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

Opcodes

Opcode/Instruction Op/En 64/32-bit Mode CPUID Feature Flag Description
66 0F 2F /r COMISD xmm1, xmm2/m64 RM V/V SSE2 Compare low double-precision floating-point values in xmm1 and xmm2/mem64 and set the EFLAGS flags accordingly.
VEX.LIG.66.0F.WIG 2F /r VCOMISD xmm1, xmm2/m64 RM V/V AVX Compare low double precision floating-point values in xmm1 and xmm2/mem64 and set the EFLAGS flags accordingly.

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
RM ModRM:reg (r) ModRM:r/m (r) NA NA

Description

Compares the double-precision floating-point values in the low quadwords of operand 1 (first operand) and operand 2 (second operand), and sets the ZF, PF, and CF flags in the EFLAGS register according to the result (unor-dered, greater than, less than, or equal). The OF, SF and AF flags in the EFLAGS register are set to 0. The unor-dered result is returned if either source operand is a NaN (QNaN or SNaN).The sign of zero is ignored for comparisons, so that –0.0 is equal to +0.0.

Operand 1 is an XMM register; 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 a SIMD floating-point invalid oper-ation 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.

In 64-bit mode, use of the REX.R prefix permits this instruction to access additional registers (XMM8-XMM15).

Note: In VEX-encoded versions, VEX.vvvv is reserved and must be 1111b, otherwise instructions will #UD.

Operation

RESULT ← OrderedCompare(DEST[63:0] ≠ SRC[63:0]) {
(* Set EFLAGS *) CASE (RESULT) OF
    UNORDERED:
                              ZF,PF,CF ← 111;
    GREATER_THAN:
                              ZF,PF,CF ← 000;
    LESS_THAN:
                              ZF,PF,CF ← 001;
    EQUAL:
                              ZF,PF,CF ← 100;
ESAC;
OF, AF, SF ← 0; }

Intel C/C++ Compiler Intrinsic Equivalents

int _mm_comieq_sd (__m128d a, __m128d b)

int _mm_comilt_sd (__m128d a, __m128d b)

int _mm_comile_sd (__m128d a, __m128d b)

int _mm_comigt_sd (__m128d a, __m128d b)

int _mm_comige_sd (__m128d a, __m128d b)

int _mm_comineq_sd (__m128d a, __m128d b)

SIMD Floating-Point Exceptions

Invalid (if SNaN or QNaN operands), Denormal.

Other Exceptions

See Exceptions Type 3; additionally

#UD If VEX.vvvv ≠ 1111B.