Interrupts

Interrupt is one of the key concepts in computer programming and system design. Interrupt is basically a feature of a CPU that forces the processor to suspend the regular flow of instruction execution and to jump to a certain program address in response to some external event. The external event is usually a change in a hardware signal from outside of the CPU. The memory location (or, in some processors, an index in an array of memory locations) where the processor jumps to, is called the interrupt vector. The piece of code where the control goes after landing on the interrupt vector, is called the interrupt service routine, or ISR. After the ISR is executed, the control goes back to the program location that was executed by the processor when it received the interrupt.

The external event that causes the interrupt can be a tick of a timer clock. Such timer interrupts are useful to organize task switching in a system where several software tasks, or programs, share the same processor, which switches between them. Another example of an external event that causes an interrupt is a signal about the completion of an input/output operation. Such I/O interrupts are useful to offload the regular program flow from constant polling of I/O registers, checking their status, as shown on Figure 1.

Figure 1. The action of an I/O interrupt. The source of the figure: http://virtualirfan.com/history-of-interrupts.

Interrupt, sometimes called more specifically hardware interrupt, is a special case of a more general term «exception». An exception is suspending the regular instruction flow and jumping to a vector in response not only to an external signal, but also to some internal conditions of the CPU, the conditions that require immediate attention. Such conditions include errors: arithmetic overflows, accessing out of range address, running a privileged instruction in non-privileged (user) mode, bus errors, and other unusual conditions. Some of those errors should terminate the offending program, while others should cause the program to recover, though the action of the exception service routine. Some of those conditions, like memory address exceptions, may be not considered errors at all, but parts of the mechanics of virtual memory implementation. There are also so-called software interrupts, the exceptions that are intentionally caused by the program to request the services of the operating system. Exceptions are also used in the processor’s debug interface. During this lab we will deal strictly with «true» hardware interrupts. To learn about the other aspects of exceptions, please consult the core and architecture documentation.

Historically, the need for exceptions and interrupts was so obvious that they appeared very early in the history of computers, as shown on Figure 2.

Figure 2. The history of exceptions and interrupts. The source of the figure: http://virtualirfan.com/history-of-interrupts.

According to the article Interrupts by Mark Smotherman (https://people.cs.clemson.edu/~mark/interrupts.html#dyseac), the first computer that employed I/O interrupts was DYSEAC, the second version of SEAC, the Standards Electronic Automatic Computer. According to Wikipedia (https://en.wikipedia.org/wiki/DYSEAC), «DYSEAC was a first-generation computer built by the National Bureau of Standards for the US Army Signal Corps. It went into operation in April 1954». According to Smotherman, DYSEAC was perhaps also the first mobile computer, carried in two tractor trailers at 12 and 8 tons, as shown on Figure 3.

Figure 3. DYSEAC, the first computer with I/O interrupts. The source of the figure: http://ed-thelen.org/comp-hist.

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