If either operand is an SNaN or is in an undefined format, or if a stack fault occurs, the invalid-operation exception is raised, and the condition bits are set to "unordered."
If either operand is a QNaN, the condition bits are set to "unordered." Unlike the ordinary compare instructions (FCOM, etc.), the unordered compare instructions do not raise the invalid-operation exception on account of a QNaN operand.
The sign of zero is ignored, so that -0.0=+0.0.