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.