Question

I’m trying to write a procedure to read a byte string from input and store it in ASCIIZ format (string ends with 0). Below is the code I have up until now; after I read the string, I can’t figure out how to access the last byte in the string and set it to 0.

READ_ASCIIZ PROC
    PUSH BP
    MOV BP, SP

    MOV AH, 0AH
    MOV DX, 4H[BP]
    INT 21H

    POP BP
    RET 2H
READ_ASCIIZ ENDP

Answer 1

You could do it in the following manner (in nasm syntax, untested):

  ; saved modified registers
  push  bx
  push  si
  ; get pointer to buffer
  mov   si, [bp+4]
  ; grab number of chars read 
  ; (zero-extending as the length is stored as a byte)
  movzx bx, byte [si+1]
  ; write zero at end of character buffer (overwriting the newline)
  ; (+2 since that's were the buffer starts)
  mov   byte [si+bx+2], 0
  ; restore registers
  pop   si
  pop   bx

(Note this isn’t the most optimized way to do it)