NX bit

The NX bit (no-execute) is a technology used in CPUs to segregate areas of a virtual address space to store either data or processor instructions.

An operating system with support for the NX bit may mark certain areas of an address space as non-executable.

The general technique, known as executable space protection, also called Write XOR Execute, 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; one class of such attacks is known as the buffer overflow attack.

Intel implemented a similar feature in its Itanium (Merced) processor—having IA-64 architecture—in 2001, but did not bring it to the more popular x86 processor families (Pentium, Celeron, Xeon, etc.).

In the x86 architecture it was first implemented by AMD, as the NX bit, for use by its AMD64 line of processors, such as the Athlon 64 and Opteron.

[2] After AMD's decision to include this functionality in its AMD64 instruction set, Intel implemented the similar XD bit feature in x86 processors beginning with the Pentium 4 processors based on later iterations of the Prescott core.