Schneiderware Pseudo ROM
The Schneiderware Pseudo ROM is a expansion "ROM" board, with 16K EPROM (read-only) and 16K battery-backed SRAM (read/write-able), published by german magazine CPC Schneider International as part of their Schneiderware DIY series. The plain PCB, or the fully assembled board were also available via mail-order.
|DFxxh|| Schneiderware RAM/EPROM - Expansion ROM bank number (W)|
Battery-backed SRAM and/or EPROM are selected when the bank-number matches the jumper-selected values; the memory is then mapped to C000h..FFFFh (in case of READing one must also enable upper ROM via Gate Array). The bank number decoding is a bit strange:
The SRAM/Write mode doesn't disable the internal RAM in the CPC, so writes are going both to SRAM and normal RAM at C000-FFFF, that no matter if upper ROM is enabled/disabled via Gate Array; the author recommended to map VRAM to 4000-7FFF via CRTC registers, in order to prevent video dirt during writing.
Note - Port DFxxh is the standard Upper ROM Bank Number register, which is also used by AMSDOS, for example. Reserved banks are 00h=Basic, and 07h=AMSDOS. Bank FFh (and XFh) can be also considred as reserved (as it's used by the "128K RAM detection" in CP/M+). Other bank numbers can be used by one (or more) Schneiderware Pseudo ROM board(s).
The magazine article contains three listings with examples on how to use the Pseudo ROM board, including a ROM version of the Schneiderware Real Time Clock driver.
- Schneiderware 8 - Pseudo ROM.pdf - SRAM and EPROM mapped as ROM - 4/1987 page 26-34, plus corrections in 5/1987 page 32-34