Screen Effects 2

SCREEN FX 2


by Damian Scattergood





Damian Scattergood, the Z80 Project Leader of Emerald Software,


nominated as Mr. Benevolence for three years in succession, was so


impressed with my screen effects special a few months back (swell)


that he decided to submit a compendium of routines himself. I was so


impressed I printed them.





Lines


The Spectrum screen display is a complicated beast, using all manner


of confusing and downright irritating structural nuances that can make


it a major hurdle for a novice. I won't go into detaiLs but briefly


there are 192 pixel lines, divided into three blocks of 64 lines,


covering memory locations 16384 to 22527; the screen addresses go in


batches of eight so that logically the address you want is never in


the right place. Well, something like that anyway.





These two routines are used to calculate the screen address of the


next scan line up or down. You'll need an assembler to enter them in,


and they'll go in any address.





Next Line Down





;Execute this routine with HL


;pointing to the screen address


;and HL will return with the


;next line down





NXTC ld a,h


and 7


cp 7


jr z,LINEE


inc h


ret


LINEE ld a,l


and 224


cp 224


jr z,NCHAR


ld de,1760


and a


sbc hl,de


ret


NCHAR ld de,32


add hl,de


ret





Next Line Up





LSTC ld a,h


and 7


jr z,SALI


dec h


ret


SALI ld a,l


and 224


jr z,SASE


ld de,1760


and a


adc hl,de


ret


SASE ld de,32


and a


sbc hl,de


ret





Nibbling


The next program is one big demo, split up into five easily digestible


parts. It combines the two previous/next scan-line routines into a


mammoth scrolling demo, shifting windows all over the place, up, down,


left and right! And to boot, it scrolls by four pixels (or "nibbles"


if you want to be confused by jargon). Very useful for faster than


fast scrolls.





The Huge Demo





;This whopper (fnurk!) is the


;demo, using the previous two


;routines





DEMO call FILL


ld b,20


LOOP1 push bc


ld hl,16384+15


ld bc,10c0h


call SLEFT


ld hl,16384+16


ld bc,10c0h


call SRIGHT


pop bc


djnz LOOP1


ld b,20


LOOP2 push bc


ld hl,16384


ld bc,6020h


call SUP4


pop bc


djnz LOOP2


ld b,20


LOOP3 push bc


ld hl,20640


ld bc,6020h


call SDOWN4


pop bc


djnz LOOP3


ret


This is the main stem of the program and addresses sub-routines with


CALL statements (rather like GOSUBs in Basic). The register HL holds


the top right screen address of the window to be scrolled, and BC


holds the height and width of that window.





FILL ld hl,16384


ld bc,6143


LOOP4 ld (hl),l


inc hl


dec bc


ld a,b


or c


jr nz,LOOP4


ret


So there's something there to scroll, this small routine simply fills


the screen with garbage. Not just any old garbage though, strategic garbage.





SLEFT push hl


DEEP push bc


xor a


LOOP5 rld


dec hl


djnz LOOP5


pop bc


pop hl


call NXTC


push hl


dec c


jr nz,DEEP


pop hl


ret


This sub-routine shifts the contents of a window left by four pixels,


using the specific instruction 'RLD'.





SRIGHT push hl


DEEP1 push bc


xor a


LOOP6 rrd


inc hl


djnz LOOP6


pop bc


pop hl


call NXTC


push hl


dec c


jr nz,DEEP1


pop hl


ret


This similar sub-routine shifts the contents of the window right by


four pixels, using the specific instruction 'RRD'.





[This scrolls up, not down. JimG]


SDOWN4 push hl


call LSTC


call LSTC


call LSTC


call LSTC


ex de,hl


pop hl


ex de,hl


LOOP7 push bc


push de


push hl


ld b,0


ldir


pop hl


call LSTC


pop de


ex de,hl


push de


call LSTC


pop de


ex de,hl


pop bc


djnz LOOP7


ret


The down scroll routine works in a different way. It calculates the


address four bytes below and block moves everything in the window to


that address, creating a "scrolling" illusion.





[This scrolls down, not up. JimG]


SUP4 push hl


call NXTC


call NXTC


call NXTC


call NXTC


ex de,hl


pop hl


ex de,hl


LOOP8 push bc


push de


push hl


ld b,0


ldir


pop hl


call NXTC


pop de


ex de,hl


push de


call NXTC


pop de


ex de,hl


pop bc


djnz LOOP8


ret


This up scroll routine is almost identical as the previous one, but


calculates the address four bytes above instead.