部件!我不能让循环退出？

Question

我无法弄清楚为什么我的程序进入无限循环时我希望它在ecx的值等于0后退出？请帮忙？

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

调用函数(例如printf)时,需要保留值ecx

http://www.x86-64.org/documentation/abi.pdf

寄存器%rbp,%rbx和%r12到%r15"属于"调用函数,并且需要被调用函数来保存它们的值.换句话说,被调用函数必须为其调用者保留这些寄存器的值.剩余寄存器"属于"被调用函数.如果调用函数想要在函数调用中保留这样的寄存器值,则它必须将值保存在其本地堆栈帧中.