Atari 2600 Programming for Newbies
Session 14: Playfield Weirdness
By Andrew Davie (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Page Table of Contents
Original Session
The diagram below shows the bizarre way that bits in the TIA playfield registers (PF0, PF2) map to the onscreen pixels in reverse order. We have already seen how PF1 works—it is shown in this diagram, too.
This strange backwardness (not to mention inconsistency!) is probably a result of the fact that it was simpler (cheaper) to design the hardware to operate in this fashion. Among other things, this layout of pixels in our TIA registers makes scrolling horizontally a major pain in the neck!
The bits marked with a cross are not used by the '2600 (including the low bit in the color registers), and you may write any value to these—it is ignored.
The diagram shows a shadowy ‘right-side’—where the 20 pixels of the left side are duplicated. Be aware that this right-side may also be mirrored, further complicating things.
- Confirm that PF0 and PF2 have reverse pixel to bit position ordering (Hint: using binary for your values will assist you to see exactly what pixel corresponds to what bit (ie: lda #%01000000, sta PF0)
- What happens if you write PF0, PF1 or PF2 in the middle of a scanline? What would you expect to happen? Can you see any use for this? (Hint: how do you think an asymmetric playfield—a different pattern on the left and right of the screen—is created?)
- Write some solid shape(s) to PF0, PF1, PF2 (ie: lda #%01011110, sta PF0, sta PF1, sta PF2) and then play with changing the playfield color several times during a scanline. How many color changes (maximum) do you think you can get on any line? Why is there a limit?
- How would a game do horizontal scrolling? This is a difficult question—but I'm trying to get you to think about the implications of the odd playfield->pixel correspondence, and the implications for game writing.
- How would you make a 'wall' which was 8 scanlines high, full screen width, followed by left and right walls just 1 pixel wide each, at extreme left/right edges of the screen, 176 scanlines high, followed by another horizontal 'wall', full screen width and 8 scanlines high? Note: this would form a 'box' border around the entire playfield.
Next session we'll walk through exactly what all this playfield oddity is about.
See you next time!
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Session 2: Television Display Basics
Sessions 3 & 6: The TIA and the 6502
Session 5: Memory Architecture
Session 7: The TV and our Kernel
Session 9: 6502 and DASM - Assembling the Basics
Session 14: Playfield Weirdness
Session 15: Playfield Continued
Session 16: Letting the Assembler do the Work
Sessions 17 & 18: Asymmetrical Playfields (Parts 1 & 2)
Session 20: Asymmetrical Playfields (Part 3)
Session 22: Sprites, Horizontal Positioning (Part 1)
Session 22: Sprites, Horizontal Positioning (Part 2)
Session 23: Moving Sprites Vertically
Session 25: Advanced Timeslicing
Atari 2600 BASIC
If assembly language is too hard for you, try batari Basic. It's a BASIC-like language for creating Atari 2600 games. It's the faster, easier way to make Atari 2600 games.
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