ASM Community

Hi!
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

_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!

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.