Changes

Two additional blocks are shown in the PSG Block Diagram which have nothing directly to do with the production of sound -- these are the two I/O ports (A and B). Since virtually all uses of microprocessor-based sound would require interfacing between the outside world and the processor, this facility has been included in the PSG. Data to/from the CPU bus may be read/written to either of two 8-bit I/O Ports without affecting any other function of the PSG. The I/O Ports are TTL-compatible and are provided with internal pull-ups on each pin. Both Ports are available on the AY-3-8910; only I/O Port A is available on the AY-3-8912; no ports are available on the AY-3-8913.

== OPERATION ==