COMISS (Compare Scalar Ordered Single-Precision Floating-Point Values and Set EFLAGS)

Opcodes

Opcode/Instruction Op/En 64/32-bit Mode CPUID Feature Flag Description
0F 2F /r COMISS xmm1, xmm2/m32 RM V/V SSE Compare low single-precision floating-point values in xmm1 and xmm2/mem32 and set the EFLAGS flags accordingly.
VEX.LIG.0F.WIG 2F /r VCOMISS xmm1, xmm2/m32 RM V/V AVX Compare low single precision floating-point values in xmm1 and xmm2/mem32 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 single-precision floating-point values in the low doublewords 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 32 bit memory location.

The COMISS instruction differs from the UCOMISS instruction in that it signals a SIMD floating-point invalid opera-tion 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.

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(SRC1[31:0] ≠ SRC2[31:0]) {
(* 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_ss (__m128 a, __m128 b)

int _mm_comilt_ss (__m128 a, __m128 b)

int _mm_comile_ss (__m128 a, __m128 b)

int _mm_comigt_ss (__m128 a, __m128 b)

int _mm_comige_ss (__m128 a, __m128 b)

int _mm_comineq_ss (__m128 a, __m128 b)

SIMD Floating-Point Exceptions

Invalid (if SNaN or QNaN operands), Denormal.

Other Exceptions

See Exceptions Type 3; additionally

#UD If VEX.vvvv ≠ 1111B.