This memory overlays RAM addresses from #C000 to #FFFF although the RAM still exists in this configuration, it is invisible to the rest of the system. It can be switched out only with the DISC DRIVE system since this expansion utilises the missing 16K of RAM for its DISC OPERATING SYSTEM, be it RANDOS, SUPERDOS or SEDORIC.

Note there is an empty socket or space for a further chip besides the 16K PROM. In the early versions of the ORIC, the system used two 8K EPROMS.

Recently, this space has been realised as being far more valuable than previously thought.

A 256K EPROM chip can now sit comfortably in the empty socket next to the existing PROM although some further circuitry is required to bank the right 16K block of memory in.