Expansion Bus Decoding b16-23 Register
From SpecNext official Wiki
| Number | $88 |
|---|---|
| Readable | Yes |
| Writable | Yes |
| Short Description | When expansion bus is enabled: Internal ports decoding mask |
| Bit | Description |
|---|---|
| 7 | (bit 23) Masking decoding: SpecDrum DAC Output ($xxDF / 223) (DAC mono Specdrum) |
| 6 | (bit 22) Masking decoding: ($xxB3) (DAC mono GS Covox) |
| 5 | (bit 21) Masking decoding: ($xxFB) (DAC mono Pentagon/ATM (sd mode 2 off)) |
| 4 | (bit 20) Masking decoding: ($xx0F) and ($xx4F) (DAC stereo Covox) |
| 3 | (bit 19) Masking decoding: ($xx3F) and ($xx5F) (DAC stereo profi Covox) |
| 2 | (bit 18) Masking decoding: ($xxF1), ($xxF3), ($xxF9) and ($xxFB) (DAC soundrive mode 2) |
| 1 | (bit 17) Masking decoding: ($xx0F), Kempston Joystick ($xx1F / 31), ($xx4F) and ($xx5F) (DAC soundrive mode 1) |
| 0 | (bit 16) Masking decoding: Turbo Sound Next Control ($FFFD / 65533) and Sound Chip Register Write ($BFFD / 49149) |
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)