x86 Instruction Set Reference

COMISD

Compare Scalar Ordered Double-Precision Floating- Point Values and Set EFLAGS

Opcode Mnemonic Description
66 0F 2F /r COMISD xmm1, xmm2/m64 Compare low double-precision floating-point values in xmm1 and xmm2/mem64 and set the EFLAGS flags accordingly.
Description

Compares the double-precision floating-point values in the low quadwords 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 64 bit memory location.

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

Operation
Result = OrderedCompare(Destination[0..63], Source[0..63]);
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;
SIMD Floating-Point Exceptions
Invalid (if SNaN or QNaN operands), Denormal.
Protected Mode Exceptions
#GP(0)For an illegal memory operand effective address in the CS, DS, ES, FS or GS segments.
#GP(0)For an illegal memory operand effective address in the CS, DS, ES, FS or GS segments.
#SS(0)For an illegal address in the SS segment.
#PF(fault-code)For a page fault.
#NMIf TS in CR0 is set.
#XMIf an unmasked SIMD floating-point exception and OSXMMEXCPT in CR4 is 1.
#UDIf an unmasked SIMD floating-point exception and OSXMMEXCPT in CR4 is 0. If EM in CR0 is set. If OSFXSR in CR4 is 0. If CPUID feature flag SSE2 is 0.
Real-Address Mode Exceptions
GP(0)If any part of the operand lies outside the effective address space from 0 to FFFFH.
GP(0)If any part of the operand lies outside the effective address space from 0 to FFFFH.
#NMIf TS in CR0 is set.
#XMIf an unmasked SIMD floating-point exception and OSXMMEXCPT in CR4 is 1.
Virtual-8086 Mode Exceptions
Same exceptions as in Real Address Mode
#PF(fault-code)For a page fault.
#PF(fault-code)For a page fault.
Instruction Latency Throughput Execution Unit
CPUID0F3n/0F2n/069n0F3n/0F2n/069n0F2n
COMISD xmm, xmm7/6/12/2/1FP_ADD FP_MISC