MO5

The MO5 stock configuration is 48KiB of RAM, of which 16 are used as video memory, and 32 as user RAM.

There is a RAM expansion that plugs to the cartridge port and adds an extra 64KiB of RAM, in the form of 4 16KiB pages.

This expansion will not work on all machines: the first generation of MO5 does not wire any write signal to the cartridge port. This was added as the extension was introduced.

This expansion was developed for use in the Nanoréseau system. It was used to load software from the master computer through the Nanoréseau network, then write locked as the software was being used. This has a few incidences on the hardware design:

• The RAM expansion plugs in the cartridge port (leaving the expansion port free for the Nanoréseau controller), and uses the cartridge space in the memory map,
• It can be write protected,
• It can not be used from BASIC, because it hides the BASIC ROM when in use.

The RAM is mapped at addresses B000-EFFF, just like a regular cartridge.

The RAM expansion is configured using a single register at address A7CB (write only).

• bits 0-1: RAM page number
• bit 2: enable RAM expansion (when disabled, the BASIC is mapped)
• bit 3: write enable

The BASIC will be mapped in when the machine boots, however, this is not the case on warm reset. To avoid a crash when resetting the machine, well-behaving software located in the RAM expansion should provide a reset vector at address EFFE-EFFF, in all banks. The monitor ROM reads the address stored there and jumps to it on reset.