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
• There is inefficient loading and saving of screen characters with manually advancing DI twice -- 80x86 has 1-byte string instructions to do that, so we'll use LODSW and STOSW. This will load the character attribute byte redundantly, but we don't care about speed, we care about size.

This shaves 10 bytes down to 39 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.

This gets the code down to 31 bytes:

                org 100h

                mov     ah, 0B8h
                mov     es, ax          ; es now points to screen segment
                mov     ds, ax          ; ds = es so we can LODS and STOS to the same place

doScreen:
                mov     ch, 08          ; (80 * 25 = 2000/07d0h)
                xor     bx, bx          ; bx is also our "num of altered chars" counter
                mov     di, bx          ; es:di now points at the screen (b800:0000)
                mov     si, di          ; ds:si = es:di, needed for lods/stos

alterChars:
                lodsw                   ; Retreive onscreen character
                cmp     al, 32          ; comp to a space character (#32)
                jz      short nextChar  ; If already a space, do nothing
                dec     al              ; If higher than a space, decrease downward
                inc     bx              ; increase "number of processed chars" counter

nextChar:
                stosw
                loop    alterChars      ; Continue processing characters
                cmp     bx, 0           ; Were any characters processed?
                jnz     short doScreen  ; If so (bx != 0), keep processing
                ret                     ; exit

Optimizing this further is left as an exercise to the reader. It may be possible to shrink it further using instructions for the 80186 and later processors, but it runs way too quickly on those systems, so if doing so you may want to insert a HLT to pace the animation at 18.2 Hz. It may also be possible to shrink it by taking advantage of the fact that ASCII character #0 is a null and shows nothing onscreen (just like the #32 space character), so instead of decrementing down to #32, you could decrement down to #0 in a more optimized form (like using the zero flag).