CODEWITHSUNDEEP
linuxassemblynasm
James
James

Reputation: 77

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

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

Upvotes: 3

Views: 1098

Answers (3)

Mayurii
Mayurii

Reputation: 26

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

Upvotes: 1

Murilo Vasconcelos
Murilo Vasconcelos

Reputation: 4827

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

Upvotes: 1

cbranch
cbranch

Reputation: 4769

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.

Upvotes: 5

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