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Chapter 24 Interrupt Control
5.Operation
5. Operation
The following section explains priority determination operation of interrupt control.
■ Priority determination
• The interrupt control circuit selects the highest priority factor from those that have been generated
simultaneously, and outputs the factor's interrupt level (ICR) and interrupt number (#) to the CPU.
• The priority level criteria of an interrupt cause are the following conditions.
• The value of the interrupt level is not 31. (31 is “interrupt disable”)
• The factors with the smallest interrupt level.
• Among these, the factor that has the smallest interrupt number.
• If nothing is applicable by the above-mentioned criteria, interrupt level 31 (11111
B
) is sent to the CPU. In this
case, the interrupt number is indeterminate.
CPU processing
Interrupt cause generated
The interrupt request flag is set to “1”.
The interrupt request is transmitted to
the interrupt control circuit.
Are interrupt requests enabled?
The interrupt level and the interrupt number
are transmitted to the CPU.
Is not the corresponding interrupt disabled?
(ICR) < 31?
Which interrupt has the lowest level
among the interrupt requests?
Which interrupt has the lowest
number (#) among the lowest
interrupt requests?
Interrupt control circuit
Interrupt level = 31
Interrupt number
= indeterminate
Wait until the executed instructions finish
Is the interrupt level higher than
the interrupt mask level?
(ICR) < (ILM)
Transition processing to interrupts
- Save to the system stack (PS and PC)
- Set an interrupt level to ILM
- System stack enabled
- Branch to the interrupt routine
(PC <= interrupt vector)
The Flow of the Interrupt Process
Are interrupts enabled?
I flag = 1
YES
YES
The lowest interrupts
The interrupt with
the lowest number
YES
YES
NO
NO
NO
Priority determination