I’ve been working a program in x86 assembly for MSDOS, that opens a file and displays it’s contents..however, i’m stumped on the reading the file part..

Basically, i dynamically allocated filesize amount of bytes using INT 21h, 48h , and i want the program to write the file’s contents into the allocated space…however, the memory address that’s returned by the interrupt is a segment address, and the interrupt responsible for reading the file’s contents exclusively requires it to be inside DS.

.MODEL SMALL
.STACK 100h

READ equ 0
SEEK_END equ 2


.DATA
  file_name db "test.txt",0
  file_handle dw ?
  file_size dw ?
  file_segment dw ?

.CODE

main PROC
  mov ax, @DATA
  mov ds, ax

  ;Return the file's handle.

  mov ah, 3Dh
  mov al, READ
  lea dx, file_name
  int 21h
  mov [file_handle], ax
  mov bx, ax

  ;Get the file's size by going to the end of the file and retrieving new the pointer location.

  mov ah, 42h
  mov al, SEEK_END
  xor dx, dx
  int 21h
  mov [file_size], ax

  ;Calculate the amount of paragraphs necessary for the allocation (1 paragraph = 16 bytes).

  add ax, 15
  mov cl, 4
  shr ax, cl

  ;Allocate the memory.

  mov bx, ax
  mov ah, 48h
  int 21h
  mov [file_segment], ax

  ;Write the file's contents into memory.

  mov ah, 3Fh
  mov bx, [file_handle]
  mov cx, [file_size]
  ;DX is supposed to be the pointer offseted by DS.
  int 21h

  ;Terminate Program.

  mov ah, 4Ch
  int 21h
main ENDP
END main

FYI, i ran Turbo Debugger and there doesn’t seem to any problem with the program, it’s just the reading file that doesn’t work…

This is the website i got my interrupt information from by the way : https://www.stanislavs.org/helppc/int_21.html

I wanted the program to write into the dynamically allocated block of memory, so i tried changing the DS register’s address inside the interrupt, i tried using ASSUME, i tried scouring throught the deepest depths of the Google results, i got so low i asked ChatGPT for help, yet none of it worked..