This may go some way to explaining why ZX Prism's codename was "Project Overkill" for a while.
Some further explanation/clarification:
In 1024K mode, the memory page at 0xC000 is chosen using port 0x7FFD in the same way as the 1024K Pentagon (as documented in the ZX Profi documentation by Velesoft):
- 7FFD bit 0 = Page # bit 0
- 7FFD bit 1 = Page # bit 1
- 7FFD bit 2 = Page # bit 2
- 7FFD bit 6 = Page # bit 3
- 7FFD bit 7 = Page # bit 4
- 7FFD bit 5 = Page # bit 5
Due to the way this locking works in ZX Prism, enabling 1024K mode from "48K mode" has the effect of re-enabling memory paging.