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In computing, a non-maskable interrupt (NMI) is a hardware interrupt that standard interrupt-masking techniques in the system cannot ignore. It typically occurs to signal attention for non-recoverable hardware errors. Some NMIs may be masked, but only by using proprietary methods specific to the particular NMI.
A hardware interrupt is a condition related to the state of the hardware that may be signaled by an external hardware device, e.g., an interrupt request (IRQ) line on a PC, or detected by devices embedded in processor logic (e.g., the CPU timer in IBM System/370), to communicate that the device needs attention from the operating system (OS) [7] or, if there is no OS, from the bare metal ...
During normal operation the computer regularly kicks Stage1 to prevent a timeout. If the computer fails to kick Stage1 (e.g., due to a hardware fault or programming error), Stage1 will eventually timeout. This event will start the Stage2 timer and, simultaneously, notify the computer (by means of a non-maskable interrupt) that a reset is imminent.
An interrupt control register, or ICR, is a hardware register in a computer chip used to configure the chip to generate interrupts—to raise a signal on an interrupt line—in response to some event occurring within the chip or a circuit connected to the chip.
The detection of an NMI or IRQ signal, as well as the execution of a BRK instruction, will cause the same overall sequence of events, which are, in order: [1] [3]. The processor completes the current instruction and updates registers or memory as required before responding to the interrupt.
This allows known-good stacks to be used in case of serious errors (NMI or Double fault for example). Previously, the entry for the exception or interrupt in the IDT pointed to a task gate, causing the processor to switch to the task that is pointed by the task gate.
When an interrupt occurs, the processor multiplies the interrupt vector by the entry size (8 for protected mode, 16 for long mode) and adds the result to the IDT base address. [4] If the address is inside the table, the DPL is checked and the interrupt is handled based on the gate type.
The Interrupt flag (IF) is a flag bit in the CPU's FLAGS register, which determines whether or not the (CPU) will respond immediately to maskable hardware interrupts. [1] If the flag is set to 1 maskable interrupts are enabled. If reset (set to 0) such interrupts will be disabled until