I/O Handling Methods: Interrupt-Driven I/O

2. Most CPUs have two types of interrupt request lines: a nonmaskable one and a maskable one, each of which is used to transfer signals for two types of interrupts. A nonmaskable interrupt is one that a CPU cannot ignore, and must take measures in order to handle the interrupt. Events that could issue a nonmaskable interrupt are usually unrecoverable memory errors, such as division by zero. A maskable interrupt might be ignored by the CPU when critical OS code that must not be interrupted is executed. While executing such critical code, the CPU would turn off the maskable interrupt request lines, so that no interrupt signals are registered while such code is running. In the diagram on the previous slide, all I/O-related interrupts have numbers ranging from 32 to 255. This is because all the described interrupts in numbers 0 to 31 are nonmaskable, while I/O interrupts are maskable and can be ignored by the CPU when needed.