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UMWAIT

The instruction suspends the execution of the processor until a specified time period has elapsed, a specified event occurs, or an interrupt is received. It allows the processor to enter a low-power state while waiting for a condition to be met.

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

DO NOT support LOCK

The UMWAIT instruction SHALL only be executed when the WAITPKG feature is supported by the processor and enabled. It is available in 64-bit mode. Execution of UMWAIT in compatibility mode or 32-bit mode may result in an invalid opcode exception if the feature is not explicitly supported for those modes.

To avoid unexpected behavior or immediate wake-up, the user SHALL ensure that the CPL (Current Privilege Level) is 3; executing UMWAIT at a higher privilege level (CPL < 3) may result in the instruction being treated as a NOP or triggering a general protection fault depending on the processor implementation. Additionally, the user SHOULD be aware that the time delay is not precise and is subject to the processor's power management state and clock frequency.