Internal Port Decoding b16-23 Register

Number	$84
Readable	Yes
Writable	Yes
Short Description	Enabling internal ports decoding

Bit	Description
7	(bit 23) Enabling SpecDrum DAC Output ($xxDF / 223) (DAC mono Specdrum)
6	(bit 22) Enabling ($xxB3) (DAC mono GS Covox)
5	(bit 21) Enabling ($xxFB) (DAC mono Pentagon/ATM (sd mode 2 off))
4	(bit 20) Enabling ($xx0F) and ($xx4F) (DAC stereo Covox)
3	(bit 19) Enabling ($xx3F) and ($xx5F) (DAC stereo profi Covox)
2	(bit 18) Enabling ($xxF1), ($xxF3), ($xxF9) and ($xxFB) (DAC soundrive mode 2)
1	(bit 17) Enabling ($xx0F), Kempston Joystick ($xx1F / 31), ($xx4F) and ($xx5F) (DAC soundrive mode 1)
0	(bit 16) Enabling Turbo Sound Next Control ($FFFD / 65533) and Sound Chip Register Write ($BFFD / 49149)

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)