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.

The program status word (PSW) is a 128-bit data area in the processor that, along with a variety of other types of registers (control registers, timing registers, and prefix registers) provides details crucial to both the hardware and the software. The current PSW includes the address of the next program instruction and control information about the program that is running. Each processor has only one current PSW. Thus, only one task can execute on a processor at a time.

The PSW controls the order in which instructions are fed to the processor, and indicates the status of the system in relation to the currently running program. Although each processor has only one PSW, it is useful to think of three types of PSWs to understand interrupt processing:

• Current PSW
• New PSW
• Old PSW

The current PSW indicates the next instruction to be executed. It also indicates whether the processor is enabled or disabled for I/O interrupts, external interrupts, machine check interrupts, and certain program interrupts. When the processor is enabled, these interrupts can occur. When the processor is disabled, these interrupts are ignored or remain pending.

There is a new PSW and an old PSW associated with each of the six types of interrupts. The new PSW contains the address of the routine that can process its associated interrupt. If the processor is enabled for interrupts when an interrupt occurs, PSWs are switched using the following technique:

1. Storing the current PSW in the old PSW associated with the type of interrupt that occurred.
2. Loading the contents of the new PSW for the type of interrupt that occurred into the current PSW.

The current PSW, which indicates the next instruction to be executed, now contains the address of the appropriate routine to handle the interrupt. This switch has the effect of transferring control to the appropriate interrupt handling routine.

Mainframe architecture provides registers to keep track of things. The PSW, for example, is a register used to contain information that is required for the execution of the currently active program.

Mainframes provide other registers, as follows:

Access registers

These registers specify the address space in which data is found.

General registers

These registers address data in storage, and also hold user data.

Floating point registers

These registers hold numeric data in floating point form.

Control registers

These registers are used by the operating system itself, for example, as references to translation tables.

Figure 1 shows the use of registers and the PSW.

Figure 1. Registers and the PSW