MELT.COM

MELT.COM was written by an unknown author in the 1980s. Originally 49 bytes in size, it performs the following cute effect:

(The video is simulated and shows how MELT performs on the old hardware it was written for.) This effect is achieved by increasing or decreasing each onscreen character's value until it reaches #32, the space character. 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)

There are some very quick wins right off the bat:

• AX is already 0, so set AX=B800 by setting AH=B8
• Replace DOS exit sequence with RET, since this is a .COM file

This shaves 4 bytes down to 45 total. At this point we can make some drastic changes that will shave bytes, but also make the program not behave exactly as it did before. I chose to do the following:

2000 is 7D0 in hex. Change MOV CX,2000 (decimal) to MOV CH,08 (hex) to shave a byte. This could result in CX being anywhere in the range 0800 to 08FF but the difference is minimal at execution time. It's also larger than the original area, but that is fine since there is there is extra screen RAM after the visible portion of the screen. The code contains a check for characters below #32 (space) and moves them upward. Most characters onscreen are going to be above #32, so this isn't really necessary and I removed the check. Even if it were, removing the check will just rotate them downward until they wrap around to 255, then go downward again until hitting #32 and stopping.