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02 — Memory paging

The Z80’s banked 16 KiB slots are windows onto physical memory selected by I/O ports.

SlotSelect portSelects
4000–7FFF (bank A)port 6 (mapBankA)a flash page (0–63, bit 7 clear) or a RAM page (bit 7 set, e.g. 0x83→RAM page 83); the bcall mechanism uses the flash case to bring routines into view
8000–BFFF (bank B)port 7 (mapBankB)a RAM or flash page; 0x81 observed = 84+ RAM mode
C000–FFFF (bank C)port 5 (memModeA)a RAM page; 0x00 observed = RAM page 80

0000–3FFF is hardwired to flash page 0. C000–FFFF is RAM in the static OS model and is normally the stack/user-RAM window; the 84+ hardware banks the high RAM slot through MemC/port 5. The dynamic trace in RAM pages confirms port-5 restores to RAM page 80 during normal OS execution. [confirmed]

How code uses it

  • bcalls to a paged body set port_mapBankA (port 6) to the target routine’s page, run it at 4000+, then restore the previous page; bcalls whose body lives on page 0 run it in place below 4000 (e.g. _JErrorNo→00:2799). See 03-bcall-mechanism.md. The helper ram:181c is the page-set primitive used by the dispatcher.
  • A routine that runs banked into 4000 must therefore be written position-fixed for 4000 — which is exactly why every overlay page in Ghidra is based at 4000.
  • cross_page_jump (ram:2b09) is the common cross-page jump trampoline: many page-0 entries and inline OS calls use it with a following .dw addr; .db page payload to reach a body on another page. [confirmed]

Modeling in Ghidra

Each physical flash page 1–63 is an overlay block page_NN based at 4000, so the same logical 4000–7FFF window exists once per page without collision. Bank B/RAM is the single RAM block 8000–FFFF. This loses the runtime “only one page visible” semantics (intentionally) so all code is statically present.