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Non-Maskable Interrupt
From SnesLab
A non-maskable interrupt (NMI) is an interrupt that cannot be ignored. The SNES normally generates an NMI at the beginning of the vertical blanking period, which is received by the Ricoh CPU.
NMIs have higher priority than IRQs, but lower priority than Reset.
References
- 7.19 Interrupt Priorities, page 53: https://www.westerndesigncenter.com/wdc/documentation/w65c816s.pdf