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STAC

Sets the Alignment Check (AC) flag in the EFLAGS register to 1.

The table after the description covers what the source and destinations can be.

DO NOT support LOCK

The instruction is available in 64-bit mode and compatibility mode. It does not require any specific architectural constraints other than being executed in a mode where the EFLAGS register is accessible.

The AC flag only triggers an alignment check exception (#AC) if the Alignment Check enable bit (CR0.AC) is also set to 1. If CR0.AC is 0, executing STAC will set the flag, but no alignment checks will occur during memory accesses.