IFF2 stores the previous state of IFF1 so that after the NMI is handled, IFF1 can be restored to its original state.
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| WZ || 16-bit || Internal temporary register pair. Also known as MEMPTR || Used for memory and address calculations.Normally, you never see the content of this register. But it leaks the info through in case of the BIT b,(HL) instruction.
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| Q || 8-bit || Internal register where it assembles the new content of the F register, before moving it back to F || When the instruction doesn't modify the flags, this register is reset instead (or in case of NEC CPUs, set to some partially undefined values).