Zach's Projects - Chetiry Technical Notes

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If you haven't seen it in the homebrew forum yet, I have posted a demo of a well-known Russian block game. The kernel mangages 10 independently colored blocks at 8 pixels wide. (Well, 9 pixels for the far right one.)

 

Although there are only 4 COLUxx registers in the TIA, VBLANK offers an often overlooked 5th color (always black).

 

There are four colors available for the first 4 cells: background, playfield, player 0, and player 1. There are also three registers that can be pre-loaded with new colors. Then there is just barely enough time to load and store the last three colors, only because I am running code in RAM and using immediate addressing. Finally, to close off the edges of the play area, I turn VBLANK on and off.

 

In order to turn on VBLANK on the right side, I chose color values whose bit #1 were set. Then whatever value was saved to COLUPF on the right could also be saved to VBLANK. Turning it off on the left side was more of a challenge. I ended up using PHP, knowing that none of the color values loaded were zero, and bit 1 of the status register would be clear.

 

I also experimented with SHX to clear VBLANK. It has the advantage of using fewer cycles and freeing the SP, but the disadvantage is that it is a little understood illegal opcode that may not be entirely stable. So far, my tests with SHX have been successful, but I'll stick to PHP unless I find another use for the SP.

 

Currenlty it takes two scanlines to update the colors for the next row. I wish I could have done it in one line, but the block data has to be compressed to fit in 128 bytes of RAM, and I don't see any way to unpack and store 10 colors in only 76 cycles.

 

Without further ado, here is the code for coloring the blocks, part of an unwrapped loop.

CODE

;------------------------------------------

pla

lda #<BackToRom1

sta SelfMod+22

lda color4;

sta COLUP1; block 4

lda color1

sta COLUBK; block 1

lda color2

sta COLUPF; block 2

lda color3

sta COLUP0; block 3

lda color5

sleep 4

jmp SelfMod

BackToRom1

;--------------------------------------------

 

SelfModCode; byte count

php ; 0; turn off vblank

sta COLUBK; 1 2; block 5

stx COLUPF; 3 4; block 6

sty COLUP0; 5 6; block 7

lda #COLOR0; 7 8

sta COLUP1; 9 10; block 8

lda #COLOR0; 11 12

sta COLUBK; 13 14; block 9

lda #COLOR0; 15 16

sta COLUPF; 17 18; block 10

sta VBLANK; 19 20

jmp BackToRom1; 21 22 23

;-------------------------------------------

 

 

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