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sierpinski: add comments

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This commit is contained in:
Vince Weaver 2021-01-31 16:17:09 -05:00
parent 2728bfc23a
commit 4686edff2d
1 changed files with 31 additions and 37 deletions
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68
graphics/gr/sier/sier.s
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@ -1,20 +1,19 @@
; fake sierpinski
; sierpinski-like demo
; based on the code from Hellmood's Memories demo
; 143 bytes -- sorta working
; 140 bytes -- enough for appleiibot plus {B11} directive
; to allow for decompression time
; the simple sierpinski you more or less just plot
; X AND Y
; x=0..39
; T =0 XX_T = 0 .. 0
; T =1 XX_T = 0 .. 39 ($0027)
; T =2 XX_T = 0 .. 78 ($004E)
; T =3 XX_T = 0 .. 117 ($0075)
; T = 128 XX_T = 0 .. 4992 ($1380)
; T = 255 XX_T = 0 .. 9945 ($26D9)
; Hellmood's you plot something more or less like
; COLOR = ( (Y-(X*T)) & (X+(Y*T) ) & 0xf0
; where T is an incrementing frame value
; to get speed on 6502/Apple II we change the multiplies to
; a series of 16-bit 8.8 fixed point adds
; just plot X AND Y
;.include "zp.inc"
.include "hardware.inc"
GBASH = $27
@ -45,20 +44,14 @@ sier:
sier_outer:
ldx #0 ; YY
ldx #0 ; YY starts at 0
stx YY_TL
stx YY_TH
sier_yloop:
; reset XX to 0
; ldy #0 ; XX
; sty XX_TL
; sty XX_TH
; calc YY_T (8.8 fixed point add)
; save space by skipping clc as it's only a slight variation w/o
; clc
lda YY_TL
adc T_L
@ -67,7 +60,7 @@ sier_yloop:
adc T_H
sta YY_TH
txa ; YY
txa ; YY ; plot call needs Y/2
lsr
bcc even_mask
@ -77,12 +70,14 @@ even_mask:
ldy #$0f
sty MASK
; txa ; YY
; lsr
jsr GBASCALC ; take Y-coord/2 in A, put address in GBASL/H ( a trashed, C clear)
lda GBASH
draw_page_smc:
adc #0
sta GBASH ; adjust for PAGE1/PAGE2 ($400/$800)
; reset XX to 0
@ -91,11 +86,6 @@ even_mask:
sty XX_TH
draw_page_smc:
adc #0
sta GBASH
sier_xloop:
; want (YY-(XX*T)) & (XX+(YY*T)
@ -129,25 +119,23 @@ sier_xloop:
and #$f0
beq red
beq green
black:
lda #00 ; black
.byte $2C ; bit trick
red:
lda #$CC ; red
green:
lda #$CC ; green
sta COLOR
; ldy XX
; XX value already in Y
jsr PLOT1 ; PLOT AT (GBASL),Y
iny ; XX
iny ; XX
cpy #40
bne sier_xloop
inx
inx ; YY
cpx #48
bne sier_yloop
@ -161,7 +149,7 @@ blah_smc:
adc #0
sta T_H
; speed up the zoom
; speed up the zoom as it goes
inc blah_smc+1
flip_pages:
@ -178,6 +166,12 @@ done_page:
jmp sier_outer ; just slightly too far???
; for maximum twitter size we enter this program
; by using the "&" operator which jumps to $3F5
; we can't load there though as the code would end up overlapping
; $400 which is the graphics area
; this is at 389
; we want to be at 3F5, so load program at 36C?
jmp sier ; entry point from &