dos33fsprogs/fire/fire_tiny.s

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; Lo-res fire animation, size-optimized
; by deater (Vince Weaver) <vince@deater.net>
; based on code described here http://fabiensanglard.net/doom_fire_psx/
; 611 bytes at first
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; 601 bytes -- strip out some unused code
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; 592 bytes -- don't init screen
; 443 bytes -- remove more dead code
; 206 bytes -- no need to clear screen
; 193 bytes -- un-cycle exact the random16 code
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; 189 bytes -- more optimization of random16 code
; 161 bytes -- move to 8-bit RNG
; 152 bytes -- reduce lookup to top half colors (had to remove brown)
; also changed maroon to pink
; 149 bytes -- use monitor GR
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; 149 bytes -- load into zero page
; 140 bytes -- start using zero-page addressing
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; 139 bytes -- rotate instead of mask for low bit
; 138 bytes -- bcs instead of jmp
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; 137 bytes -- BIT nop trick to get rid of jump
; 135 bytes -- push/pull instead of saving to zero page
; 134 bytes -- replace half of lookup table with math
; 119 bytes -- replace that half of lookup table with better math
; 134 bytes -- replace other half of lookup table with math
; 132 bytes -- replace stack with zp loads
; 129 bytes -- use Y instead of X
; 125 bytes -- optimize low byte generationw with branch
; 113 bytes -- make get_row a subroutine
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; 112 bytes -- regrettable change to the low-byte code
; 109 bytes -- replace BIT/OR in low calc with an ADC
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; 107 bytes -- replace self-modifying load/store absolute with Y-indirect
; 106 bytes -- assume bit 8 is as random as bit 0
; 105 bytes -- qkumba points out that GR leaves BASL:BASH pointing at line 23
; so we can use Y-indirect of BASL to draw the bottom white line
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; Zero Page
SEEDL = $4E
TEMP = $00
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TEMPY = $01
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OUTL = $02
OUTH = $03
INL = $04
INH = $05
BASL = $28
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; 100 = $64
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; Soft Switches
SET_GR = $C050 ; Enable graphics
FULLGR = $C052 ; Full screen, no text
LORES = $C056 ; Enable LORES graphics
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; monitor routines
ASOFT_GR = $F390
MON_SETGR = $FB40
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fire_demo:
; GR part
jsr MON_SETGR ; 3
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bit FULLGR ; 3
;==========
; 6
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; Setup white line on bottom
; GR leaves BASL pointing at hline 23
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lda #$ff ; 2
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ldy #39 ; 2
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white_loop:
sta (BASL),Y ; hline 23 (46+47) ; 3
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dey ; 1
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bpl white_loop ; 2
;============
; 10
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fire_loop:
ldx #22 ; 22 ; 2
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yloop:
stx TEMPY ; txa/pha not any better ; 2
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; setup the load/store addresses
; using Y-indirect is smaller than self-modifying code
jsr get_row ; 3
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sty OUTL ; 2
sta OUTH ; 2
inx ; 1
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jsr get_row ; 3
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sty INL ; 2
sta INH ; 2
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ldy #39 ; 2
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xloop:
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;=============================
; random8
;=============================
; 8-bit 6502 Random Number Generator
; Linear feedback shift register PRNG by White Flame
; http://codebase64.org/doku.php?id=base:small_fast_8-bit_prng
random8:
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lda SEEDL ; 2
beq doEor ; 2
asl ; 1
beq noEor ; if the input was $80, skip the EOR ; 2
bcc noEor ; 2
doEor: eor #$1d ; 2
noEor: sta SEEDL ; 2
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; end inlined RNG
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bmi no_change ; assume bit 8 is as random as bit 0 ; 2
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lda (INL),Y ; load value at row+1 ; 2
and #$7 ; mask off ; 2
tax ; 1
lda <(color_progression),X ; 2
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.byte $2c ; BIT trick, nops out next instruction ; 1
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no_change:
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lda (INL),Y ; load value at row+1 ; 2
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smc_sta:
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sta (OUTL),Y ; 2
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dey ; 1
bpl xloop ; 2
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ldx TEMPY ; 2
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dex ; 1
bpl yloop ; 2
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bmi fire_loop ; 2
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color_progression:
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.byte $00 ; 8->0 ; 1000 0000
.byte $bb ; 9->11 ; 1001 1011
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.byte $00 ; 10->0 ; 1010 0000
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.byte $aa ; 11->10 ; 1011 1010
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.byte $00 ; 12->0 ; 1100 0000
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.byte $99 ; 13->9 ; 1101 1001
.byte $00 ; 14->0 ; 1110 0000
.byte $dd ; 15->13 ; 1111 1101
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; Take row X and convert to address A:Y
get_row:
; get low byte
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; 000X X00O
; lsr
; 0000 XX00 O
; adc #1
; 0000 XXOx
; lsr
; 0000 0XXO
; lsr
; 0000 00XX O
; ror
txa ; 1
and #$19 ; 2
lsr ; 1
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adc #1 ; 2
lsr ; 1
lsr ; 1
tay ; 1
lda <low_offsets,Y ; 2
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ror ; 1
tay ; 1
;===========
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; 13
; high byte of store address
txa ; 1
and #$7 ; 2
lsr ; 1
ora #$4 ; 2
;===========
; 6
rts ; 1
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; 14+6 = 20, as opposed to full lookup table which would be
; at least 24
; down to 13+3 at the expense of readability
; these are shifted left by one due to the algorithm
low_offsets:
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.byte $00,$50,$a0