I'm a total asm newbie. I'm using NASM on GNU/Linux. I'm trying to push 3 characters into stack and then print them, it does print 3 characters, but not the right ones.
strace output:
 strace ./write
write(1, "A\0\0", 3A)                    = 3

section .data

section .text
global _start


push 'C'
push 'B'
push 'A'

mov eax,4         
mov ebx,1         
mov ecx,esp     
mov edx,3   
int 80h       

mov eax,1   
mov ebx,0   
int 80h

Here's the write system call:
ssize_t write(int fd, const void *buf, size_t count);
I'm not sure why it doesn't print ABC. It last argument in strace output '3A' looks strage also, it should be just 3 I guess. Thank you!
Posted on 2006-10-17 12:51:18 by Hyaku_
push 'A' is in fact push dword 0x00000040

you should do push 'ABC', this is push dword 0x00424140
Posted on 2006-10-17 14:18:17 by vid
By looking at the head of the "Write" procedure, you would be able to say that the second parameter is the pointer to the beginning of the characters not the characters themselves. Therefore, you should put those characters in adjacent places in the same segment and pass the pointer to the one with the memory location less than the others as the second parameter to the "write" routine.

Here is an example of a procedure which fills the value of the AL, AH and the DL registers with the correct parameters.

  GetParams PROC NEAR
    PUSH    ES                          ; Push the extra segment onto the stack
    PUSH    BP                          ; Push the base pointer onto the stack
    MOV    BP , SP                    ; Move the stack pointer to the base pointer
    MOV    BX , WORD PTR       ; BX now points to the first parameter's offset
    MOV    ES , WORD PTR       ; ES now points to the segment parameter
    MOV    AL , BYTE PTR ES:      ; AL is the first byte inside the parameter
    MOV    AL , BYTE PTR ES: ; AH is the second byte inside the parameter
    MOV    AL , BYTE PTR ES: ; DL is the third byte inside the parameter
    POP    BP                          ; Restore the base pointer
    POP    ES                          ; Restore the extra segment
    RET    0004h                      ; Return and remove 2 WORDs from the stack
  GetParams ENDP
Good luck.
Posted on 2006-10-17 14:39:32 by XCHG
mov bp,B800
mov es,bp
mov di,722
push "A"
push "B"
push "C"
pop ax
mov ah,0F
pop ax
mov ah,0F
pop ax
mov ah,0F
Posted on 2006-10-17 21:37:44 by eek