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153 lines
3.9 KiB
ArmAsm
153 lines
3.9 KiB
ArmAsm
; Apple ][ Lo-res fire animation, size-optimized to 64 bytes
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; by deater (Vince Weaver) <vince@deater.net>
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; originally based on code described here
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; http://fabiensanglard.net/doom_fire_psx/
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; but this is a slightly different algorithm (but still cool looking)
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; Optimization history:
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; 80 bytes -- the size of tire_tiny.s
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; 64 bytes -- use the firmware SCROLL routine to handle the scrolling
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; this adds some flicker but saves many bytes
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; 63 bytes -- qkumba noted that SCROLL leaves BAS2L:BAS2H pointing at $7d0
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; 66 bytes -- tried on real hardware. Found that the original firmware
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; never sets MIXCLR on boot, and my II+ machine was leaving
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; it off so for correctness need to add it on.
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; 66 bytes -- wasted a lot of time trying to either find clever ways to
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; get $C050/C052 accessed, or to shorten the lookup table
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; in half by lsr. Nothing panned out.
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; 64 bytes -- realized I could save/restore the random SEEDL on the stack!
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; doesn't seem to affect the output pattern either.
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; Zero Page
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WNDTOP = $22
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WNDBTM = $23
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CV = $25
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BASL = $28
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BASH = $29
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BAS2L = $2A
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BAS2H = $2B
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SEEDL = $4E
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; Soft Switches
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SET_GR = $C050 ; Enable graphics
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MIXCLR = $C052 ; Full screen, no text at bottom
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LORES = $C056 ; Enable LORES graphics
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; Monitor ROM routines. Try to use well-known entry points as
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; these did sometimes change with newer models
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SCROLL = $FC70
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VTAB = $FC22 ; takes row in CV, Result is in BASL:BASH ($28/$29)
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VTABZ = $FC24 ; VTABZ variant takes row in Accumulator
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fire_demo:
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; Set lores graphics
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bit SET_GR ; force graphics mode ; 3
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; LORES and LOWSCR (lo-res page1)
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; are set by the boot rom
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bit MIXCLR ; want full-screen graphics w/o mixed ; 3
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; text the boot process doesn't set this
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; (it doesn't matter in text mode)
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; and while IIe and emulators come up clear
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; my Apple II+ doesn't
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; Set window. This seems to be set properly at boot though
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; lda #0
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; sta WNDTOP
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; lda #24
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; sta WNDBTM
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scroll_loop:
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jsr SCROLL ; scrolls screen up one row ; 3
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;======================================
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; re-draw bottom line (row 23) as white
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; Y is left at 40 after SCROLL
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; also BAS2L:BAS2H is left at $7d0
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; this code over-writes $7F8 (the MSLOT screen hole value)
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; but this should not matter for this demo
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lda #$ff ; top/bottom white ; 2
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w_loop:
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sta (BAS2L),Y ; hline 23 (46+47) ; 2
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dey ; 1
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bpl w_loop ; 2
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;============
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; 8
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fire_loop:
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lda #22 ; start at line 22 ; 2
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sta CV ; store in Row location ; 2
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yloop:
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; puts address of row CV into BASL:BASH
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jsr VTAB ; 3
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; loop for X values 39 ... 0
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ldy #39 ; 2
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xloop:
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;=============================
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; random8
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;=============================
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; 8-bit 6502 Random Number Generator
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; Linear feedback shift register PRNG by White Flame
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; http://codebase64.org/doku.php?id=base:small_fast_8-bit_prng
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random8:
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pla
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; lda SEEDL ; 2
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beq doEor ; 2
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asl ; 1
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beq noEor ; if the input was $80, skip the EOR ; 2
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bcc noEor ; 2
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doEor: eor #$1d ; 2
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noEor: ;sta SEEDL ; 2
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pha
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; end inlined RNG
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; Randomly either use current value, or decay by one
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bmi no_change ; assume bit 7 is as random as bit 0 ; 2
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; Lookup the new color in table
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lda (BASL),Y ; load value ; 2
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and #$7 ; mask off ; 2
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tax ; 1
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lda <(color_progression),X ; 2
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sta (BASL),Y ; 2
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no_change:
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dey ; 1
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bpl xloop ; 2
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dec CV ; 2
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bpl yloop ; 2
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bmi scroll_loop ; 2
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color_progression:
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.byte $00 ; 8->0 ; 1000 -> 0000 ; needed
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.byte $bb ; 9->11 ; 1001 -> 1011
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.byte $00 ; 10->0 ; 1010 -> 0000 ; needed
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.byte $aa ; 11->10 ; 1011 -> 1010
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.byte $00 ; 12->0 ; 1100 -> 0000 ; don't care
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.byte $99 ; 13->9 ; 1101 -> 1001
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.byte $00 ; 14->0 ; 1110 -> 0000 ; don't care
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.byte $dd ; 15->13 ; 1111 -> 1101
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