Memory Paging Control
From SpecNext official Wiki
| Number | $7FFD |
|---|---|
| Decimal | |
| Short desc. | Selects active RAM, ROM, and displayed screen. |
| Bit Mask | %01-- ---- ---- --0- |
| Readable | No |
| Writable | Yes |
| 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 - this is ULA layer shadow, not related to Layer 2 RAM Shadow Page Register ($13) |
| 0-2 | Bank number for slot 4 ($C000) |
Since core3.0 the Layer 2 can be used together with shadow ULA screen from Bank 7 (previous cores did disable Layer 2).