Expansion Bus Decoding b16-23 Register

Number	TBRegisterNumber::$88
Readable	TBRegisterReadable::Yes
Writable	TBRegisterWritable::Yes
Short Description	ShortDesc::When expansion bus is enabled: Internal ports decoding mask

Bit	Description
7	(bit 23) Masking decoding: {{#ask: PortNumber::$xxDF }} ($xxDF{{#ask: PortNumber::$xxDF |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) (DAC mono Specdrum)
6	(bit 22) Masking decoding: {{#ask: PortNumber::$xxB3 }} ($xxB3{{#ask: PortNumber::$xxB3 |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) (DAC mono GS Covox)
5	(bit 21) Masking decoding: {{#ask: PortNumber::$xxFB }} ($xxFB{{#ask: PortNumber::$xxFB |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) (DAC mono Pentagon/ATM (sd mode 2 off))
4	(bit 20) Masking decoding: {{#ask: PortNumber::$xx0F }} ($xx0F{{#ask: PortNumber::$xx0F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) and {{#ask: PortNumber::$xx4F }} ($xx4F{{#ask: PortNumber::$xx4F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) (DAC stereo Covox)
3	(bit 19) Masking decoding: {{#ask: PortNumber::$xx3F }} ($xx3F{{#ask: PortNumber::$xx3F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) and {{#ask: PortNumber::$xx5F }} ($xx5F{{#ask: PortNumber::$xx5F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) (DAC stereo profi Covox)
2	(bit 18) Masking decoding: {{#ask: PortNumber::$xxF1 }} ($xxF1{{#ask: PortNumber::$xxF1 |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}), {{#ask: PortNumber::$xxF3 }} ($xxF3{{#ask: PortNumber::$xxF3 |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}), {{#ask: PortNumber::$xxF9 }} ($xxF9{{#ask: PortNumber::$xxF9 |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) and {{#ask: PortNumber::$xxFB }} ($xxFB{{#ask: PortNumber::$xxFB |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) (DAC soundrive mode 2)
1	(bit 17) Masking decoding: {{#ask: PortNumber::$xx0F }} ($xx0F{{#ask: PortNumber::$xx0F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}), {{#ask: PortNumber::$xx1F }} ($xx1F{{#ask: PortNumber::$xx1F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}), {{#ask: PortNumber::$xx4F }} ($xx4F{{#ask: PortNumber::$xx4F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) and {{#ask: PortNumber::$xx5F }} ($xx5F{{#ask: PortNumber::$xx5F |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) (DAC soundrive mode 1)
0	(bit 16) Masking decoding: {{#ask: PortNumber::$FFFD }} ($FFFD{{#ask: PortNumber::$FFFD |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }}) and {{#ask: PortNumber::$BFFD }} ($BFFD{{#ask: PortNumber::$BFFD |mainlabel=- |headers=hide |intro= /  |?NumberDec#- }})

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)