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There's some history for this instruction. It is not in the official instruction documentations from AMD or Intel but it is supported on almost every x87 supporting CPU (I think there was some Cyrix that missed it?) This was originally implemented by accident on the Intel 80287 CPU. It was then periodically documented in random documentation. The instruction behaves exactly like FFREE but then pops the stack after the operation. For our current implementation of FFREE, which is a NOP, we just need to pop the stack.