Internal Port Decoding b0-7 Register

Number	$82
Readable	Yes
Writable	Yes
Short Description	Enabling internal ports decoding

Bit	Description
7	Enabling Kempston Joystick 2, Joystick I/O ($xx37 / 55) (also MD2 controller)
6	Enabling Kempston Joystick ($xx1F / 31) (also MD1 controller)
5	Enabling Datagear DMA Port ($xx6B / 107)
4	Enabling +3 floating bus
3	Enabling Plus 3 Memory Paging Control ($1FFD / 8189)
2	Enabling Next Memory Bank Select ($DFFD / 57341)
1	Enabling Memory Paging Control ($7FFD / 32765)
0	Enabling Timex Sinclair Video Mode Control ($xxFF / 255)

since core3.1.1: all bits are set to 1 when: soft reset and bit 31 is set OR hard reset and bit 31 is clear (otherwise content is kept intact). 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)