To clarify what Sour was saying (because your response is a bit confusing):
The 6502's
i flag (controlled with instructions like
sei/cli or
plp)
does not affect NMI. NMI stands for "non-maskable interrupt", which in this case means it's an externally-generated hardware interrupt. The
i bit can only be used to inhibit what's coming on the IRQ line/wire. (You should be familiar with this from the fact that the 6502 provides an NMI vector as well as an IRQ/BRK vector. If you want to know more about the later, just ask.)
Don't confuse IRQ and NMI. They're separate things (physically separate pins on the 6502 CPU, thus physically separate traces/wires). The confusion lies in the fact that both are types of interrupts. :-)
Like many video games systems, the NES ties/wires PPU VBlank to NMI, and provides an MMIO register to control that behaviour:
bit 7 of $2000. It sounds like what Mesen is showing you in the info box is just whether or not bit 7 of $2000 is set or not.
Mesen generates an NMI when the emulator reaches/hits scanline 241 (which is when VBlank starts on the NES; see
PPU rendering).
If you're programming on the NES or trying to figure it out: be aware that if you write to $2000, you are also affecting some very critical internal PPU addressing bits (particularly through bits 1 and 0 of $2000), which can affect scrolling, despite not being immediately obvious. That brings into focus
this document which will cause you to pull your hair out in confusion. :)
(Related-yet-not: the one thing I've seen several emulators offer, which I've never understood the purpose of, is what Mesen has under Debug -> PPU Viewer ->
When emulation is running, show PPU data at scanline X and cycle Y where you can select X and Y yourself, defaulting to 241 and 0. I don't know what "show PPU data" means in this context. FCEUX has something under Debug -> PPU Viewer called
Display on scanline: X where X defaults to 0. It also has the same thing under Name Table Viewer.)