x86-64.net

Instruction

KORB

Computes the bitwise OR of two 8-bit masks.

Extensions
Forms
1 (1 VEX)
Data
Intel XED v2026.08.23

KORB ORs the low 8 bits of two mask registers into a third and clears bits 63:8. It is KORW for masks of eight elements, the doubles or quadwords of a ZMM register, and that page describes when compilers need an OR of masks and when the OR only decides a branch. KORB needs AVX512DQ; KORD and KORQ are the AVX512BW forms.

Recorded on Zen 5

  • Only the low bytes. korb k1, k2, k3 with two sources whose low bytes were 00h gave 0, whatever bits 63:8 held, and FFFFFFFFFFFFFF55h with AAh gave FFh.
  • One register twice. korb k7, k7, k7 kept the low byte of K7 and cleared the rest: 504477124C59A3E0h became E0h.
  • Random bits. 74h with A0252EC369F4EB79h gave 7Dh, and FFFFFFFFFFFFFF6Eh with 0Eh gave 6Eh.

In compiled code

With GCC 14 and Clang 23 at -O2 and -march=x86-64-v4, the OR of two compares of doubles used as the zero mask of an add, _mm512_cmp_pd_mask(a, b, _CMP_LT_OQ) | _mm512_cmp_pd_mask(a, c, _CMP_LT_OQ), is two VCMPPD and a KORB in both; with only -mavx512f both use KORW. For a > b || a > c on signed quadwords Clang compares once with the minimum of b and c (VPMINSQ), as KORW’s page shows for doublewords, while GCC keeps two VPCMPQ and the KORB. When the OR only decides whether any element matched, Clang tests both masks with KORTESTB and GCC moves them out with KMOVB and ORs AL and DL. _kor_mask8 of two arguments is KMOVB twice, KORB and a KMOVB back in GCC, and OR on EAX in Clang.

AMD’s Zen 5 latency spreadsheet gives KORB one cycle and two per cycle on pipes FP0 and FP3, as KORW.

Forms

Every encoding of KORB that Intel XED knows, one row per XED instruction form (iform). How to read the notation.
InstructionEncodingRequires
KORB k, k, k KORB_MASKmskw_MASKmskw_MASKmskw_AVX512VEX.L1.66.0F.W0 45 /rAVX10.1 or AVX512DQ

Latency and throughput

What the scheduling models of LLVM 23.1.2 give for each form, not measurements: latency / reciprocal throughput in cycles, the ports under them. A row is a model, or models that agree; a dash, a form that its processors lack. Memory forms: latency from the address, ports without the load and store units. Which processors each model covers.
Modelk, k, k
Skylake-SP, Ice Lake, Sapphire Rapids1 / 1p0
Zen 41 / 0.5FP2 FP3

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

k1.b[0] = k2.b[0] | k3.b[0]
# bits 63:8 of k1 are cleared

Run on AMD Zen 5 (EPYC 9645): 11 cases.

Operands

What each form reads and writes, as Intel XED records it: the operands of its syntax, in order, and the implicit ones that the syntax does not name. How to read.
OperandKindAction
KORB k, k, k
kexplicitwrite
kexplicitread
kexplicitread

Flags

None: XED records no flag that the forms test or change.

Exceptions

The exception class is XED's name for the set of exception conditions that a form follows. How to read.

All forms
AVX512-K20

Intrinsics

The C intrinsics of Clang 23.1.2 that compile to KORB, each with the form it compiles to and the Clang options it needs. The forms come from compiling every intrinsic and decoding the result with Intel XED; all intrinsics A–Z explains the method.

IntrinsicCompiles to
_kor_mask8 __mmask8 _kor_mask8(__mmask8 __A, __mmask8 __B)

KORB k, k, k VEX.L1 -mavx512dq

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

CPUID requirements and processors of the XED ISA sets above. Alternatives are separated by “or”; processors follow XED’s chip model, whose AMD processors end with Zen 2, and the CPU data of this site for Zen 3 to Zen 6.
ISA setCPUIDProcessors
AVX512DQ_KOPB

AVX10.1
CPUID.(EAX=07H,ECX=1):EDX[19] AVX10
CPUID.(EAX=24H,ECX=0):EBX[7:0] ≥ 1 AVX10 version

or

AVX512DQ
CPUID.(EAX=07H,ECX=0):EBX[17] AVX512DQ

Cannon Lake, Ice Lake (client), Tiger Lake, Nova Lake, Skylake-SP, Cascade Lake, Cooper Lake, Ice Lake (server), Sapphire Rapids, Emerald Rapids, Granite Rapids, Diamond Rapids, Zen 4, Zen 5, Zen 6

More AVX10.1 instructions

All 686 AVX10.1 instructions

Sources

  1. Intel XED v2026.08.23 (commit 0bcb6237345c): forms, encodings, operands, flags, exception classes, attributes, ISA sets, CPUID bits and chip model.
  2. The CPU data of x86-64.net: Zen 3 to Zen 6, from GCC’s -march feature sets and AMD’s Processor Programming References, which the page of each processor cites.
  3. Clang's x86 intrinsic headers, LLVM 23.1.2 (commit 85ac56026243): the intrinsics, their signatures and doc comments, and the Clang of that release, which compiled each of them.
  4. LLVM 23.1.2's x86 scheduling models (commit 85ac56026243): the latency, reciprocal throughput and ports of each form, as llvm-mca reports them.
  5. Intel Advanced Vector Extensions 10.2 Architecture Specification, revision 7.0 (361050-007, April 2026)
  6. Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 2 (325383-093, September 2026): Volume 2A, page 3-522 (KORW/KORB/KORQ/KORD)
  7. Software Optimization Guide for the AMD Zen5 Microarchitecture (58455, rev. 1.00, August 2024), Wayback Machine copy of AMD's ZIP

The tables are derived from Intel XED, Copyright Intel Corporation, licensed under the Apache License 2.0; x86-64.net converted and reformatted them. The intrinsics and their signatures come from Clang and the timings from LLVM's scheduling models, both part of the LLVM Project, licensed under the Apache License 2.0 with LLVM Exceptions. The text is our own.