ARM architecture

ARM, previously Advanced RISC Machine, originally Acorn RISC Machine, is a family of (RISC)  for, configured for various environments. develops the architecture and licenses it to other companies, who design their own products that implement one of those architectures—‌including s (SoC) and s (SoM) that incorporate memory, interfaces, radios, etc. It also designs that implement this  and licenses these designs to a number of companies that incorporate those core designs into their own products.

Processors that have a RISC architecture typically require fewer s than those with a (CISC) architecture (such as the  processors found in most s), which improves cost, power consumption, and heat dissipation. These characteristics are desirable for light, portable, battery-powered devices—‌including s, s and s, and other s. For s, which consume large amounts of electricity, ARM could also be a power-efficient solution.

Arm Holdings periodically releases updates to the architecture. Architecture versions ARMv3 to ARMv7 support  (pre-ARMv3 chips, made before Arm Holdings was formed, as used in the, had 26-bit address space) and 32-bit arithmetic; most architectures have 32-bit fixed-length instructions. The Thumb version supports a variable-length instruction set that provides both 32- and 16-bit instructions for improved. Some older cores can also provide hardware execution of s; and newer ones have one instruction for. Released in 2011, the ARMv8-A architecture added support for a address space and 64-bit arithmetic with its new 32-bit fixed-length instruction set.

With over 100 billion ARM processors produced, ARM is the most widely used and the instruction set architecture produced in the largest quantity. Currently, the widely used Cortex s, older "classic" cores, and specialized cores variants are available for each of these to include or exclude optional capabilities.

History
The British computer manufacturer first developed the Acorn RISC Machine architecture (ARM) in the 1980s to use in its personal computers. Its first ARM-based products were coprocessor modules for the series of computers. After the successful BBC Micro computer, Acorn Computers considered how to move on from the relatively simple processor to address business markets like the one that was soon dominated by the, launched in 1981. The  (ABC) plan required that a number of s be made to work with the BBC Micro platform, but processors such as the and  were considered unsuitable, and the 6502 was not powerful enough for a.

According to, all the processors tested at that time performed about the same, with about a 4 Mbit/second bandwidth.

After testing all available processors and finding them lacking, Acorn decided it needed a new architecture. Inspired by papers from the project, Acorn considered designing its own processor. A visit to the in, where the 6502 was being updated by what was effectively a single-person company, showed Acorn engineers  and Sophie Wilson they did not need massive resources and state-of-the-art  facilities.

Wilson developed the instruction set, writing a simulation of the processor in that ran on a BBC Micro with a. This convinced Acorn engineers they were on the right track. Wilson approached Acorn's CEO,, and requested more resources. Hauser gave his approval and assembled a small team to implement Wilson's model in hardware.

Acorn RISC Machine: ARM2
The official Acorn RISC Machine project started in October 1983. They chose as the silicon partner, as they were a source of ROMs and custom chips for Acorn. Wilson and Furber led the design. They implemented it with a similar efficiency ethos as the 6502. A key design goal was achieving low-latency input/output (interrupt) handling like the 6502. The 6502's memory access architecture had let developers produce fast machines without costly (DMA) hardware.

The first samples of ARM silicon worked properly when first received and tested on 26 April 1985.

The first ARM application was as a second processor for the BBC Micro, where it helped in developing simulation software to finish development of the support chips (VIDC, IOC, MEMC), and sped up the used in ARM2 development. Wilson subsequently rewrote BBC BASIC in ARM assembly language. The in-depth knowledge gained from designing the instruction set enabled the code to be very dense, making ARM BBC BASIC an extremely good test for any ARM emulator. The original aim of a principally ARM-based computer was achieved in 1987 with the release of the. In 1992, Acorn once more won the for the ARM.

The ARM2 featured a,  address space and 27 32-bit. Eight bits from the register were available for other purposes; the top six bits (available because of the 26-bit address space) served as status flags, and the bottom two bits (available because the program counter was always ) were used for setting modes. The address bus was extended to 32 bits in the ARM6, but program code still had to lie within the first 64 of memory in 26-bit compatibility mode, due to the reserved bits for the status flags. The ARM2 had a of just 30,000, compared to Motorola's six-year-older 68000 model with around 40,000. Much of this simplicity came from the lack of (which represents about one-quarter to one-third of the 68000) and from (like most CPUs of the day) not including any. This simplicity enabled low power consumption, yet better performance than the. A successor, ARM3, was produced with a 4 cache, which further improved performance.

