Expansion Bus Decoding b16-23 Register
From SpecNext Wiki
| Next Register Number | $88 |
|---|---|
| Readable | Yes |
| Writable | Yes |
| Short Description | When expansion bus is enabled: Internal ports decoding mask |
| Bit | Description |
|---|---|
| 7 | (bit 23) Masking decoding: NextPort:$xxDF (DAC mono Specdrum) |
| 6 | (bit 22) Masking decoding: NextPort:$xxB3 (DAC mono GS Covox) |
| 5 | (bit 21) Masking decoding: NextPort:$xxFB (DAC mono Pentagon/ATM (sd mode 2 off)) |
| 4 | (bit 20) Masking decoding: NextPort:$xx0F and NextPort:$xx4F (DAC stereo Covox) |
| 3 | (bit 19) Masking decoding: NextPort:$xx3F and NextPort:$xx5F (DAC stereo profi Covox) |
| 2 | (bit 18) Masking decoding: NextPort:$xxF1, NextPort:$xxF3, NextPort:$xxF9 and NextPort:$xxFB (DAC soundrive mode 2) |
| 1 | (bit 17) Masking decoding: NextPort:$xx0F, NextPort:$xx1F, NextPort:$xx4F and NextPort:$xx5F (DAC soundrive mode 1) |
| 0 | (bit 16) Masking decoding: NextPort:$FFFD and NextPort:$BFFD |
since core3.1.1: all bits are set to 1 when: soft reset and bit 31 is clear OR hard reset and bit 31 is set (otherwise content is kept intact). 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)