Changes
/* Ext Port */
|6||not used
|-
|5||if 0, then AY is accessed when read/write to 8255 port A, if 1 then real-time clock is accessed with read/write to 8255 port A. When real-time clock is selected, bits 32..0 of PPI port C are used to define real-time clock operation. DS bit 2, AS bit 1, R/W bit 0. So combinations are 2, write address, 4 write data, 5 read data.
|-
|4||Enable 8253 timer. Any I/O write then accesses 8253 with data comming from 8255 port A. Bit A0/A1 of I/O port defines 8253 register
