Difference between revisions of "MELT.COM"
From SizeCoding
| Line 11: | Line 11: | ||
mov es, ax ; es now points to screen segment | mov es, ax ; es now points to screen segment | ||
| − | doScreen: | + | doScreen: |
mov cx, 2000 ; Going to loop over all 2000 characters | mov cx, 2000 ; Going to loop over all 2000 characters | ||
; (80 * 25 = 2000) | ; (80 * 25 = 2000) | ||
| Line 18: | Line 18: | ||
mov di, bx ; es:di now points at the screen (b800:0000) | mov di, bx ; es:di now points at the screen (b800:0000) | ||
| − | alterChars: | + | alterChars: |
mov ah, es:[di] ; Retreive onscreen character | mov ah, es:[di] ; Retreive onscreen character | ||
cmp ah, 32 ; comp to a space character (#32) | cmp ah, 32 ; comp to a space character (#32) | ||
| Line 29: | Line 29: | ||
; --------------------------------------------------------------------------- | ; --------------------------------------------------------------------------- | ||
| − | upToSpace: | + | upToSpace: |
inc ah ; Increase character upwards towards a space | inc ah ; Increase character upwards towards a space | ||
mov es:[di], ah ; Store altered character back to screen | mov es:[di], ah ; Store altered character back to screen | ||
inc bx ; increase "number of processed chars" counter | inc bx ; increase "number of processed chars" counter | ||
| − | nextChar: | + | nextChar: |
inc di | inc di | ||
inc di ; es:di now points to next character | inc di ; es:di now points to next character | ||
| Line 42: | Line 42: | ||
mov ah, 4Ch ; Otherwise, get ready to terminate | mov ah, 4Ch ; Otherwise, get ready to terminate | ||
int 21h ; DOS - 2+ - QUIT WITH EXIT CODE (EXIT) | int 21h ; DOS - 2+ - QUIT WITH EXIT CODE (EXIT) | ||
| − | |||
</syntaxhighlight> | </syntaxhighlight> | ||
Revision as of 03:12, 7 August 2016
MELT.COM was written by an unknown author in the 1980s. Originally 49 bytes in size, it performs the following cute effect:
The original source is lost to history, but here's a quick commented disassembly:
org 100h
mov ax, 0B800h
mov es, ax ; es now points to screen segment
doScreen:
mov cx, 2000 ; Going to loop over all 2000 characters
; (80 * 25 = 2000)
xor bx, bx ; bx = 0
; bx is also our "num of altered chars" counter
mov di, bx ; es:di now points at the screen (b800:0000)
alterChars:
mov ah, es:[di] ; Retreive onscreen character
cmp ah, 32 ; comp to a space character (#32)
jz short nextChar ; If already a space, do nothing
jl short upToSpace ; If lower than a space, increase upward
dec ah ; If higher than a space, decrease downward
mov es:[di], ah ; Store altered character back to screen
inc bx ; increase "number of processed chars" counter
jmp short nextChar ; Keep processing characters
; ---------------------------------------------------------------------------
upToSpace:
inc ah ; Increase character upwards towards a space
mov es:[di], ah ; Store altered character back to screen
inc bx ; increase "number of processed chars" counter
nextChar:
inc di
inc di ; es:di now points to next character
loop alterChars ; Continue processing characters
cmp bx, 0 ; Were any characters processed?
jnz short doScreen ; If so (bx != 0), keep processing
mov ah, 4Ch ; Otherwise, get ready to terminate
int 21h ; DOS - 2+ - QUIT WITH EXIT CODE (EXIT)
