Interrupts

As you know from our previous discussion, there are many circumstances under which it is important to interrupt the normal flow of a program in the computer to react to special events. An unexpected user command from the keyboard or other external input, an abnormal situation, such as a power failure, that requires immediate attention from the computer, an attempt to execute an illegal instruction, a request for service from a network controller, or the completion of an I/O task initiated by the program: all of these suggest that it is necessary to include some means to allow the computer to take special actions when required. Interrupt capabilities are also used to make it possible to time share the CPU between several different programs or program segments at once.

Modern computers provide interrupt capability by providing one or more special control lines to the central processor known as interrupt lines. For example, the standard I/O for a modern PC may contain as many as thirty-two interrupt lines, labeled IRQ0 through IRQ31. (IRQ stands for Interrupt ReQuest.) The messages sent to the computer on these lines are known as interrupts. The presence of a message on an interrupt line will cause the computer to suspend the program being executed and jump to a special interrupt processing program.(ISR- Interrupt Service Routine).

For Example, in a large, multiuser system there may be hundreds of keyboards being used with the computer at any given time. Since any of these keyboards could generate input to the computer at any time, it is necessary that the computer be aware of any key that is struck from any keyboard in use. This process must take place quickly, before another key is struck on the same keyboard, to prevent data loss from occurring when the second input is generated.

Theoretically, though impractically, it would be possible for the computer to perform this task by checking each keyboard for input in rotation, at frequent intervals. This technique is known as polling. The interval would have to be shorter than the time during which a fast typist could hit another key. Since there may be hundreds of keyboards in use, this technique may result in a polling rate of thousands of samples per second. Most of these samples will not result in new data; therefore, the computer time spent in polling is largely wasted.

This is a situation for which the concept of the interrupt is well suited. The goal is achieved more productively by allowing the keyboard to notify the CPU by using an interrupt when it has input. When a key is struck on any keyboard, it causes the interrupt line to be activated, so that the CPU knows that an I/O device connected to the interrupt line requires action. Interrupts satisfy the requirement for external input controls, and also provide the desirable feature of freeing the CPU from waiting for events to occur.

Servicing Interrupts

Since the computer is capable only of executing programs, interrupt actions take the form of special programs, executed whenever triggered by an interrupt signal. Interrupt procedures follow the form shown in Figure 9.5.

Specifically, the interrupt causes the temporary suspension of the program in progress. All the pertinent information about the program being suspended, including the location of the last instruction executed, and the values of data in various registers, is saved in a known part of memory, either in a special area associated with the program, known as the process control block (PCB), or in a part of memory known as the stack area. This information is known as the program’s context, and will make it possible to restart the program exactly where it left off, without loss of any data or program state. Many computers have a single instruction that saves all the critical information at once. The memory belonging to the original program is kept intact. The computer then branches to a special interrupt handler program elsewhere in memory; the interrupt handler program is also known as an interrupt routine. The interrupt handler program determines the appropriate course of action. This process is known as servicing the interrupt. Since many interrupts exist to support I/O devices, most of the interrupt handling programs are also known as device drivers.

When the interrupt routine completes its task, it normally would return control to the interrupted program, much like a subroutine. Original register values would be restored, and the original program would resume execution exactly where it left off, and in its identical state, since all the registers were restored to their original values. There are some circumstances when this is not the case, however, since actions taken by the interrupt routine may make a difference in what the original program is supposed to do. For example, a printer interrupt indicating that the printer is out of paper would require a different action by the original program (perhaps a message to the screen telling the user to load more paper); it would not be useful for the program to send more characters!

SOFTWARE INTERRUPTS In addition to the actual hardware interrupts already discussed, modern CPU instruction sets include an instruction that simulates an interrupt. In the Intel x86 architecture, for example, this instruction has the mnemonic INT, for INTerrupt. The IBM System z uses the mnemonic SVC for SUPERVISOR CALL. The interrupt instruction works in the same way as a hardware interrupt, saving appropriate registers and transferring control to an interrupt handling procedure. The address space of the INT instruction can be used to provide a parameter that specifies which interrupt is to be executed. The software interrupt is very similar to a subroutine jump to a known, fixed location.

Software interrupts make the interrupt routines available for use by other programs. Programs can access these routines simply by executing the INT instruction with the appropriate parameter.

An important application for software interrupts is to centralize I/O operations. One way to assure that multiple programs do not unintentionally alter another program’s files or intermingle printer output is to provide a single path for I/O to each device. Generally, the I/O paths are interrupt routines that are a part of the operating system software. Software interrupts are used by each program to request I/O from the operating system software. As an example, a software interrupt was used in Figure 9.7 to initiate printing.