NX Bit

The NX bit, which stands for Never eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions (or code) or for storage of data, a feature normally only found in Harvard architecture processors. However, the NX bit is being increasingly used in conventional von Neumann architecture processors, for security reasons.

An operating system with support for the NX bit may mark certain areas of memory as non-executable. The processor will then refuse to execute any code residing in these areas of memory. The general technique, known as executable space protection, is used to prevent certain types of malicious software from taking over computers by inserting their code into another program's data storage area and running their own code from within this section; this is known as a buffer overflow attack.

Intel markets the feature as the XD bit, for eXecute Disable. AMD uses the name Enhanced Virus Protection. The ARM architecture refers to the feature as XN for eXecute Never; it was introduced in ARM v6.