IFEL RISC OS ROM carrier board

The Acorn Archimedes A300 originally shipped with the Arthur OS and later, RISC OS 2 which was delivered on ROMs with a 512Kbit capacity. The maximum capacity ROM size a standard A300 series machine could support was 1Mbit.

The IFEL RISC OS ROM carrier board provided the ability for the A300 series to be upgraded with ROM images up to 4Mbit in size. Due to the larger capacities of the newly supported ROMs, extra select lines were required in order to fully access them. This was achieved by using three extra signals taken from the motherboard to deliver the compatibility required.

The size of the ROMs being used was defined by the use of jumpers on the board and the photo below shows the board configured to take the 4Mbit ROM image containing RISC OS 3.10. You can also see the extra select lines connecting to the carrier board to provide full support for the larger ROM capacities.

There is no need to remove the ROM carrier board when downgrading the operating system to the earlier issues. Using the jumpers on the board, the machine could be configured to cater for the different capacity ROMs that were fitted in the earlier versions of the Archimedes such as Arthur and RISC OS 2.

RISC OS 2

The RISC OS 2 and Arthur ROMs are physically smaller than the RISC OS 3 ROMS and you can see how they fit into the RISC OS 3 capable ROM carrier board below. Due to the physical differences in the chips, it is important that the jumpers are configured correctly for each OS before powering on otherwise damage may occur. 

Arthur OS

The jumper settings for fitting the Arthur OS should be the same as those for RISC OS 2 although I have not yet tested this by fitting my Arthur OS ROMs.