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Interrupts can be categorized into these different types: Processors typically have an internal interrupt mask which allows the software to ignore all external hardware interrupts while it is set.
Setting or clearing this mask may be faster than accessing an interrupt mask register (IMR) in a PIC or disabling interrupts in the device itself.
Different devices are usually associated with different interrupts using a unique value associated with each interrupt.
This makes it possible to know which hardware device caused which interrupts.
The MIT Lincoln Laboratory TX-2 system (1957) was the first to provide multiple levels of priority interrupts.
A level-triggered interrupt is an interrupt signaled by maintaining the interrupt line at a high or low logic level.
Hardware interrupts were introduced as an optimization, eliminating unproductive waiting time in polling loops, waiting for external events.
These interrupt values are often called IRQ lines, or just interrupt lines. Unlike interrupts, exceptions occur synchronously with respect to the processor clock.
That is why sometimes they are referred to as synchronous interrupts.
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.
If implemented in hardware as a distinct component, an interrupt controller circuit such as the IBM PC's Programmable Interrupt Controller (PIC) may be connected between the interrupting device and the processor's interrupt pin to multiplex several sources of interrupt onto the one or two CPU lines typically available.