The Playfield on Programming the Atari VCS in Assembly

The Three Registers

Mon, 01 Jan 0001 00:00:00 +0000

The Three Registers: PF0, PF1, PF2#

Everything the playfield does starts with three 8-bit registers — PF0, PF1, PF2 — and the 20 bits of them the TIA actually draws. This page lays out exactly which bit lights which pixel. We will stay on the left half of the screen the whole way; the other half turns out to be a story of its own.

What one “pixel” is#

First, what these bits paint. A playfield pixel is a virtual block, generated on the fly by the TIA as the beam passes — not a dot in any frame buffer. Each is 4 color clocks wide and exactly one scanline tall. The playfield has no vertical resolution of its own: a pixel is one scanline high, and a taller block is just the same bits held unchanged across consecutive lines — exactly how the Christmas-tree kernel grows its shape.

Symmetry: Reflection & Repetition

Mon, 01 Jan 0001 00:00:00 +0000

Symmetry: Reflection & Repetition#

The three registers gave you 20 bits — the left 20 pixels of a 40-pixel-wide screen. You never wrote the other 20. This page is where they come from.

The TIA generates the right half automatically from those same 20 bits, and a single flag decides whether it does so as a copy or a mirror. That is why the playfield is inherently symmetric: one write of PF0/PF1/PF2 per line paints the whole 40-pixel width, but both halves are built from the same bits.

The Asymmetric Playfield

Mon, 01 Jan 0001 00:00:00 +0000

The Asymmetric Playfield#

Symmetry is the playfield’s default because its 20 bits describe only the left half and the TIA mirrors or copies them onto the right. To show two different things on the two halves — a genuinely asymmetric playfield — you have to break that deal. The escape hatch is timing.

The TIA reads the registers as it goes#

The crucial fact: the TIA does not snapshot PF0/PF1/PF2 at the start of the line. It reads them continuously as the beam sweeps across, consulting each register at the exact moment the beam reaches the pixels it controls. So if you rewrite the registers partway across the line — after the beam has drawn the left half, but before it reaches the right half — the right half is built from the new values. The two halves now differ.

Building a Scoreboard

Mon, 01 Jan 0001 00:00:00 +0000

Building a Scoreboard#

A two-player scoreboard is the textbook use of an asymmetric playfield — two different numbers, one on each half of the screen — and it pulls together nearly everything in this chapter: digit graphics drawn from the playfield, bit masking to pack two digits into a byte, the no-multiply ×5 index trick, and BCD arithmetic to keep the count in decimal.

The code here is illustrative — the repository doesn’t ship a scoreboard project yet. Treat the snippets as sketches of the technique, not a drop-in kernel.