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Execute Disable (NX)

Architecture referenceFeex

Execute Disable, also called NX or XD in x86 terminology, allows page tables to mark memory as non-executable. It is a hardware building block used by operating-system execution-protection policies.

What you need to know

Execute Disable, also called NX or XD in x86 terminology, allows page tables to mark memory as non-executable. It is a hardware building block used by operating-system execution-protection policies.

How it works

When enabled in the applicable paging environment, an instruction fetch from a page marked non-executable faults instead of executing its bytes. Page permissions distinguish execution from data access; software is responsible for assigning suitable permissions and handling transitions for legitimate generated code.

Where it is useful

An OS can map a data buffer without execution permission so writing machine-code bytes into that buffer does not make it an executable code page. JIT runtimes use documented memory-management APIs and permission policies for generated code.

How to check support and enablement

CPUID.80000001H:EDX bit 20 enumerates the x86 execute-disable capability under the relevant vendor definition. Actual enforcement also depends on the paging mode and OS enablement. Inspect the OS's documented protection status rather than inferring it from a CPU model.

Capability, enablement and enforcement must be checked separately. A processor can support a protection while the OS, application or virtual-machine policy does not activate it. A database support field is not an attestation of a running system.

What the feature does not establish

NX does not establish that an executable file is safe, block every code-reuse attack or prove Windows eligibility. Data remains readable or writable according to separate permissions. A feature flag does not mean every memory mapping follows a strict write/execute policy.

Compatibility and performance in practice

Keep the OS, firmware and affected runtime current. If a protection exposes a compatibility problem, use vendor diagnostics and supported updates first. A broad protection toggle can affect unrelated applications and should follow an explicit deployment and recovery policy.

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

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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