mirror of
https://github.com/sehugg/8bitworkshop.git
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141 lines
3.2 KiB
Plaintext
141 lines
3.2 KiB
Plaintext
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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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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;
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; We're going to set the player's coarse and fine position
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; at the same time using a clever method.
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; We divide the X coordinate by 15, in a loop that itself
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; is 15 cycles long. When the loop exits, we are at
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; the correct coarse position, and we set RESP0.
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; The accumulator holds the remainder, which we convert
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; into the fine position for the HMP0 register.
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; This logic is in a subroutine called SetHorizPos.
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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SpriteHeight equ 8
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Variables segment
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seg.u Variables
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org $80
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XPos .byte
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YPos .byte
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Code segment
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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 #5
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sta YPos
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sta XPos
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NextFrame
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lsr SWCHB ; test Game Reset switch
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bcc Start ; reset?
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; 1 + 3 lines of VSYNC
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VERTICAL_SYNC
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; 35 lines of underscan
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ldx #35
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LVBlank sta WSYNC
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dex
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bne LVBlank
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; animate X and Y coordinates
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; NOTE: when X is too close to the right side,
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; we risk using an extra scanline
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inc XPos
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inc YPos
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; the next two scanlines
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; position the player horizontally
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lda XPos ; get X coordinate
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ldx #0 ; player 0
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jsr SetHorizPos ; set coarse offset
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sta WSYNC ; sync w/ scanline
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sta HMOVE ; apply fine offsets
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; 192 lines of frame
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ldx #192 ; X = 192 scanlines
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LVScan
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txa ; X -> A
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sec ; set carry for subtract
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sbc YPos ; local coordinate
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cmp #SpriteHeight ; in sprite?
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bcc InSprite ; yes, skip over next
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lda #0 ; not in sprite, load 0
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InSprite
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tay ; local coord -> Y
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lda Frame0,y ; lookup color
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sta WSYNC ; sync w/ scanline
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sta GRP0 ; store bitmap
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lda ColorFrame0,y ; lookup color
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sta COLUP0 ; store color
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dex ; decrement X
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bne LVScan ; repeat until 192 lines
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; 29 lines of overscan
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ldx #29
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LVOver sta WSYNC
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dex
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bne LVOver
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; total = 262 lines, go to next frame
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jmp NextFrame
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; SetHorizPos routine
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; A = X coordinate
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; X = player number (0 or 1)
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SetHorizPos
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sta WSYNC ; start a new line
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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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; Cat-head graphics data
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Frame0
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.byte #0 ; zero padding, also clears register
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.byte #%00111100
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.byte #%01000010
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.byte #%11100111
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.byte #%11111111
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.byte #%10011001
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.byte #%01111110
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.byte #%11000011
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.byte #%10000001
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; Cat-head color data
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ColorFrame0
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.byte #0 ; unused (for now)
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.byte #$AE
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.byte #$AC
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.byte #$A8
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.byte #$AC
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.byte #$8E
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.byte #$8E
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.byte #$98
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.byte #$94
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Epilogue
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org $fffc
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.word Start ; reset vector
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.word Start ; BRK vector
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