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.
Chastity's Chess Challenge 