4.2: Mechanisms of interruption- recognition of vector, and interrupt priority
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The following section introduces the learner to Interruptions that occur in Programmed I/O
In system programming, an interrupt is a signal to the processor emitted by hardware or software indicating an event that needs immediate attention. An interrupt is a signal from a device attached to a computer or from a program within the computer that causes the main program that operates the computer (the operating system) to stop and figure out what to do next. An interrupt alerts the processor to a high-priority condition requiring the interruption of the current code the processor is executing. The processor responds by suspending its current activities, saving its state, and executing a function called an interrupt handler (or an interrupt service routine, ISR) to deal with the event. This interruption is temporary, and, after the interrupt handler finishes, the processor resumes normal activities. There are two types of interrupts: hardware interrupts and software interrupts.
Hardware interrupts are used by devices to communicate that they require attention from the operating system. Internally, hardware interrupts are implemented using electronic alerting signals that are sent to the processor from an external device, which is either a part of the computer itself, such as a disk controller, or an external peripheral.
For example, pressing a key on the keyboard or moving the mouse triggers hardware interrupts that cause the processor to read the keystroke or mouse position. Unlike the software type (described below), hardware interrupts are asynchronous and can occur in the middle of instruction execution, requiring additional care in programming. The act of initiating a hardware interrupt is referred to as an interrupt (IRQ).
A software interrupt is caused either by an exceptional condition in the processor itself, or a special instruction in the instruction which causes an interrupt when it is executed. The former is often called a trap or exception and is used for errors or events occurring during program executions that are exceptional enough that they cannot be handled within the program itself.
For example, if the processor’s arithmetic logic unit is commanded to divide a number by zero, this impossible demand will cause a divide-by-zero exception, perhaps causing the computer to abandon the calculation or display an error message. Software interrupt instructions function similarly to subroutine calls and are used for a variety of purposes, such as to request services from low-level system software such as device drivers. For example, computers often use software interrupt instructions to communicate with the disk controller to request data be read or written to the disk.
Each interrupt has its own interrupt handler. The number of hardware interrupts is limited by the number of interrupt request (IRQ) lines to the processor, but there may be hundreds of different software interrupts. Interrupts are a commonly used technique for computer multitasking, especially in real-time computing. Such a system is said to be interrupt-driven
Interrupts can be categorized into these different types:
- Maskable interrupt (IRQ): a hardware interrupt that may be ignored by setting a bit in an interrupt mask register’s (IMR) bit-mask.
- Non-maskable interrupt (NMI): a hardware interrupt that lacks an associated bit- mask, so that it can never be ignored. NMIs are used for the highest priority tasks such as timers, especially watchdog timers.
- Inter-processor interrupt (IPI): a special case of interrupt that is generated by one processor to interrupt another processor in a multiprocessor system.
- Software interrupt: an interrupt generated within a processor by executing an instruction. Software interrupts are often used to implement system calls because they result in a subroutine call with a CPU ring level change.
- Spurious interrupt: a hardware interrupt that is unwanted. They are typically generated by system conditions such as electrical interference on an interrupt line or through incorrectly designed hardware.
This section has introduced learners to the different categories of I/O interrupts, that is the hardware and software interrupts
Briefly describe hardware and software Interruptions
Hardware interruptions are generated by certain events which come up during the execution of a program. This type of interruptions is managed on their totality by the hardware and it is not possible to modify them.
A clear example of this type of interruptions is the one which actualizes the counter of the computer internal clock, the hardware makes the call to this interruption several times during a second in order to maintain the time up to date.
External interruptions are generated by peripheral devices, such as keyboards, printers, communication cards, etc. They are also generated by coprocessors. It is not possible to deactivate external interruptions.
These interruptions are not sent directly to the CPU but they are sent to an integrated circuit whose function is to exclusively handle this type of interruptions
Software interruptions can be directly activated by the assembler invoking the number of the desired interruption with the INT Instruction.
The use of interruptions helps us in the creation of programs and by using them our programs gets shorter. It is easier to understand them and they usually have a better performance mostly due to their smaller size. This type of interruptions can be separated in two categories: the operative system DOS interruptions and the BIOS interruptions.