| Bits | Description |
|---|
| 7 | 1 to enable alt rom |
| 6 | 1 to make alt rom visible only during writes, otherwise replaces rom during reads |
| 5 | 1 to lock ROM1 (48K rom) |
| 4 | 1 to lock ROM0 (128K rom) |
| 3 | 1 to enable alt rom |
| 2 | 1 to make alt rom visible only during writes, otherwise replaces rom during reads |
| 1 | 1 to lock ROM1 (48K rom) |
| 0 | 1 to lock ROM0 (128K rom) |
Notes:
IMMEDIATE
AFTER SOFT RESET (copied into bits 7-4)
The locking mechanism also applies if the alt rom is not enabled. For the +3 and zx next, if the two lock bits are not
zero, then the corresponding rom page is locked in place. Other models use the bits to preferentially lock the corresponding
48K rom or the 128K rom.