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lovebyte: sierzoom: more comments

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Vince Weaver 2021-03-25 19:14:55 -04:00
parent fe720a30fa
commit d9fd1e96bc
1 changed files with 22 additions and 25 deletions
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47
demos/lovebyte2021/sierzoom/sierzoom.s
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@ -1,4 +1,5 @@
; 128 byte sierpinski-like rotozoomer intro ; 122 byte sierpinski-like rotozoomer intro
; for Apple II lo-res 40x48 mode
; based on the code from Hellmood's Memories demo ; based on the code from Hellmood's Memories demo
; by Vince `deater` Weaver <vince@deater.net> ; by Vince `deater` Weaver <vince@deater.net>
@ -92,8 +93,8 @@ sier:
bit LORES ; drop down to lo-res bit LORES ; drop down to lo-res
; lda #0 ; start with multiplier 0 ; lda #0 ; start with multiplier 0
; sta T_L ; sta T_L ; since we are executing in zero page we
; sta T_H ; sta T_H ; don't need to init these
sier_outer: sier_outer:
@ -105,8 +106,8 @@ sier_outer:
sier_yloop: sier_yloop:
; calc YY_T (8.8 fixed point add) ; calc YY_T (8.8 fixed point add)
; save space by skipping clc as it's only a slight variation w/o ; clc ; save byte by ignoring carry (slightly
; clc ; changes results but not by much)
lda YY_TL lda YY_TL
adc T_L adc T_L
sta YY_TL sta YY_TL
@ -114,21 +115,15 @@ sier_yloop:
adc T_H adc T_H
sta YY_TH sta YY_TH
txa ; YY ; plot call needs Y/2 txa ; YY ; plot call needs Y/2
lsr lsr
php php ; save the bit we shifted into carry
; bcc even_mask jsr GBASCALC ; take Y-coord/2 in A, put address in GBASL/H
; ldy #$f0 ; ( A trashed, C clear)
; .byte $2C ; bit hack
;even_mask:
; ldy #$0f
; sty MASK
jsr GBASCALC ; take Y-coord/2 in A, put address in GBASL/H ( a trashed, C clear) lda GBASH ; adjust GBASH for draw page
lda GBASH
draw_page_smc: draw_page_smc:
adc #0 adc #0
sta GBASH ; adjust for PAGE1/PAGE2 ($400/$800) sta GBASH ; adjust for PAGE1/PAGE2 ($400/$800)
@ -150,15 +145,16 @@ sier_xloop:
; SAVED = XX+(Y*T) ; SAVED = XX+(Y*T)
; clc ; clc ; skip setting carry to save byte
tya ; XX
adc YY_TH
sta SAVED
tya ; XX lives in Y
adc YY_TH
sta SAVED ; save XX+YY_T
; calc XX*T ; calc XX*T
; clc ; clc ; skip setting carry to save byte
lda XX_TL
lda XX_TL ; XX_T=XX_T+T
adc T_L adc T_L
sta XX_TL sta XX_TL
lda XX_TH lda XX_TH
@ -167,7 +163,7 @@ sier_xloop:
; calc (YY-X_T) ; calc (YY-X_T)
txa ; lda YY txa ; YY lives in X
sec sec
sbc XX_TH sbc XX_TH
@ -207,7 +203,7 @@ blah_smc:
inc T_H inc T_H
no_carry: no_carry:
; speed up the zoom as it goes ; speed up the zoom as time goes on
inc blah_smc+1 inc blah_smc+1
@ -223,7 +219,8 @@ done_page:
eor #$4 ; flip draw page between $400/$800 eor #$4 ; flip draw page between $400/$800
sta draw_page_smc+1 ; DRAW_PAGE sta draw_page_smc+1 ; DRAW_PAGE
jmp sier_outer ; what can we branch on? jmp sier_outer ; can't guarantee a flag value for a 2-byte
; branch :(
T_L: .byte $00 T_L: .byte $00
T_H: .byte $00 T_H: .byte $00