The TIA (as seen by the programmer)

10.0 Collisions

The TIA detects collisions between any of the 6 objects it generates (the playfield and 5 moveable objects). There are 15 possible two-object collisions which are stored in 15 one bit latches. Each collision register contains two of these latches which are read by the microprocessor on D6 and D7 of the data bus for easy access. A "1" on the data line indicates the collision it records has occurred. The collision registers could be read at any time but is usually done during vertical blank after all possible collisions have occurred. The collision registers are all reset simultaneously by writing to the collision reset register (CXCLR).

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See also: 'TIA 1A - TELEVISION INTERFACE ADAPTOR (MODEL 1A) - 7. Collision Detection Latches'