Screen Compress

SCREEN COMPRESS


by Luca Rasca





The only listing I received was this assembly listing, so rather than


work it out as a hex dump (argh! all those swirling bytes!), I am


setting this month's challenge to improve the screen packing of this


routine. You can write your own one from scratch if you want. Send


these and any other brilliant programs and routines to: Program


Pitstop, Your Sinclair, 14 Rathbone Place, London W1P 1DE. That's it!


See you soon!





Screen Compress


Here is a fairly simple screen compression sub-routine from Luca


Rasca. It scans through your screen and stores a compressed version at


50000. It does this by taking note of all the zeros and recording how


many it finds. Since it scans across the screen it doesn't find 255s


which are frequently as common as zeros. But here it is:





Assembly Listing


0000 00000 ;Screen Compress


0000 00000 ;by Luca Rasca


0000 00000 ;from Your Sinclair, Feb'88


0000 00000 ;


0000 00000 ;I added the FINISH step to


0000 00000 ;load BC with the end address,


0000 00000 ;so that USR 65200-50000 will


0000 00000 ;give the compressed screen


0000 00000 ;length. [JimG]


0000 00000 org 65200


FEB0 210040 00000 COMP ld hl,16384


FEB3 1150C3 00000 ld de,50000


FEB6 7E 00000 LOOP1 ld a,(hl)


FEB7 A7 00000 and a


FEB8 12 00000 ld (de),a


FEB9 280A 00000 jr z,ZERO


FEBB 23 00000 inc hl


FEBC 13 00000 CONT inc de


FEBD 3E5B 00000 ld a,91


FEBF BC 00000 cp h


FEC0 20F4 00000 jr nz,LOOP1


FEC2 42 00000 FINISH ld b,d


FEC3 4B 00000 ld c,e


FEC4 C9 00000 ret


FEC5 4F 00000 ZERO ld c,a


FEC6 13 00000 inc de


FEC7 23 00000 LOOP2 inc hl


FEC8 3E5B 00000 ld a,91


FECA BC 00000 cp h


FECB 79 00000 ld a,c


FECC 12 00000 ld (de),a


FECD 28F3 00000 jr z,FINISH


FECF AF 00000 xor a


FED0 BE 00000 cp (hl)


FED1 20E9 00000 jr nz,CONT


FED3 0C 00000 inc c


FED4 20F1 00000 jr nz,LOOP2


FED6 18E4 00000 jr CONT


FED8 00000 ;


FED8 00000 org 65248


FEE0 F3 00000 EXPAND di


FEE1 110040 00000 ld de,16384


FEE4 2150C3 00000 ld hl,50000


FEE7 7E 00000 LOOP3 ld a,(hl)


FEE8 A7 00000 and a


FEE9 12 00000 ld (de),a


FEEA 2009 00000 jr nz,RICONT


FEEC 23 00000 inc hl


FEED 46 00000 ld b,(hl)


FEEE B8 00000 cp b


FEEF 2804 00000 jr z,RICONT


FEF1 13 00000 LOOP4 inc de


FEF2 12 00000 ld (de),a


FEF3 10FC 00000 djnz LOOP4


FEF5 13 00000 RICONT inc de


FEF6 23 00000 inc hl


FEF7 3E5B 00000 ld a,91


FEF9 BA 00000 cp d


FEFA 20EB 00000 jr nz,LOOP3


FEFC FB 00000 ei


FEFD C9 00000 ret