| IFF1 || 1-bit || Interrupt enable flag || Set when interrupts are enabled, cleared on disable
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| IFF2 || 1-bit || Backup of IFF1 during NMIs buffer || Restores IFF1 Allows interrupts to be enabled after returning from an NMIthe instruction following EI
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| WZ || 16-bit || Internal temporary register pair || Used for memory and address calculations (not user-accessible)