These are classified as hardware interrupts or software interrupts, respectively. Interrupt signals may be issued in response to hardware or software events. The MIT Lincoln Laboratory TX-2 system (1957) was the first to provide multiple levels of priority interrupts. The IBM 704 was the first to use interrupts for debugging, with a "transfer trap", which could invoke a special routine when a branch instruction was encountered. The National Bureau of Standards DYSEAC (1954) was the first to use interrupts for I/O. Earlier, on the UNIVAC I (1951) "Arithmetic overflow either triggered the execution of a two-instruction fix-up routine at address 0, or, at the programmer's option, caused the computer to stop." The IBM 650 (1954) incorporated the first occurrence of interrupt masking. The UNIVAC 1103A computer is generally credited with the earliest use of interrupts in 1953. The first system to use this approach was the DYSEAC, completed in 1954, although earlier systems provided error trap functions. Hardware interrupts were introduced as an optimization, eliminating unproductive waiting time in polling loops, waiting for external events. Systems that use interrupts in these ways are said to be interrupt-driven. Interrupts are also commonly used to implement computer multitasking, especially in real-time computing. Interrupts are commonly used by hardware devices to indicate electronic or physical state changes that require time-sensitive attention. This interruption is often temporary, allowing the software to resume normal activities after the interrupt handler finishes, although the interrupt could instead indicate a fatal error. If the request is accepted, the processor will suspend its current activities, save its state, and execute a function called an interrupt handler (or an interrupt service routine, ISR) to deal with the event. In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to interrupt currently executing code (when permitted), so that the event can be processed in a timely manner.
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