| Bits | Description |
|---|
| 7:6 | Reserved, must be 0 |
| 5 | Propagate port 0xeff7 io cycles (hard reset = 0) |
| 4 | Propagate port 0xff io cycles (hard reset = 0) |
| 3 | Propagate port 0x1ffd io cycles (hard reset = 0) |
| 2 | Propagate port 0xdffd io cycles (hard reset = 0) |
| 1 | Propagate port 0x7ffd io cycles (hard reset = 0) |
| 0 | Propagate port 0xfe io cycles (hard reset = 0) |
Notes:
If any of the bits are set, io 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.
Port 0xfe 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 0xfe.