x86 Instruction Set Reference

Compares the single-precision floating-point values in the low doublewords of source operand 1 (first operand) and source operand 2 (second operand), and sets the ZF, PF, and CF flags in the EFLAGS register according to the result (unordered, greater than, less than, or equal). The OF, SF, and AF flags in the EFLAGS register are set to 0. The unordered result is returned if either source operand is a NaN (QNaN or SNaN).

Source operand 1 is an XMM register; source operand 2 can be an XMM register or a 32 bit memory location.

The COMISS instruction differs from the UCOMISS instruction in that it signals an SIMD floating-point invalid operation exception (#I) when a source operand is either a QNaN or SNaN. The UCOMISS instruction signals an invalid numeric exception only if a source operand is an SNaN.

The EFLAGS register is not updated if an unmasked SIMD floating-point exception is generated.

Result = OrderedCompare(Source1[0..31], Source2[0..31]);
switch(Result) {
	case ResultUnordered:
		ZF = 1;
		PF = 1;
		CF = 1;
		break;
	case ResultGreaterThan:
		ZF = 0;
		PF = 0;
		CF = 0;
		break;
	case ResultLessThan:
		ZF = 0;
		PF = 0;
		CF = 1;
		break;
	case ResultEqual:
		ZF = 1;
		PF = 0;
		CF = 0;
		break;
}
OF = 0;
AF = 0;
SF = 0;