mirror of
https://github.com/sehugg/8bitworkshop.git
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275 lines
6.2 KiB
Plaintext
275 lines
6.2 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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; Given all the heavy lifting it requires to put a single
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; sprite on the screen, we'd like to abstract away the process
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; of putting multiple sprites on the screen so we can actually
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; make a game. This is called the "kernel" because it is
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; particularly timing-sensitive and the CPU will spend a lot
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; of time in that loop.
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; We'll start with drawing two overlapping sprites with full
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; resolution (1 scanline per sprite line) and full color
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; (1 color per scanline). To do this our kernel (DrawSprites)
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; will have to update four TIA registers after each WSYNC,
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; pulling from four lookup tables, and handling edge cases
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; where sprites overlap partially in the Y-dimension.
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; This will use up almost an entire scanline of CPU time.
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; Right now we'll just hard-code the horizontal position
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; of each sprite, limiting ourselves to two sprites.
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seg.u Variables
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org $80
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Counter byte ; increments each frame
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Scanline byte ; scanline to draw next
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PData0 word ; pointer (lo/hi) to player 0 bitmap data
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PData1 word ; pointer to player 1 bitmap data
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PColr0 word ; pointer to player 0 color data
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PColr1 word ; pointer to player 1 color data
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SIndx0 byte ; index into player data (0 = inactive)
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SIndx1 byte ; index into player data (0 = inactive)
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SSize0 byte ; sprite size for each player
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SSize1 byte ; sprite size for each player
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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seg Code
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org $f000
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; Initialize and set initial offsets of objects.
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Start CLEAN_START
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; Hard-code the horizontal player positions.
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lda #70
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ldx #0
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jsr SetHorizPos ; set player 0 horiz. pos
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lda #73
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ldx #1
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jsr SetHorizPos ; set player 1 horiz. pos
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sta WSYNC
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sta HMOVE
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; Next frame loop
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NextFrame
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TIMER_SETUP 37
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VERTICAL_SYNC
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; Set some colors
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lda #$80
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sta COLUBK ; set the background color
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; Setup the two sprites
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lda #50
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sta SIndx0
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lda Counter ; changes every frame
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sta SIndx1
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lda #17
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sta SSize0
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sta SSize1
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lda #<Frame0
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sta PData0
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lda #>Frame0
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sta PData0+1
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lda #<ColorFrame0
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sta PColr0
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lda #>ColorFrame0
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sta PColr0+1
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lda #<Frame1
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sta PData1
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lda #>Frame1
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sta PData1+1
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lda #<ColorFrame0
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sta PColr1
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lda #>ColorFrame0
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sta PColr1+1
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TIMER_WAIT
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; Scanline loop
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lda #0
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sta Scanline
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NextScanline
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jsr DrawSprites
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inc Scanline
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; WSYNC and then clear sprite data
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sta WSYNC
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lda #0
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stx GRP0
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stx GRP1
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; repeat until all scanlines drawn
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lda Scanline
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cmp #192
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bcc NextScanline
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; Clear all colors to black before overscan
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stx COLUBK
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stx COLUP0
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stx COLUP1
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stx COLUPF
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; 30 lines of overscan
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TIMER_SETUP 30
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TIMER_WAIT
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; Jump to next frame
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inc Counter
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jmp NextFrame
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; Here is our sprite kernel.
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; Inputs:
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; ssize0, ssize1 (size in lines of each sprite)
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; sindx0, sindx1 (# scanlines to bottom of sprites)
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; pdata0, pdata1 (ptr to sprite bitmaps)
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; pcolr0, pcolr0 (ptr to sprite color tables)
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DrawSprites
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; Find out the maximum # of lines to draw
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; by taking the maximum of the two sprite heights
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lda SIndx0
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cmp SIndx1
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bpl Cmp1 ; sindx0 < sindx1
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lda SIndx1
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Cmp1 tax ; X = # of lines left to draw
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cmp #0 ; no lines?
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beq NoSprites
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; add total draw height to scanline count
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clc
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adc Scanline
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sta Scanline
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DrawNextScanline
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; Fetch next pixels/colors for sprite 0
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ldy SIndx0
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cpy SSize0
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bcs Inactive0 ; index >= size? (unsigned)
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lda (PData0),y
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; Do WSYNC and then quickly store pixels/colors for player 0
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sta WSYNC
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sta GRP0
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lda (PColr0),y
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sta COLUP0
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DrawSprite1
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; Fetch next pixels/colors for sprite 1
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ldy SIndx0+1
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cpy SSize0+1
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bcs Inactive1 ; index >= size? (unsigned)
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lda (PData1),y
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sta GRP1
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lda (PColr1),y
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sta COLUP1
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Inactive1
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; Decrement the two sprite indices
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dey
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sty SIndx0+1
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dec SIndx0
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dex
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bne DrawNextScanline
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NoSprites
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; Clear player data on exit
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rts
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Inactive0
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; Alternate sprite 0 path when inactive
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lda #0
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sta WSYNC
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sta GRP0
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sta COLUP0
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beq DrawSprite1 ; always taken due to lda #0
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; SetHorizPos - Sets the horizontal position of an object.
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; The X register contains the index of the desired object:
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; X=0: player 0
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; X=1: player 1
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; X=2: missile 0
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; X=3: missile 1
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; X=4: ball
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; This routine does a WSYNC and HMOVE before executing,
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; so whatever you do here will not take effect until you
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; call the routine again or do your own WSYNC and HMOVE.
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SetHorizPos
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sta WSYNC ; start a new line
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sta HMOVE ; apply the previous fine position(s)
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sta HMCLR ; reset the old horizontal position(s)
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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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; Height of our sprite in lines
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SpriteHeight equ 18
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; To avoid nasty timing issues,
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; we'll start the bitmap data at a page boundary
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org $f800
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; Bitmap data "standing" position
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Frame0
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.byte #0
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.byte #%01101100;$F6
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.byte #%00101000;$86
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.byte #%00101000;$86
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.byte #%00111000;$86
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.byte #%10111010;$C2
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.byte #%10111010;$C2
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.byte #%01111100;$C2
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.byte #%00111000;$C2
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.byte #%00111000;$16
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.byte #%01000100;$16
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.byte #%01111100;$16
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.byte #%01111100;$18
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.byte #%01010100;$18
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.byte #%01111100;$18
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.byte #%11111110;$F2
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.byte #%00111000;$F4
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; Bitmap data "throwing" position
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Frame1
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.byte #0
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.byte #%01101100;$F6
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.byte #%01000100;$86
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.byte #%00101000;$86
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.byte #%00111000;$86
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.byte #%10111010;$C2
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.byte #%10111101;$C2
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.byte #%01111101;$C2
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.byte #%00111001;$C2
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.byte #%00111000;$16
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.byte #%01101100;$16
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.byte #%01111100;$16
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.byte #%01111100;$18
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.byte #%01010100;$18
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.byte #%01111100;$18
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.byte #%11111110;$F2
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.byte #%00111000;$F4
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; Color data for each line of sprite
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ColorFrame0
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.byte #$FF;
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.byte #$F6;
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.byte #$86;
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.byte #$86;
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.byte #$86;
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.byte #$C2;
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.byte #$C2;
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.byte #$C2;
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.byte #$C2;
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.byte #$16;
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.byte #$16;
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.byte #$16;
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.byte #$18;
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.byte #$18;
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.byte #$18;
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.byte #$F2;
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.byte #$F4;
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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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