The "Smrnmi" Extension for Resumable Non-Maskable Interrupts adds support for a resumable non-maskable interrupt (RNMI) to the processor. The extension adds four new CSRs (mnepc, mncause, mnstatus, and mnscratch) to hold the interrupted state, and one new instruction, MNRET, to resume from the RNMI handler.
Two custom CSRs (mnivec and mnevec) are also provided for the interrupt and exception handler vector base-addresses.
The extension is only enabled when C_TRAP_ENHANCEMENT > 1 and C_USE_MMU < 2.