@Norbert, post #301
@Rafał A-3000, post #302
@luka, post #315
@luka, post #319
@luka, post #321
„Mystery“ Header (courtesy of Dave Haynie)
„There is a 12 pin header (DIL, J975) near the mouse ports. This feeds the 'extra' shift register in Lisa. Unlike OCS/ECS systems, which simply multiplex the four quadrature signals from each mouse port with one another, the AA systems serialize all eight bits of mouse data. While they were at it, the AA designers added a second 8-bits to the mouse port registers. These don't hook into mouse logic or anything, but they can be read by the CPU.
So we used them for configuration. The first two bits (not on that header) tell the OS about the AA system – is it 16 or 32 bit, is it single or double access per cycle. These are hard-wired on the motherboard. The remaining six bits read high when unjumpered, low when jumpered. I had recommended using one to say 'VGA only,' but there wasn't enough ROM space.“
@sq7bti, post #322