Internal Port Decoding b24-31 Register: Difference between revisions
core 3.0 changes/refresh |
core 3.0 changes/refresh |
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| Line 8: | Line 8: | ||
! Bit !! Description | ! Bit !! Description | ||
|- | |- | ||
| 7-1 || (bit 31-25) Reserved, use 1 | | 7-1 || (bit 31-25) Reserved, preserve the current value (or at least use 1) | ||
|- | |- | ||
| 0 || (bit 24) Enabling {{PortNo|$BF3B}} and {{PortNo|$FF3B}} (ULA+) | | 0 || (bit 24) Enabling {{PortNo|$BF3B}} and {{PortNo|$FF3B}} (ULA+) | ||
Revision as of 10:26, 12 November 2019
| Number | TBRegisterNumber::$85 |
|---|---|
| Readable | TBRegisterReadable::Yes |
| Writable | TBRegisterWritable::Yes |
| Short Description | ShortDesc::Enabling internal ports decoding |
| Bit | Description |
|---|---|
| 7-1 | (bit 31-25) Reserved, preserve the current value (or at least use 1) |
| 0 | (bit 24) Enabling {{#ask:
PortNumber::$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 soft reset.
The internal port decoding enables always apply.
When the expansion bus is on, the expansion port decoding (Next Registers $86-$89) enables are 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)