114 lines
2.4 KiB
ArmAsm
114 lines
2.4 KiB
ArmAsm
; Apple ][ Lo-res rasterbars, size-optimized
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; by deater (Vince Weaver) <vince@deater.net>
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; Zero Page
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WNDTOP = $22
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WNDBTM = $23
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CV = $25
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BASL = $28
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BASH = $29
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BAS2L = $2A
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BAS2H = $2B
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SEEDL = $4E
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; Soft Switches
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SET_GR = $C050 ; Enable graphics
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MIXCLR = $C052 ; Full screen, no text at bottom
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LORES = $C056 ; Enable LORES graphics
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; Monitor ROM routines. Try to use well-known entry points as
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; these did sometimes change with newer models
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SCROLL = $FC70
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VTAB = $FC22 ; takes row in CV, Result is in BASL:BASH ($28/$29)
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VTABZ = $FC24 ; VTABZ variant takes row in Accumulator
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raster_demo:
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; Set lores graphics
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bit SET_GR ; force graphics mode ; 3
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; LORES and LOWSCR (lo-res page1)
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; are set by the boot rom
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bit MIXCLR ; want full-screen graphics w/o mixed ; 3
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; text the boot process doesn't set this
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; (it doesn't matter in text mode)
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; and while IIe and emulators come up clear
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; my Apple II+ doesn't
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; Set window. This seems to be set properly at boot though
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; lda #0
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; sta WNDTOP
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; lda #24
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; sta WNDBTM
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reset_scroll:
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jsr random8
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sta CV
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scroll_loop:
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jsr SCROLL ; scrolls screen up one row ; 3
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;======================================
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; re-draw bottom line (row 23) as white
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; Y is left at 40 after SCROLL
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; also BAS2L:BAS2H is left at $7d0
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; this code over-writes $7F8 (the MSLOT screen hole value)
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; but this should not matter for this demo
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lda CV
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and #7
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tax
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lda <color_progression,X
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; lda #$ff ; top/bottom white ; 2
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w_loop:
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sta (BAS2L),Y ; hline 23 (46+47) ; 2
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dey ; 1
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bpl w_loop ; 2
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;============
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; 8
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no_change:
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dec CV ; 2
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bpl reset_scroll ; 2
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bmi scroll_loop ; 2
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color_progression:
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.byte $00 ; 8->0 ; 1000 -> 0000 ; needed
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.byte $bb ; 9->11 ; 1001 -> 1011
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.byte $00 ; 10->0 ; 1010 -> 0000 ; needed
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.byte $aa ; 11->10 ; 1011 -> 1010
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.byte $00 ; 12->0 ; 1100 -> 0000 ; don't care
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.byte $99 ; 13->9 ; 1101 -> 1001
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.byte $00 ; 14->0 ; 1110 -> 0000 ; don't care
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.byte $dd ; 15->13 ; 1111 -> 1101
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;=============================
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; random8
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;=============================
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; 8-bit 6502 Random Number Generator
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; Linear feedback shift register PRNG by White Flame
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; http://codebase64.org/doku.php?id=base:small_fast_8-bit_prng
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random8:
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lda SEEDL ; 2
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beq doEor ; 2
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asl ; 1
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beq noEor ; if the input was $80, skip the EOR ; 2
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bcc noEor ; 2
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doEor: eor #$1d ; 2
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noEor: sta SEEDL ; 2
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rts
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