MagicDesk-like External ROM cartridge for C128
This is a clone of Magic Desk 512k by Marko Šolajić converted to KiCad 6 format and modified to work as an External ROM cartridge for C128 and map 32K of ROM at a time.
There are two projects Magic Desk 128 and Gmod2 (128) with a similar goal, but both map only 16K at a time. This makes it impossible e.g. to put The Servant on such cartridge.
Why
It just seems much more useful in C128 mode to have whole 32K available - just like Internal Function ROM, but easily switched. Since MMU can bank in/out both 16K halves ($8000-$BFFF) and ($C000-$FFFF) separately it seems that e.g. GEOS 128 could run directly from ROM bypassing any boot or loading parts.
Hardware
The schematic is here.
The main difference to C64 version is that this cartridge is meant for C128, so /EXROM signal is no longer connected to prevent from entering C64 mode automatically.
Two diodes and a pullup resistor perform AND function on /ROML and /ROMH to map 32K at a time ($8000-$FFFF).
Because the bank size is now 32K instead of 8K the lowest two bits of control register are not connected.
kicad folder contains all the KiCad 6.0 project files and kicad/plots all the Gerber files needed to produce PCB.
Please read the notes from the original project. The jumper configuration for various EPROMS/EEPROMS/FLASH chips is exactly the same, as well as possible expansion to 1MB.
Programming
Data from ROM will appear as External Function ROM mapped in $8000-$FFFF area (32K).
ROM banks can be changed from BASIC.
BANK15:POKEDEC("DE00"),4*<rom bank>
There are 16 banks in a 512K EEPROM and 32 banks if you piggy-back two of them and connect /CE2 signal as described for C64 Magic Desk 512K.
RESET will always enable bank 0.
It's easy to check the ROM data from C128 monitor, just use 8 as the bank number:
M 88000 880FF
Reading from $DE00 register will switch bank to a random value (what happened to be on the data bus), don't do that.
Cartrige ROM generator
This forked and modified version of the Magic cartridge generator supports 32K banks, use -832 option instead of -8 when building ROM image.
A simple code snippet for a true C128 cartridge (internal/external function ROM) code to load and start BASIC program can be found on World of Jani page.
Gmod2 Commodore 128 technical manual is also a useful resource.
You can also put The Servant on it
Emulator Support
None yet :)