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BMI1

Architecture referenceFeex

BMI1 is an x86 extension for scalar bit manipulation, including bit extraction, bit isolation and trailing-zero counting. It operates on general-purpose integer registers rather than vector lanes.

What you need to know

BMI1 is an x86 extension for scalar bit manipulation, including bit extraction, bit isolation and trailing-zero counting. It operates on general-purpose integer registers rather than vector lanes.

How it works

The extension includes ANDN, BEXTR, BLSI, BLSMSK, BLSR and TZCNT. These operations combine masking, selection or counting steps that otherwise need longer sequences. Instruction-specific flag effects and edge cases are part of the contract, particularly when an input is zero.

Where it is useful

A bitset iterator can clear its lowest set bit with BLSR, while a decoder can extract a field with BEXTR. Whether that helps depends on dependencies and the target implementation. BEXTR is not an arbitrary gather of noncontiguous bits.

How to check support and enablement

Check CPUID.07H.00H:EBX bit 3 for BMI1. Use feature dispatch rather than testing whether TZCNT appears to execute. The zero-input result and flag behavior must match the intended instruction semantics.

Check the exact instruction and operand form rather than inferring support from a neighboring extension. Scalar integer features generally use general-purpose registers; an encoding associated with vectors does not automatically mean a vector-state requirement.

What the feature does not establish

BMI1 does not imply BMI2, POPCNT, LZCNT or AVX OS-state support. TZCNT shares an encoding relationship with an older instruction, so unsupported execution may produce different semantics instead of a clean fault.

Compatibility and performance in practice

Correctness depends on edge cases as well as speed. Zero input, overflow, carry flags, operand width, alignment and memory ordering can change semantics. Prefer compiler or library interfaces that document these details and provide a supported fallback.

A practical verification checklist

  1. Record the exact processor or platform model and the application, OS and runtime versions involved. Preserve the wording of the actual error or requirement.
  2. Check the manufacturer reference for that exact model, then inspect the effective capability and enabled state in the environment where the program runs.
  3. Compare every requirement of the selected binary or backend. A similarly named feature, alias or broad technology label is not enough.
  4. Confirm the application selected the intended implementation. Use its documented diagnostics and a supported fallback when requirements are not met.
  5. Make one supported change at a time and repeat the same workload. Keep the original configuration and recovery path available.

If the feature is listed but software cannot use it

Start with the exact failing component. A hardware database describes recorded capability; the operating system or hypervisor can expose a different effective environment. An illegal-instruction error also needs investigation of the actual faulting instruction and binary build, because corruption or an unrelated unsupported instruction can produce a similar symptom.

Install the application, driver or runtime version supported for the exact platform, and use the vendor's documented baseline or fallback build if needed. A software update cannot create an instruction that the processor lacks. Firmware and security-policy changes should follow the system manufacturer's procedure and a recovery plan rather than a generic optimization recipe.

Sources and scope of this explanation

Intel 64 and IA-32 Architectures Software Developer's Manual · x86-64 psABI: processor architecture and microarchitecture levels

These references establish architectural behavior and software requirements. They do not independently verify every feature value on a linked hardware model page. For model support, use that page's manufacturer evidence and check the effective system configuration. This article does not supply a benchmark, certification or independent review date.

Sources and verification

Editorial ownership: Feex. No independent review date is recorded. How we research.

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