx]
;mov rdx,[r12]
xor eax,eax ;must clear eax when using printf
call printf
cmp ecx, 0
jz .done
lea rdi,[x_greater]
xor eax,eax ;must clear eax when using printf
call printf
lea rdi,[fmt]
lea rsi,[ecx]
;mov rdx,[r12]
xor eax,eax ;must clear eax when using printf
call printf
dec ecx
;sub rcx,[one]
jmp .myloop
;jmp .done
.done:
leave
;xor eax, eax
ret;exit program
;leave ;destroy stack frame
section .bss
section .data
prompt db "This is a practice program to test what I know!",0x0a,0
y_less db "Y < X",0x0a,0
x_greater db "X > Y ",0x0a,0
x db 10
y db 1
count dq 3
zero db 0
one dq 1
fmt db "R11 %d ",0x0a,0

Answer 1

When calling functions (e.g., printf), you need to preserve the value of ecx
http://www.x86-64.org/documentation/abi.pdf
Registers %rbp, %rbx and
%r12 through %r15 “belong” to the calling function and the called function is
required to preserve their values. In other words, a called function must preserve
these registers’ values for its caller. Remaining registers “belong” to the called
function. If a calling function wants to preserve such a register value across a
function call, it must save the value in its local stack frame.

Answer 2

I have made a loop called WHILE-DO with Printf and just checked on Microsoft Visual Studio That RAX,RCX,RDX,R9,R8 are used by Call Printf. So I added our number to EBX. It's working without poping, pushing, stacking :). I hope it will help some people.
extrn ExitProcess: PROC
extrn printf: PROC
.data
fmt db '%s',10,0 ; The printf format, "\n", '0'
check db 'HALO',0
.code
Start PROC
xor eax,eax
mov rbx,1
start_while:
cmp rbx,10
jge end_while
lea rcx,check
lea rdx,fmt
call printf
add rbx,1
jmp start_while
end_while:
xor eax,eax
xor rbx,rbx
call ExitProcess
Start ENDP
End

Answer 3

The printf call may change the value of ecx register. So what you have to do one of the following:
Push it to the stack before the printf calls and pop it after the printf call;
Use a callee-saved register as the loop counter; or
Save it in a callee-saved register and restores it.
Example of the first option:
.myloop:
lea rdi,[fmt]
lea rsi,[ecx]
xor eax,eax ;must clear eax when using printf
push ecx ; saved
call printf
pop ecx ; restored
cmp ecx, 0
jz .done
lea rdi,[x_greater]
push ecx ; saved
xor eax,eax ;must clear eax when using printf
call printf
pop ecx ; restored
lea rdi,[fmt]
lea rsi,[ecx]
push ecx ; saved
xor eax,eax ;must clear eax when using printf
call printf
push ecx ; restored
dec ecx
jmp .myloop