Changes

* When switching input/output of port A, on a normal 8255, the outputs are all cleared to 0. This doesn't happen on the emulated 8255. This is another source of keyboard reading bugs.

An automatic process has been added to feed data to the sound generator from three instruction streams in main RAM without CPU intervention. Three separate channels each fetch one 16-bit instruction during horizontal retrace time. These instructions must be in usual Z-80 format, i.e. least significant bit first and must be aligned to word boundaries (i.e. address of first byte must be even). Once the three instructions have been captured , they are then executed sequentially. The maximum achievable update rate to the PSG is thus equal to the horizontal scan rate of 15.625 kHz per channel.