使用 eax 的 Fasm 循环不起作用

Question

I am trying to make a for loop in assembly where the EAX register is set to 5, and increased until it is greater than 10. Each time it increases, it outputs it's current value. When I execute my program, it goes in an infinite loop which only outputs 4. Why is EAX 's value 4? Why is register EAX not increasing as it should?

include 'include/macro/import32.inc'
format PE console
entry start

section '.text' code readable executable

start:

mov eax,5
loop1:
    inc eax
    push eax
    push msg2
    call [printf]
    cmp eax,10
    jb loop1

call [getchar]
push 0
call [exit]

section '.data' data readable writable
msg2  db "%i",0dh,0ah,0

section 'idata' import data readable
library msvcrt,"msvcrt.dll"
import msvcrt,printf,"printf",getchar,"getchar",exit,"exit"

Answer 1

The output from printf is returned in eax which contains the number of characters printed: 3 in your case (the digit, CR, and LF). Since that is less than 10, you loop up, add 1 (which makes it 4), print that, and repeat.

What you need to do is store eax ( push eax ) before setting up the printf call, then restore it ( pop eax ) after printf returns like:

loop1:
    inc  eax
    push eax        ; store eax
    push eax
    push msg2
    call [printf]
    add  esp,8      ; clean the stack from the printf call 
    pop  eax        ; restore eax
    cmp  eax,10
    jb   loop1

Or use a different register such as ebx for your loop variable.

Answer 2

Always preserve EAX before using printf. printf destroys your EAX

inc eax
push eax
...call to printf
pop eax
cmp eax,10