Expansion Bus Decoding b24-31 Register
From SpecNext official Wiki
| Number | $89 |
|---|---|
| Readable | Yes |
| Writable | Yes |
| Short Description | When expansion bus is enabled: Internal ports decoding mask |
| Bit | Description |
|---|---|
| 7-1 | (bit 31-25) Reserved, preserve the current value (or at least use 1) |
| 0 | (bit 24) Masking decoding: ($BF3B) and ($FF3B) (ULA+) |
All bits are set to 1 upon hard reset.
When the expansion bus is on, the expansion port decoding mask (Next Registers $86-$89) is logically ANDed with the internal enables (Next Registers $82-$85). A zero bit indicates the internal device is disabled.
If the expansion bus is on, this allows I/O cycles for disabled internal ports to propagate to the expansion bus, otherwise corresponding I/O cycles to the expansion bus are filtered.
(note: Next registers with number higher than $7F are inaccessible from Copper code)