nonmaskable

Nonmaskable is an adjective describing something that cannot be masked or suppressed. In computing, the term is commonly applied to nonmaskable interrupts (NMIs), which are hardware signals that cannot be disabled by the normal interrupt masking mechanisms.

In most CPU architectures, interrupt handling differentiates between maskable interrupts, which software can disable or enable, and non-maskable interrupts, which have the highest priority and bypass the standard interrupt mask. The NMI line or signal preempts current execution and transfers control to a dedicated handler. While some designs may temporarily inhibit NMIs in exceptional circumstances, they are not typically controlled by ordinary software masks.

Typical purposes for NMIs include reporting critical hardware errors (such as memory parity or ECC errors), power or reset events, watchdog timer expiration, or other system-critical conditions. In x86-based PCs, the NMI is a well-known mechanism used to signal hardware faults and can invoke a debugger or crash dump; many motherboards provide a physical NMI button or jumper for debugging or recovery.

Implementation and behavior vary by architecture. Some systems allow limited masking or gating of NMIs in rare situations, but in general the NMI handler must be robust and fast, since it interrupts high-priority operations. Handling usually involves minimal work in the interrupt routine and a swift path to a specialized NMI handler, with the system either resuming normal operation or entering a safe state if required.

See also: non-maskable interrupt, maskable interrupt, machine check, parity error, watchdog timer.