NX bit

The NX bit, which stands for No-eXecute, is a technology used in CPUs to segregate areas of memory for use by either storage of processor instructions (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; one class of such attacks is known as the buffer overflow attack.

Intel markets the feature as the XD bit, for eXecute Disable. AMD uses the marketing term Enhanced Virus Protection. The ARM architecture refers to the feature as XN for eXecute Never; it was introduced in ARM v6.^[1]

Hardware background

x86 processors, since the 80286, included a similar capability implemented at the segment level. However, current operating systems implementing the flat memory model cannot use this capability. There was no 'Executable' flag in the page table entry (page descriptor) in the 80386 and later x86 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.

The page-level mechanism has been supported for years in various other processor architectures such as DEC (now HP) Alpha, Sun SPARC, and IBM System/370-XA, System/390, z/Architecture and PowerPC. 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. The term NX bit itself is sometimes used to describe similar technologies in other processors.

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.^[2] 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) and 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/enable execution.

References

↑ "APX and XN (execute never) bits have been added in VMSAv6 [Virtual Memory System Architecture]", ARM Architecture Reference Manual, retrieved 2009/12/01.
↑ "Data Execution Prevention" (PDF). Hewlett Packard. 2005. Retrieved 2014-03-23.

External links

CPU technologies

Architecture	Von Neumann Harvard (Modified Harvard) Dataflow TTA

Instruction set	ASIP CISC RISC EDGE EPIC MISC OISC VLIW NISC ZISC TRIPS Comparison

Word size	1-bit 4-bit 8-bit 9-bit 10-bit 12-bit 15-bit 16-bit 18-bit 22-bit 24-bit 25-bit 26-bit 27-bit 31-bit 32-bit 33-bit 34-bit 36-bit 39-bit 40-bit 48-bit 50-bit 60-bit 64-bit 128-bit 256-bit 512-bit Variable

Execution	Instruction pipelining Bubble Operand forwarding Out-of-order execution Register renaming Speculative execution Branch predictor Memory dependence prediction Hazards

Parallel level	Bit Bit-serial Word Instruction Scalar Superscalar Task Thread Process Data Vector Memory

Multithreading	Temporal Simultaneous Preemptive Cooperative

Flynn's taxonomy	SISD SIMD MISD MIMD SPMD Addressing mode

Types	Digital signal processor (DSP) GPGPU Microcontroller Physics processing unit System on a chip (SoC) Cellular

Components	Address generation unit (AGU) Arithmetic logic unit (ALU) Barrel shifter Floating-point unit (FPU) Back-side bus Multiplexer Demultiplexer Registers Memory management unit (MMU) Translation lookaside buffer (TLB) Cache Register file Microcode Control unit Clock rate

Power management	APM ACPI Dynamic frequency scaling Dynamic voltage scaling Clock gating

CPU hardware security	Non-executable memory (NX bit) Bounds checking (Intel MPX) Hardware restriction (firmware) Trusted Execution Technology Secure cryptoprocessor Hardware security module Hengzhi chip

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