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79 lines
1.9 KiB
Plaintext
79 lines
1.9 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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org $f000
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; Sprites are a tricky beast on the 2600.
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; You only have two of them.
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; They are 8 bits wide and 1 bit high.
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; There's no way to position the X or Y coordinate directly.
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; To position X, you have to wait until the TIA clock hits a
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; given X position and then write to a register to lock it in.
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; To position Y, you simply wait until the desired scanline and
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; set the bits of your sprite to a non-zero value.
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; Having fun yet? :)
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Counter equ $81
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Start CLEAN_START
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NextFrame
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; This macro efficiently gives us 3 lines of VSYNC
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VERTICAL_SYNC
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; 37 lines of VBLANK
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ldx #37
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LVBlank sta WSYNC
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dex
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bne LVBlank
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; For the last VBLANK line, we'll lock in our sprites' X position.
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; The first one will be 20 TIA clocks from the left, plus whatever the
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; DEX and BNE instructions in the previous loop took.
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; There are 68 TIA clocks *before* the scanline starts, and there are
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; 3 TIA clocks per CPU cycle, so counting clocks (or trial-and-error)
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; is neccessary to get exact positioning.
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SLEEP 20 ; this macro hard-codes a 20 cycle delay (60 TIA clocks)
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sta RESP0 ; set position of sprite #1
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SLEEP 35 ; wait 35 more cycles (105 TIA clocks)
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sta RESP1 ; set position of sprite #2
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; Draw the 192 scanlines
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; We'll draw both sprites all the way down the screen
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ldx #192
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lda #0 ; changes every scanline
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ldy Counter ; changes every frame
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ScanLoop
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sta WSYNC ; wait for next scanline
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sta COLUBK ; set the background color
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sta GRP0 ; set sprite 0 pixels
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sta GRP1 ; set sprite 1 pixels
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adc #1 ; increment A to cycle through colors and bitmaps
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dex
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bne ScanLoop
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; Clear the background color and sprites before overscan
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; (We don't need to use VBLANK neccessarily...)
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stx COLUBK
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stx GRP0
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stx GRP1
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; 30 lines of overscan
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ldx #30
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LVOver sta WSYNC
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dex
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bne LVOver
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; Cycle the sprite colors for the next frame
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inc Counter
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lda Counter
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sta COLUP0
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sta COLUP1
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jmp NextFrame
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org $fffc
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.word Start
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.word Start
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