An interesting platform that can be used to develop cartridge games that require a lot of ROM space, additional RAM, and non-volatile writable memory.

The Magic Desk Plus radically expands the specifications of the classic format. In addition to supporting 256/512/1024 KBytes of ROM memory, it introduces a 128 KByte static SRAM backed by a battery, alongside an 8/32 KByte EEPROM memory. A configuration with only the SRAM or only the EEPROM is allowed.

To manage this additional space while keeping the $DE00 register unchanged, the cartridge introduces a dedicated data transit window and two new registers in the I/O 1 area:

Data I/O Window ($DF00 - $DFFF): A fixed 256-byte access window mapped directly onto the currently active SRAM or EEPROM memory page.

Page Register ($DE01): Write-only register responsible for selecting the 256-byte page (values from 0 to 255) to project into the transit window.

Control Register ($DE03): Write-only register dedicated to enabling and configuring the SRAM/EEPROM.

The $DF00 - $DFFF window will be the direct access point, both for reading and writing, to the memory (SRAM or EEPROM) even with the main cartridge ROM disabled.

All infos: https://github.com/crystalct/MagicDeskPlus