Memory Paging Control
From SpecNext Wiki
| Number | PortNumber::$7FFD |
|---|---|
| Decimal | NumberDec:: |
| Short desc. | ShortDesc::Selects active RAM, ROM, and displayed screen. |
| Bit Mask | [[PortMask::%01-- ---- ---- --0-]] |
| Readable | Readable::No |
| Writable | Writable::Yes |
| Subsystem | Subsystem::Memory map |
Selects the active memory page in slot 4 at $C000. See Memory map.
Any values written to this port should also be stored at $5B5C if any OS routines are in use.
| Bit | Effect |
|---|---|
| 6-7 | Undocumented |
| 5 | Lock memory paging (setting to 1 locks pages and cannot be unlocked until next reset) |
| 4 | ROM select (low bit of ROM select on +2/+3) |
| 3 | Shadow Screen toggle
When 1, the "Layer 2" is disabled due to ULA screen being read from other memory bank. For double-buffered ULA screen together with Layer 2 functionality the {{#ask: PortNumber::$xxFF }} ($xxFF{{#ask: PortNumber::$xxFF |mainlabel=- |headers=hide |intro= / |?NumberDec#- }}) is suggested. |
| 0-2 | Bank number for slot 4 ($C000) |