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int 21

A multipurpose DOS interrupt used for various functions including reading the keyboard and writing to the console and printer. It was also used to read and write disks using the earlier File Control Block (FCB) method. See interrupt.


A signal that gets the attention of the CPU and is usually generated when I/O is required. For example, hardware interrupts are generated when a key is pressed or when the mouse is moved. Software interrupts are generated by a program requiring disk input or output.

An internal timer may continually interrupt the computer several times per second to keep the time of day current or for timesharing purposes.

When an interrupt occurs, control is transferred to the operating system, which determines the action to be taken. Interrupts are prioritized; the higher the priority, the faster the interrupt will be serviced.

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