Heartbeat interrupt

Like the original Expansion Interface, MIRE provides an interrupt to the CPU every 25 milliseconds. Among other purposes, this is used for updating DOS software clocks.

The interrupt service routine resets the interrupt by reading any of the memory addresses 0x37E0-0x37E3 or 0x37F0-0x37F3, although reading 0x37E0 is the only documented method and is therefore preferred.

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