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Concept
NX bit
Summary
The NX bit (no-execute) is a technology used in CPUs to segregate areas of a virtual address space for use by either storage of processor instructions or for storage of data. An operating system with support for the NX bit may mark certain areas of an address space as non-executable. The processor will then refuse to execute any code residing in these areas of the address space. 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.
The term NX bit originated with Advanced Micro Devices (AMD), as a marketing term. Intel markets the feature as the XD bit (execute disable). The MIPS architecture refers to the feature as XI bit (execute inhibit). The ARM architecture refers to the feature, which was introduced in ARMv6, as XN (execute never). The term NX bit itself is sometimes used to describe similar technologies in other processors.
x86 processors, since the 80286, included a similar capability implemented at the segment level. However, almost all operating systems for the 80386 and later x86 processors implement the flat memory model, so they cannot use this capability. There was no 'Executable' flag in the page table entry (page descriptor) in those processors, until, to make this capability available to operating systems using the flat memory model, AMD added a "no-execute" or NX bit to the page table entry in its AMD64 architecture, providing a mechanism that can control execution per page rather than per whole segment.
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.
Official source
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