Expansion Bus Decoding b24-31 Register
| Number | TBRegisterNumber::$89 |
|---|---|
| Readable | TBRegisterReadable::Yes |
| Writable | TBRegisterWritable::Yes |
| Short Description | ShortDesc::When expansion bus is enabled: Internal ports decoding mask |
| Bit | Description |
|---|---|
| 7-1 | (bit 31-25) Reserved, preserve the current value (or at least use 1) |
| 0 | (bit 24) Masking decoding: {{#ask:
PortNumber::$BF3B }} ($BF3B{{#ask: PortNumber::$BF3B |mainlabel=- |headers=hide |intro= / |?NumberDec#- }}) and {{#ask: PortNumber::$FF3B }} ($FF3B{{#ask: PortNumber::$FF3B |mainlabel=- |headers=hide |intro= / |?NumberDec#- }}) (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)