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Private and Shared Memory

The memory contained into the Master Controller Unit (MC) is said to be "private" because it is isolated from the EXT-BUS by the mean of 3rd-state buffers so no external device can gain access to it in any way.

Memory can be extended (up to 64KB total) by building an external Memory Unit. That memory would said to be "shared" because is accessible from both the CPU and DMA capable peripherals.

The use of a separate private and shared memory allows for concurrent operations between DMA operations and normal software execution as long as the later takes place in private memory.