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The Interrupt Enabled condition

The necessity of recording the Interrupt Enabled condition as a flag has to do with nested interrupts. When an interrupt service routine (lets call it: ISR1) gets interrupted by a highest priority interrupt (ISR2), the Interrupt Controller disables interrupts before passing control to ISR2. If this routine failed to enable interrupts back, then interrupts would remain disabled when control returns to ISR1, which is not the intended behavior. But, by remembering the Interrupt Enable condition in the Flags register, interrupts will be automatically restored upon return since F had been saved to the Stack.

Thus there is a flip-flop in F for the Interrupt Enable condition, only that its input (SI) and output (CSI) are not wired with the S-BUS and CS-BUS respectively but treated as point-to-point connections. This is because only a few circuits will make use of them; those circuits are: the Interrupt Controller, the IDS card responsible for the DI and EI instructions and the Control Panel.