Interrupt System
The Nord-10 had a multi-program system with 16 priority program levels. Each program level had its own set of registers, including a program counter and a status word. The levels running could be shown on the front panel by pressing the button "active levels". Levels 0 through 9 were used for programs. Internal hardware status interrupts were assigned to level 14, whilst level 15 was reserved for extremely fast user interrupts (this was colloquially called the "synchrotron level", since the only program ever to have used it was the program controlling the synchrotron at CERN)
Levels 10, 11, 12, and 13 were reserved for external devices. Each device had its own unique identification vector. In all 2048 such vectors were available. The "ident" instruction determined which device was giving an interrupt. The identification of an interrupt took 1.7 microseconds, including the time taken to enable and disable the registers.
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