NMI Blue Screen

Ever had a blue screen pop up with a message like this?

*** Hardware Malfunction

Call your hardware vendor for support

*** The system has halted ***

Windows makes this screen look something like this:

There are a couple of other specific errors that can sometimes pop up in these messages:

NMI: Parity Check / Memory Parity Error
NMI: Channel Check / IOCHK

Note: There are some other assorted specific errors you can get on this screen outside of the two above, however they're only possible if you're on a machine with an EISA bus.

As the message indicates, you've got a hardware problem of some sort. These screens are generated when your system encounters a Non Maskable Interrupt (NMI). The long and short of it is that NMIs are hardware interrupts which have to be serviced immediately. Windows has a concept of IRQ levels, or IRQLs. The highest IRQL is always serviced, preempting any lower level interrupts which are currently being serviced. The preemptive behavior here is called masking the interrupt. So, an NMI is an interrupt which must be serviced immediately. Generally you get an NMI when there's a major hardware fault that prevents the operating system from continuing.

Some systems allow you to trigger an NMI either via a physical switch on the machine, or via something like an iLO.