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RDRAND

Loads a random number from the hardware random number generator into the destination operand.

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

DO NOT support LOCK

The instruction is available in 32-bit and 64-bit operating modes. It is not supported in 16-bit mode. The instruction is only available on processors that implement the RDRAND instruction; execution on unsupported processors results in an invalid opcode exception.

The instruction sets the Carry Flag (CF) to 1 if the hardware random number generator failed to provide a number. The software SHALL check the Carry Flag to verify that a random value was successfully returned. If CF is set, the destination register contains undefined data, and the software SHOULD retry the operation.

The hardware random number generator may have a limited entropy rate. If the instruction is executed in a tight loop without any other instructions, it may return CF=1 once the internal entropy pool is exhausted.