Tag: python

DOS Assembly putstring function

This is a small program which uses the putstring function I wrote. This function is one of 4 ultimate functions I have created which make up “chastelib”. DOS programming is simpler than Windows and is not that different from Linux in that system calls are done with an interrupt.

org 100h

main:

mov ax,text
call putstring

mov ax,4C00h
int 21h

text db 'Hello World!',0Dh,0Ah,0

;This section is for the putstring function I wrote.
;It will print any zero terminated string that register ax points to

stdout dw 1 ; variable for standard output so that it can theoretically be redirected

putstring:

push ax
push bx
push cx
push dx

mov bx,ax                  ;copy ax to bx for use as index register

putstring_strlen_start:    ;this loop finds the length of the string as part of the putstring function

cmp [bx], byte 0           ;compare this byte with 0
jz putstring_strlen_end    ;if comparison was zero, jump to loop end because we have found the length
inc bx                     ;increment bx (add 1)
jmp putstring_strlen_start ;jump to the start of the loop and keep trying until we find a zero

putstring_strlen_end:

sub bx,ax                  ; sub ax from bx to get the difference for number of bytes
mov cx,bx                  ; mov bx to cx
mov dx,ax                  ; dx will have address of string to write

mov ah,40h                 ; select DOS function 40h write 
mov bx,[stdout]            ; file handle 1=stdout
int 21h                    ; call the DOS kernel

pop dx
pop cx
pop bx
pop ax

ret

Anyone can assemble and run this source code, but you will need a DOS emulator like DOSBox in order for it to work. In fact, I have a video showing me assembling and running it inside of DOSBox.

Lately I have been having a programming phase and am working on a book about programming in DOS. There is no money involved in this because nobody except nerds like me care about DOS. Speaking of nerds, if you follow my blog, don’t forget that this site was set up for teaching Chess. Leave me a comment if you play Chess online or live in Lee’s Summit. I am still playing Chess every day although some of my time has been taken up with programming in Assembly language because it is so much fun.

If you like this post, you may be interested in my much longer post/book that is all about Assembly programming in DOS.

Assembly Arithmetic Algorithms 16-bit DOS Edition

October 19, 2025

Chastity’s Hex Compare Tool

Welcome to Chastity’s Hex Compare program also known as “chastecmp”. Enter two filenames as command line arguments such as:

./chastecmp file1.txt file2.txt

It works for any binary files too, not just text. In fact for text comparison you want entirely different tools. This tool can be used to find the tiny differences between files in hexadecimal. It shows only those bytes which are different. I wrote it as a solution to a reddit user who asked how to compare two files in hexadecimal.

It is an improvement over the Linux “cmp” tool which displays the offsets in decimal and the bytes in octal. Aside from using two different bases in the data, it falls short of usefulness because there are more hex editors than octal editors.

Here are also some graphical tools I can recommend if you are looking for a GUI instead of my command line program.

vbindiff
wxHexeditor

Below is the full source code:

#include <stdio.h>
#include <stdlib.h>
 
int main(int argc, char *argv[])
{
 int argx,x;
 FILE* fp[3]; /*file pointers*/
 int c1,c2;
 long flength[3]; /*length of the file opened*/
   
 /*printf("argc=%i\n",argc);*/

 if(argc<3)
 {
  printf("Welcome to Chastity's Hex Compare program also known as \"chastecmp\".\n\n");
  printf("Enter two filenames as command line arguments such as:\n");
  printf("%s file1.txt file2.txt\n",argv[0]);
  return 0;
 }

 argx=1;
 while(argx<3)
 {
   fp[argx] = fopen(argv[argx], "rb"); /*Try to open the file.*/
   if(!fp[argx]) /*If the pointer is NULL then this becomes true and the file open has failed!*/
   {
    printf("Error: Cannot open file \"%s\": ",argv[argx]);
    printf("No such file or directory\n");
    return 1;
   }
  /*printf("File \"%s\": opened.\n",argv[argx]);*/

  printf("fp[%X] = fopen(%s, \"rb\");\n",argx,argv[argx]);
  argx++;
 }

 printf("Comparing files %s and %s\n",argv[1],argv[2]);

 argx=1;
 while(argx<3)
 {
  fseek(fp[argx],0,SEEK_END); /*go to end of file*/
  flength[argx]=ftell(fp[argx]); /*get position of the file*/
  printf("length of file fp[%X]=%lX\n",argx,flength[argx]);
  fseek(fp[argx],0,SEEK_SET); /*go back to the beginning*/
  argx++;
 }

 x=0;
 while(x<flength[1])
 {
  c1 = fgetc(fp[1]);
  c2 = fgetc(fp[2]);
  if(c1!=c2)
  {
   printf("%08X: %02X %02X\n",x,c1,c2);
  }
  x++;  
 }

 argx=1;
 while(argx<3)
 {
  fclose(fp[argx]);
  printf("fclose(fp[%X]);\n",argx);
  argx++;
 }

 return 0;
}

October 6, 2025