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Daisy Chained single-line Interrupt Architecture

The bus connecting all units in the LC-81 has only one line for interrupt request (IRQ) and one for interrupt acknowledge (IAK). Multiple nested interrupts are possible, however, because those lines are daisy-chained among units. The peripheral asserting the interrupt request opens the chain behind it to prevent others to interrupt, taking priority this way in the event of simultaneous interrupt requests.

This contrasts with having multiple IRQ lines in the bus and presents the advantage of simplicity in the bus at the expense of making things more complex in the peripheral design.