CPU
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
s to segregate areas of memory for use by either storage of processor instructions 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, also called
Write XOR Execute
W^X ("write xor execute", pronounced ''W xor X'') is a security feature in operating systems and virtual machines. It is a memory protection policy whereby every page in a process's or kernel's address space may be either writable or executab ...
, 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
ARM (stylised in lowercase as arm, formerly an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computer (RISC) instruction set architectures for computer processors, configured ...
refers to the feature, which was introduced in
ARMv6
ARM (stylised in lowercase as arm, formerly an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computer (RISC) instruction set architectures for computer processors, configured ...
, as XN (execute never). The term ''NX bit'' itself is sometimes used to describe similar technologies in other processors.
Architecture support
x86
x86 processors, since the 80286, included a similar capability implemented at the segment level. However, almost all operating systems for the
80386
The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsflat 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
The Athlon 64 is a ninth-generation, AMD64-architecture microprocessor produced by Advanced Micro Devices (AMD), released on September 23, 2003. It is the third processor to bear the name ''Athlon'', and the immediate successor to the Athlon XP. T ...
and Opteron.
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. The NX bit specifically refers to bit number 63 (i.e. the most significant bit) of a 64-bit entry in the page table. If this bit is set to 0, then code can be executed from that page; if set to 1, code cannot be executed from that page, and anything residing there is assumed to be data. It is only available with the long mode (64-bit mode) or legacy Physical Address Extension (PAE) page-table formats, but not x86's original 32-bit page table format because page table entries in that format lack the 63rd bit used to disable and enable execution.
Windows XP SP2 and later support Data Execution Prevention (DEP).
ARM
In
ARMv6
ARM (stylised in lowercase as arm, formerly an acronym for Advanced RISC Machines and originally Acorn RISC Machine) is a family of reduced instruction set computer (RISC) instruction set architectures for computer processors, configured ...
, a new page table entry format was introduced; it includes an "execute never" bit. For ARMv8-A, VMSAv8-64 block and page descriptors, and VMSAv8-32 long-descriptor block and page descriptors, for stage 1 translations have "execute never" bits for both privileged and unprivileged modes, and block and page descriptors for stage 2 translations have a single "execute never" bit(two bits due to ARMv8.2-TTS2UXN feature); VMSAv8-32 short-descriptor translation table descriptors at level 1 have "execute never" bits for both privileged and unprivileged mode and at level 2 have a single "execute never" bit.
Alpha
As of the Fourth Edition of the Alpha Architecture manual, DEC (now HP)
Alpha
Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whic ...
has a Fault on Execute bit in page table entries with the
OpenVMS
OpenVMS, often referred to as just VMS, is a multi-user, multiprocessing and virtual memory-based operating system. It is designed to support time-sharing, batch processing, transaction processing and workstation applications. Customers using Ope ...
,
Tru64 UNIX
Tru64 UNIX is a discontinued 64-bit UNIX operating system for the Alpha instruction set architecture (ISA), currently owned by Hewlett-Packard (HP). Previously, Tru64 UNIX was a product of Compaq, and before that, Digital Equipment Corporation ( ...
, and Alpha Linux
PALcode PALcode (Privileged Architecture Library code) is the name used by DEC in the Alpha instruction set architecture (ISA) for a set of functions in the System Reference Manual (SRM) or AlphaBIOS firmware, providing a hardware abstraction layer for ...
.
SPARC
The SPARC Reference MMU for SunSPARC version 8 has permission values of Read Only, Read/Write, Read/Execute, and Read/Write/Execute in page table entries, although not all SPARC processors have a SPARC Reference MMU.
A SPARC version 9 MMU may provide, but is not required to provide, any combination of read/write/execute permissions. A Translation Table Entry in a Translation Storage Buffer in Oracle SPARC Architecture 2011, Draft D1.0.0 has separate Executable and Writable bits.
PowerPC
PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...
's hashed page tables have a no-execute page bit. Page table entries for radix-tree page tables in the Power ISA have separate permission bits granting read/write and execute access.
PA-RISC
Translation lookaside buffer (TLB) entries and page table entries in PA-RISC 1.1 and PA-RISC 2.0 support read-only, read/write, read/execute, and read/write/execute pages.
Itanium
TLB entries in Itanium support read-only, read/write, read/execute, and read/write/execute pages.
z/Architecture
As of the twelfth edition of the z/Architecture Principles of Operation, z/Architecture processors may support the Instruction-Execution Protection facility, which adds a bit in page table entries that controls whether instructions from a given region, segment, or page can be executed.