comiss
Compare Scalar Ordered Single-Precision
Compares low float and sets EFLAGS (Signaling NaN raises exception).
Pseudocode Operation
src1_val ← src1[0:31] (as float); src2_val ← src2[0:31] (as float); if (IsNaN(src1_val) || IsNaN(src2_val)) { ZF ← 1; PF ← 1; CF ← 1; OF ← 0; AF ← 0; SF ← 0; } else if (src1_val < src2_val) { CF ← 1; ZF ← 0; PF ← 0; OF ← 0; AF ← 0; SF ← 0; } else if (src1_val == src2_val) { ZF ← 1; CF ← 0; PF ← 0; OF ← 0; AF ← 0; SF ← 0; } else { ZF ← 0; CF ← 0; PF ← 0; OF ← 0; AF ← 0; SF ← 0; }Encoding
Operands
-
src1
XMM -
src2
XMM/Mem
Related
More in SSE
Reference
Instruction Forms
| Opcode | Instruction | Op/En | 64/32-bit Mode | CPUID | Description |
|---|---|---|---|---|---|
| NP 0F 2F /r | COMISS xmm1, xmm2/m32 | A | 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 | A | V/V | AVX | Compare low single precision floating-point values in xmm1 and xmm2/mem32 and set the EFLAGS flags accordingly. |
| EVEX.LLIG.0F.W0 2F /r | VCOMISS xmm1, xmm2/m32{sae} | B | V/V | AVX512F OR AVX10.1 | Compare low single precision floating-point values in xmm1 and xmm2/mem32 and set the EFLAGS flags accordingly. |
Instruction Operand Encoding
| Op/En | Tuple Type | Operand 1 | Operand 2 | Operand 3 | Operand 4 |
|---|---|---|---|---|---|
| A | N/A | ModRM:reg (w) | ModRM:r/m (r) | N/A | N/A |
| B | Tuple1 Scalar | ModRM:reg (w) | ModRM:r/m (r) | N/A | N/A |
Description
Compares the single 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 (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 operation exception only if a source operand is an SNaN. The EFLAGS register is not updated if an unmasked SIMD floating-point exception is generated. VEX.vvvv and EVEX.vvvv are reserved and must be 1111b, otherwise instructions will #UD. Software should ensure VCOMISS is encoded with VEX.L=0. Encoding VCOMISS with VEX.L=1 may encounter unpredictable behavior across different processor generations.
Operation
COMISS (All Versions)
RESULT :=OrderedCompare(DEST[31:0] <> SRC[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 Equivalent
VCOMISS int _mm_comi_round_ss(__m128 a, __m128 b, int imm, int sae); VCOMISS int _mm_comieq_ss (__m128 a, __m128 b) VCOMISS int _mm_comilt_ss (__m128 a, __m128 b) VCOMISS int _mm_comile_ss (__m128 a, __m128 b) VCOMISS int _mm_comigt_ss (__m128 a, __m128 b) VCOMISS int _mm_comige_ss (__m128 a, __m128 b) VCOMISS int _mm_comineq_ss (__m128 a, __m128 b)