Subject: Microprocessors and Applications

Topic: 80806 Interrupts

Difficulty: Medium

Interrupt :- An interrupt is an event that alters the sequence in which the processor executes instructions.

An interrupt might be planned (specifically requested by the currently running program) or unplanned (caused by an event that might or might not be related to the currently running program). z/OS® uses six types of interrupts, as follows:

• Supervisor calls or SVC interrupts - These interrupts occur when the program issues an SVC to request a particular system service. An SVC interrupts the program being executed and passes control to the supervisor so that it can perform the service. Programs request these services through macros such as OPEN (open a file), GETMAIN (obtain storage), or WTO (write a message to the system operator).

• I/O interrupts - These interrupts occur when the channel subsystem signals a change of status, such as an input/output (I/O) operation completing, an error occurring, or an I/O device such as a printer has become ready for work.

• External interrupts - These interrupts can indicate any of several events, such as a time interval expiring, the operator pressing the interrupt key on the console, or the processor receiving a signal from another processor.

• Restart interrupts - These interrupts occur when the operator selects the restart function at the console or when a restart SIGP (signal processor) instruction is received from another processor.

• Program interrupts - These interrupts are caused by program errors (for example, the program attempts to perform an invalid operation), page faults (the program references a page that is not in central storage), or requests to monitor an event.

• Machine check interrupts - These interrupts are caused by machine malfunctions. When an interrupt occurs, the hardware saves pertinent information about the program that was interrupted and, if possible, disables the processor for further interrupts of the same type. The hardware then routes control to the appropriate interrupt handler routine. The program status word or PSW is a key resource in this process.

Applications of interrupts :-

These are powerful inputs typically used to -

• Service hardware timers, handle keyboard strokes and mouse actions
• Respond quickly to time-sensitive or real-time events
• Data transfer to and from peripheral devices
• Responds to high-priority tasks such as power-down signals, traps, and watchdog timers
• Indicates abnormal events of CPU
• Power-off interrupt foresees the loss of power, enabling orderly shut-down of the system
• Periodic interrupts to keep track of absolute time