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Vol. 2A 3-159
INSTRUCTION SET REFERENCE, A-M
COMISS—Compare Scalar Ordered Single-Precision Floating-Point Values and Set EFLAGS
COMISS—Compare Scalar Ordered Single-Precision Floating-Point
Values and Set EFLAGS
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 (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).
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 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.
In 64-bit mode, use of the REX.R prefix permits this instruction to access additional
registers (XMM8-XMM15).
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)
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
0F 2F /r COMISS xmm1,
xmm2/m32
Valid Valid Compare low single-precision
floating-point values in xmm1 and
xmm2/mem32 and set the EFLAGS
flags accordingly.