ADX
ADX adds x86 integer addition instructions that allow independent carry chains. It is useful for carefully written multiprecision arithmetic and is independent of BMI2.
What you need to know
ADX adds x86 integer addition instructions that allow independent carry chains. It is useful for carefully written multiprecision arithmetic and is independent of BMI2.
How it works
ADCX uses the carry flag and ADOX uses the overflow flag as separate carry inputs and outputs. A big-integer routine can interleave two chains when the data dependencies allow it. Initial flag values, register allocation and the interaction with other flag-writing instructions must be controlled.
Where it is useful
A large-number multiplication kernel can combine partial products with independent carry chains, sometimes alongside BMI2 MULX. That is an implementation choice inside a library, not a processor-wide mode that makes every integer addition faster.
How to check support and enablement
CPUID.07H.00H:EBX bit 19 indicates ADX. Dispatch independently for any additional instruction family used by the routine. Validate carry propagation and boundary cases as well as ordinary numeric results.
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
ADX does not guarantee a constant-time cryptographic implementation or accelerate all big-integer operations. It does not imply BMI2, and using another instruction that overwrites a required flag can break the algorithm.
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
- Record the exact processor or platform model and the application, OS and runtime versions involved. Preserve the wording of the actual error or requirement.
- Check the manufacturer reference for that exact model, then inspect the effective capability and enabled state in the environment where the program runs.
- Compare every requirement of the selected binary or backend. A similarly named feature, alias or broad technology label is not enough.
- Confirm the application selected the intended implementation. Use its documented diagnostics and a supported fallback when requirements are not met.
- 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
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.
- Intel 64 and IA-32 Architectures Software Developer's Manual · Volumes 1 and 2 describe the named instruction families and CPUID; Volume 3 describes state, paging, VMX and protection controls. Architecture documentation does not establish support for a particular model.
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