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Internal Interrupts

Internal interrupts can be generated by Software, from the Timer or when a Fault condition is detected. All internal interrupts are vectored (8-bits vector).

Vectors occupy all together the first 256-words block of the addressable space: 0x0000-0x00ff. Vector 0 is at 0x0000, vector 1 is at 0x0001 and so on. Each location contains the actual address of the corresponding Interrupt Service Routine.

Certain circuits in the CPU are capable of detecting fault conditions such as illegal instruction. When a Fault is detected, the Interrupt Controller is notified. It reacts by taking control of the CPU for saving PC and F to the stack, then loading the Program Counter (PC) with the content of memory location 0x0001. This behavior, however, can be changed from the computer Console so the computer simply halts on Fault.

This interrupt is not affected by the DI instruction.

The Timer is located in the Interrupt Controller. It is a simple counter triggered from the System Clock and wired to produce interrupts at long intervals (for example, 1 ms) set by DIP switches.

The address for the Timer Service routine is indicated in memory location 0x0000. When the timer interrupts, the Interrupt Controller loads the Program Counter (PC) with the content of that memory location (after have saved PC and F to the Stack).

This interrupt is not affected by the DI instruction but it can be disabled from the Console.

The instruction INT generates a Software Interrupt. This instructions takes one argument (embedded within the operational code) which is the 8-bits interrupt vector.

Notice that address 0x0000 is the one for the Timer and 0x0001 is the one for Fault. This means that both Timer and Fault interrupts can be simulated by software (INT 0 and INT 1 respectively).