Compares the contents of register ST(0) and source value and sets condition code flags C0, C2, and C3 in the FPU status word according to the results (see the table below). The source operand can be a data register or a memory location. If no source operand is given, the value in ST(0) is compared with the value in ST(1). The sign of zero is ignored, so that -0.0 is equal to +0.0.
|ST(0) > Source||0||0||0|
|ST(0) < Source||0||0||1|
|ST(0) = Source||1||0||0|
|NOTE: * Flags not set if unmasked invalid-arithmetic-operand (#IA) exception is generated.|
This instruction checks the class of the numbers being compared (see "FXAM-Examine" in this chapter). If either operand is a NaN or is in an unsupported format, an invalid-arithmeticoperand exception (#IA) is raised and, if the exception is masked, the condition flags are set to "unordered." If the invalid-arithmetic-operand exception is unmasked, the condition code flags are not set.
The FCOMP instruction pops the register stack following the comparison operation and the FCOMPP instruction pops the register stack twice following the comparison operation. To pop the register stack, the processor marks the ST(0) register as empty and increments the stack pointer (TOP) by 1.
The FCOM instructions perform the same operation as the FUCOM instructions. The only difference is how they handle QNaN operands. The FCOM instructions raise an invalid-arithmetic- operand exception (#IA) when either or both of the operands is a NaN value or is in an unsupported format. The FUCOM instructions perform the same operation as the FCOM instructions, except that they do not generate an invalid-arithmetic-operand exception for QNaNs.