Question

I have a (seemingly) simple question to read in a string and print it out again using x86 interrupt based assembly. The problem i’m having is accessing the string that has been read in properly. The variable – input db 20, 0, " " is my initial string. After I call the input interrupt, the 0 should now hold the length of the string, which I need to store and pass to cx when I call the print interrupt. 20 is the max length of input. I end up with two problems – how do I access the length of the string (I’ve using an arbitrary number, which either chops it short or prints garbage after the end) and how do I access the string without the number bit at the start? Any help appreciated, my attempt is:

(I use tasm & Tlink under win 7 32 bit, and also under dos box emulation)

;7. Read in a String of characters and Print the string back out.


.model small
.stack 100h
.data


    colour db 00001111b
    input db 20, 0, "                    "
    strlen dw 20; this should be ?


.code

main:

    call initsegs
    call readstring
    call printstring
    call exit



PROC printstring 

    push ax bx cx dx bp

    mov ah, 13h ; int 13h of 10h, print a string
    mov al, 1 ; write mode: colour on bl    
    mov bh, 0 ; video page zero
    mov bl, colour; colour attribute
    mov cx, strlen; getting this is the problem
    mov dh, 10; row
    mov dl, 10; column
    mov bp, offset input ; es:bp needs to point at string..this points to string but includes its max and length at the start

    int 10h;


    pop bp dx cx bx ax 

    ret

ENDP printstring






PROC readstring 

    push ax dx


    mov ah, 0ah ; function a of 21h - read a string
    mov dx, offset input ; reads string into DS:DX so DX needs be offset of string variable

    int 21h ; call the interrupt
    ;mov strlen ....something
    pop dx ax

    ret

ENDP readstring



PROC exit

    mov ah, 4ch
    INT 21h

    RET

ENDP Exit


PROC initsegs

    push ax

    mov ax, @DATA
    mov ds, ax
    mov es, ax

    pop ax

    RET

ENDP initsegs



end main

Answer 1

What you have here is known as Pascal String. The original version (used in 16bit Pascal language) used first byte to hold the length of the string, and the rest of bytes contaned the actual string (not zero terminated). This gives maximum length of 255 bytes.

The version uses 32bit Delphi used slighly different approach:

struct {
  DWORD allocated_size;
  DWORD used_size;
  char* buff;
};

It’s similar to your case, but you use BYTE for size instead of DWORD.
The standard way to work with them is to keep the pointer to the actual character string, and to use negative offsets for special fields, like in:

lea ax, [input + 2]  //; standard string, could need a trailing '\0'
mov al, BYTE PTR [input+2 - 1] //; strlen
mov al, BYTE PTR [input+2 - 2] //; allocated buff size