2018-08-13 22:17:36 +00:00
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2016-12-16 01:21:51 +00:00
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processor 6502
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include "vcs.h"
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include "macro.h"
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include "xmacro.h"
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seg.u Variables
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org $80
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Temp .byte
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X1 .byte ; start X coordinate of line
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Y1 .byte ; start Y coordinate of line
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Y2 .byte ; end Y coordinate of line
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Slope .word ; 16-bit slope
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XFrac .byte ; X fractional
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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seg Code
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org $f000
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Start
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CLEAN_START
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lda #80
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sta X1
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lda #80
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sta Y1
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lda #120
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sta Y2
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lda #0
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sta Slope
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lda #0
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sta Slope+1
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NextFrame
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VERTICAL_SYNC
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TIMER_SETUP 37
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; Set missile 0 X start position
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lda X1 ; starting X
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ldx #2 ; missile 0
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jsr SetHorizPos
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sta WSYNC
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sta HMOVE
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TIMER_WAIT
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TIMER_SETUP 192
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lda #$54
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sta COLUP0
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ldy #0
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sty XFrac
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ScanLoop
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cpy Y1
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bcc NoLine ; out of bounds (< Y1)?
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cpy Y2
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bcs NoLine ; out of bounds (> Y2)?
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; Add slope to X fractional error
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lda XFrac
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clc
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adc Slope ; this sets carry flag
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sta XFrac
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lda Slope+1
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adc #7 ; 7 + carry flag
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tax ; -> X
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lda DotWidths,x ; lookup register for missile width
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sta Temp ; -> Temp
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lda HMoveTable,x ; lookup HMOVE register for X offset
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sta WSYNC
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sta HMOVE ; apply moves on previous scanline
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ldx #2
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stx ENAM0 ; enable missile
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ldx Temp
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stx NUSIZ0 ; set missile width
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sta HMM0 ; set HMM0 for next scanline
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NextScan
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iny
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cpy #192
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bcc ScanLoop ; any more scanlines?
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beq DoneLine ; branch always taken
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NoLine
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sta WSYNC
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lda #0
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sta ENAM0 ; hide missile
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jmp NextScan
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DoneLine
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TIMER_SETUP 30
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; Change slope of line
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lda Slope
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clc
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adc #3
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sta Slope
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lda Slope+1
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adc #0
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cmp #2
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bmi NoLineReset
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lda #$fe
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NoLineReset
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sta Slope+1
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TIMER_WAIT
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jmp NextFrame
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SetHorizPos subroutine
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sta WSYNC ; start a new line
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bit 0 ; waste 3 cycles
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sec ; set carry flag
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DivideLoop
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sbc #15 ; subtract 15
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bcs DivideLoop ; branch until negative
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eor #7 ; calculate fine offset
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asl
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asl
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asl
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asl
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sta RESP0,x ; fix coarse position
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sta HMP0,x ; set fine offset
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rts ; return to caller
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; HMOVE table from -7 to +8
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HMoveTable
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hex 7060504030201000f0e0d0c0b0a09080
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; Table for NUSIZ registers
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DotWidths
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hex 40403030201000000010203030404040
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; Epilogue
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org $fffc
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.word Start
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.word Start
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