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; 6502 assembler Sierpinsky Triangle ver.2
; by Magnus Wedmark 2007-05-02
; This program is especially written for
; the 6502asm.com competition and
; uses the 32*32 pixel display used in that
; virtual platform. The sierpinsky
; fractal is one of the simplest to
; implement. Here is a walk-through:
; 1) Specify 3 points that form a triangle
; 2) Choose one of them as a starting point
; 3) Choose one of them as targetpoint randomly
; 4) Set the new current position half-way
; between the current point and the target
; point.
; 5) Goto 3
LDX #0
LDY #0
new_rnd:
LDA $FE ; random 0-255
AND #3 ; only 0-3 left
CMP #3
BNE good_rnd
JMP new_rnd
good_rnd:
; random = 0-2
PHA
; transform X and Y values according to:
; X=X/2+(P*8) and Y=Y/2+(P*16)
ASL
ASL
ASL
STA $F3 ; P*8
PLA
AND #1
ASL
ASL
ASL
ASL
STA $F4 ; (P AND 1)*16
TXA
LSR
ADC $F3
TAX
TYA
LSR
ADC $F4
TAY
JSR set_point ; use and restore regs
JMP new_rnd
set_point: ; uses both X,Y,A and restores them
PHA ; backup all reg-value (X,Y,A)
TXA
PHA
TYA
PHA
PHA
PHA ; triple Y push, two for int. use
STX $F2 ; transfer X to Y using $F2
LDY $F2
LDA #0
STA $F0
LDA #$2
STA $F1 ; set base vector to $200
LDA #0
PLA ; transfer the pushed Y-coord to A
AND #$07 ; the value %0000'0111
ASL
ASL
ASL
ASL
ASL
CLC
ADC $F0
STA $F0
BCC no_carry
INC $F1
no_carry:
CLC
PLA ; transfer the pushed Y-coord to A
AND #$18
LSR
LSR
LSR
ADC $F1
STA $F1
CLC
TYA
ADC $F0
ADC $F1
LDA #1 ;1 = white for trouble-shooting
JSR set_toning_point ; use for shading
STA ($F0),Y ; set pixel
PLA ; restore all reg-value (X,Y,A)
TAY
PLA
TAX
PLA
RTS
; sub routine to shade the current pixel ($F0),Y
; lighter on a scale: $0, $B, $C, $F, $1
; Black, DarkGrey, Grey, LightGrey, White
set_toning_point:
LDA ($F0),Y
CMP #$00
BNE not_black
LDA #$0B
RTS
not_black:
CMP #$0B
BNE not_dgrey
LDA #$0C
RTS
not_dgrey:
CMP #$0C
BNE not_grey
LDA #$0F
RTS
not_grey:
CMP #$0F
BNE not_lgrey
LDA #$01
RTS
not_lgrey:
; white stays white
RTS
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