192 lines
2.3 KiB
ArmAsm
192 lines
2.3 KiB
ArmAsm
; Parallax
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; by deater (Vince Weaver) <vince@deater.net>
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; Zero Page
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GBASL = $26
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GBASH = $27
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FRAME = $EA
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FRAME2 = $EB
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FRAME4 = $EC
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X2 = $FB
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COLORS = $FC
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PAGE = $FD
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; Soft Switches
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SET_GR = $C050 ; Enable graphics
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FULLGR = $C052 ; Full screen, no text
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PAGE1 = $C054 ; Page1
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PAGE2 = $C055 ; Page2
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LORES = $C056 ; Enable LORES graphics
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; ROM routines
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GBASCALC= $F847 ; take Y-coord/2 in A, put address in GBASL/H ( a trashed, C clear)
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SETGR = $FB40
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WAIT = $FCA8 ;; delay 1/2(26+27A+5A^2) us
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parallax:
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;===================
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; init screen
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jsr SETGR ; lores graphics
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bit FULLGR ; full screen
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lda #$0 ; start on page1
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sta PAGE
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parallax_forever:
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inc FRAME ; increment frame
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lda FRAME ; also have frame/2 and frame/4
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lsr
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sta FRAME2
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lsr
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sta FRAME4
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;==========================
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; flip page
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lda PAGE ; get current page
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pha ; save for later
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lsr ; switch visible page
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lsr
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tay
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lda PAGE1,Y
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pla ; save old draw page
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eor #$4 ; toggle to other draw page
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sta PAGE
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;========================
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; setup for 23 lines
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ldx #23 ; 23 lines (double high)
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yloop:
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;==============
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; point GBASL/GBSAH to current line
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txa ; get line addr in GBASL/H
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jsr GBASCALC
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lda GBASH
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clc
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adc PAGE ; adjust for page
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sta GBASH
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;==============
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; current column (work backwards)
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ldy #39
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xloop:
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lda #0 ; default color black
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sta COLORS
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; calculate colors
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; color = (XX-FRAME)^(YY)
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;===========================
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; SMALL
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sec ; subtract frame from Y
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tya
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sbc FRAME4
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sta X2
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txa
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eor X2
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and #$2
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beq skip_color_small
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lda #$4c
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; lda #$1b
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sta COLORS
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skip_color_small:
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;===========================
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; MEDIUM
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sec ; subtract frame from Y
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tya
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sbc FRAME2
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sta X2
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txa
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clc
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adc #1
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eor X2
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and #$4
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beq skip_color_medium
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lda #$26
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sta COLORS
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skip_color_medium:
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;===========================
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; LARGE
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sec ; subtract frame from Y
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tya
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sbc FRAME
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sta X2
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txa
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clc
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adc #2
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eor X2
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and #$8
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beq skip_color_large
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lda #$1B
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; lda #$4c
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sta COLORS
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skip_color_large:
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;========================
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; actually draw color
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lda COLORS ; store out pixel
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sta (GBASL),Y
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dey ; loop for X-coord
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bpl xloop ;
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dex ; loop for Y-coord
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bpl yloop ;
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bmi parallax_forever ; bra
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; for bot
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; $3F5 - 125 + 3 = $37B
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jmp parallax
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