Difference between revisions of "MELT.COM"
From SizeCoding
| Line 6: | Line 6: | ||
<syntaxhighlight lang=nasm> | <syntaxhighlight lang=nasm> | ||
| − | ; | + | org 100h |
| + | |||
| + | mov ax, 0B800h | ||
| + | mov es, ax ; es now points to screen segment | ||
| + | |||
| + | doScreen: ; CODE XREF: start+2B�j | ||
| + | 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: ; CODE XREF: start+26�j | ||
| + | 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: ; CODE XREF: start+14�j | ||
| + | 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: ; CODE XREF: start+12�j start+1C�j | ||
| + | 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) | ||
| + | |||
</syntaxhighlight> | </syntaxhighlight> | ||
Revision as of 02:12, 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: ; CODE XREF: start+2B�j
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: ; CODE XREF: start+26�j
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: ; CODE XREF: start+14�j
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: ; CODE XREF: start+12�j start+1C�j
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)
