Very simple - One Megabyte of memory on an expansion card.
The memory could not be accessed from Basic but could be paged in at machine code level and was most useful to use as a RAM Disc with MasterDOS.
Most users with the One Meg also had a SamBus or a Two-up/Three-up interface expansion unit.
On the circuit board there is a jumper connection (bottom left above the capacitor) to allow a one from four position selection (which is factory set to position four). This enables up to four 1 Mb External Interfaces to be connected together giving a possible total memory capability of 4.5 Megabytes of RAM (4,718,592 bytes)
