Expansion Bus I/O Propagate Register

Number	$8A
Readable	Yes
Writable	Yes
Short Description	Monitoring internal I/O or adding external keyboard

Bit	Description
7-6	Reserved, must be 0
5	Propagate port 0xeff7 io cycles (hard reset = 0)
4	(since core3.1.2) Propagate port Timex Sinclair Video Mode Control ($xxFF / 255) I/O cycles (hard reset = 0)
3	Propagate port Plus 3 Memory Paging Control ($1FFD / 8189) I/O cycles (hard reset = 0)
2	Propagate port Next Memory Bank Select ($DFFD / 57341) I/O cycles (hard reset = 0)
1	Propagate port Memory Paging Control ($7FFD / 32765) I/O cycles (hard reset = 0)
0	Propagate port Keyboard ($**FE / 254) I/O cycles (hard reset = 1)

core3.1.4 change: bit 0 is now set to "1" upon hard reset

If any of the bits are set, I/O cycles for the corresponding ports are propagated to the expansion bus when the expansion bus is on.

If the internal port decode is still active, any response sent by devices on the expansion bus will be ignored.

The purpose here is to allow external peripherals to monitor changes in state inside the ZX Next.

Keyboard ($**FE / 254) is treated specially so that external keyboards can be attached. When its propagate bit is set, the value read from the bus will be mixed into keyboard reads on port $FE.

(note: Next registers with number higher than $7F are inaccessible from Copper code)