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 o