The Collision Registers

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

The Collision Registers#

The TIA tracks collisions between its six drawable objects — the two players, two missiles, the ball, and the playfield. Six objects make 15 possible pairs, and the chip reports them across eight read-only registers.

Detection is geometric, and it latches#

As the beam paints each visible pixel, the TIA asks a simple question: are two objects both lit here? If so, it sets the corresponding collision bit. Two things follow from that:

Reading & Clearing

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

Reading & Clearing#

The TIA does the detecting; your job is two small, regular acts: read the latches once the frame has been drawn, and clear them before the next one. Get that rhythm right and collisions are nearly free; get it wrong and they either never fire or never stop.

Testing a latch with `BIT`#

Each collision flag sits in bit 7 (and bit 6) of its register, which is exactly where BIT is most useful: it copies a memory location’s bit 7 into the N flag and bit 6 into the V flag, without changing any register (see Registers & Flags). So one BIT tests both pairs at once: