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Interrupt Control switches

Heritage/1 accepts software interrupts as well as three types of hardware interrupts: Timer, External and Fault.

The Timer Interrupt is generated by the CPU in a timely basis; it has the highest priority and cannot be masked by software. External interrupts are requested by interrupter peripherals via the external bus (U-BUS); there can be up to 65,536 external interrupts and can be disabled or enabled all together by software (instructions EI and DI respectively). The Fault interrupt is generated by different circuits in the CPU capable of detecting fault condition such as invalid instruction code; faults cannot be masked by software. For more details on Interrupts see: Interrupt Architecture in the Engineer's Manual.

The switches in the Interrupt Control section of the Console allows you to handle those interrupts in a privileged way with total independence of software.

The TMR switch must be active (up) for the timer to function. Similarly, external interrupts won't get the CPU attention if the INT switch is not in its active position. The switch Halt-On-Fault (HOF) allows to fall in Halt automatically as soon a fault condition is detected.

It is recommended to start the computer with all of these switches inactive. Once the system is in normal execution, activate TMR and/or INT if your system is making use of those interrupts. The switch HOF is only used for debugging purposes.

Remember that in Halt mode, Timer and External interrupts will be disabled automatically; this is independent from the TMR and/or INT switches positions. When  execution is resumed (either in Normal or Step mode) interrupts will be available or not according with the TMR and INT switches positions.

Software interrupts cannot be defeated from the Console.