Editing PDP-8 (section)

==Interrupts==
There is a single [[interrupt]] line on the PDP-8 I/O bus. The processor handles any interrupt by disabling further interrupts and executing a <code>JMS</code> to location 0000. As it is difficult to write reentrant subroutines, it is difficult to nest interrupts and this is usually not done; each interrupt runs to completion and re-enables interrupts just before executing the <code>JMP I 0</code> instruction that returns from the interrupt.

Because there is only a single interrupt line on the I/O bus, the occurrence of an interrupt does not inform the processor of the source of the interrupt. Instead, the interrupt service routine has to serially poll each active I/O device to see if it is the source. The code that does this is called a ''skip chain'' because it consists of a series of PDP-8 "test and skip if flag set" I/O instructions. (It was not unheard-of for a skip chain to reach its end without finding '''any''' device in need of service.) The relative interrupt priority of the I/O devices is determined by their position in the skip chain: If several devices interrupt, the device tested earlier in the skip chain is serviced first.