Advanced RISC Machines Ltd. — ARM6
In the late 1980s, and  started working with Acorn on newer versions of the ARM core. In 1990, Acorn spun off the design team into a new company named Advanced RISC Machines Ltd., which became ARM Ltd when its parent company, plc, floated on the  and  in 1998. The new Apple-ARM work would eventually evolve into the ARM6, first released in early 1992. Apple used the ARM6-based ARM610 as the basis for their PDA.

Early licensees
In 1994, Acorn used the ARM610 as the main (CPU) in their  computers. licensed the ARMv4 architecture and produced the. At 233, this CPU drew only one watt (newer versions draw far less). This work was later passed to Intel as part of a lawsuit settlement, and Intel took the opportunity to supplement their line with the StrongARM. Intel later developed its own high performance implementation named XScale, which it has since sold to. Transistor count of the ARM core remained essentially the same throughout these changes; ARM2 had 30,000 transistors, while ARM6 grew only to 35,000.

Market share
In 2005, about 98% of all mobile phones sold used at least one ARM processor. In 2010, producers of chips based on ARM architectures reported shipments of 6.1 billion, representing 95% of s, 35% of s and and 10% of s. In 2011, the 32-bit ARM architecture was the most widely used architecture in mobile devices and the most popular 32-bit one in embedded systems. In 2013, 10 billion were produced and "ARM-based chips are found in nearly 60 percent of the world’s mobile devices".

Core licence
Arm Holdings' primary business is selling s, which licensees use to create s (MCUs), s, and based on those cores. The combines the ARM core with other parts to produce a complete device, typically one that can be built in existing s (fabs) at low cost and still deliver substantial performance. The most successful implementation has been the with hundreds of millions sold. has been a precursor design center in the ARM7TDMI-based embedded system.

The ARM architectures used in smartphones, PDAs and other s range from ARMv5 to ARMv7-A, used in low-end and midrange devices, to ARMv8-A used in current high-end devices.

In 2009, some manufacturers introduced netbooks based on ARM architecture CPUs, in direct competition with netbooks based on. According to analyst firm IHS iSuppli, by 2015, ARM s may be in 23% of all laptops.

Arm Holdings offers a variety of licensing terms, varying in cost and deliverables. Arm Holdings provides to all licensees an integratable hardware description of the ARM core as well as complete software development toolset and the right to sell manufactured  containing the ARM CPU.

SoC packages integrating ARM's core designs include Nvidia Tegra's first three generations, CSR plc's Quatro family, ST-Ericsson's Nova and NovaThor, Silicon Labs's Precision32 MCU, Texas Instruments's OMAP products, Samsung's Hummingbird and products, Apple's, , and , and Freescale's i.MX.

licensees, who wish to integrate an ARM core into their own chip design, are usually only interested in acquiring a ready-to-manufacture verified. For these customers, Arm Holdings delivers a description of the chosen ARM core, along with an abstracted simulation model and test programs to aid design integration and verification. More ambitious customers, including integrated device manufacturers (IDM) and foundry operators, choose to acquire the processor IP in   form. With the synthesizable RTL, the customer has the ability to perform architectural level optimisations and extensions. This allows the designer to achieve exotic design goals not otherwise possible with an unmodified netlist (, very low power consumption, instruction set extensions, etc.). While Arm Holdings does not grant the licensee the right to resell the ARM architecture itself, licensees may freely sell manufactured product such as chip devices, evaluation boards and complete systems. can be a special case; not only are they allowed to sell finished silicon containing ARM cores, they generally hold the right to re-manufacture ARM cores for other customers.

Arm Holdings prices its IP based on perceived value. Lower performing ARM cores typically have lower licence costs than higher performing cores. In implementation terms, a synthesizable core costs more than a hard macro (blackbox) core. Complicating price matters, a merchant foundry that holds an ARM licence, such as Samsung or Fujitsu, can offer fab customers reduced licensing costs. In exchange for acquiring the ARM core through the foundry's in-house design services, the customer can reduce or eliminate payment of ARM's upfront licence fee.

Compared to dedicated semiconductor foundries (such as and ) without in-house design services, Fujitsu/Samsung charge two- to three-times more per manufactured. For low to mid volume applications, a design service foundry offers lower overall pricing (through subsidisation of the licence fee). For high volume mass-produced parts, the long term cost reduction achievable through lower wafer pricing reduces the impact of ARM's NRE (Non-Recurring Engineering) costs, making the dedicated foundry a better choice.

Companies that have designed chips with ARM cores include 's subsidiary,, ,  (now: ), , , ,  (now ), , , , , , , , ,  and.

Built on ARM Cortex Technology licence
In February 2016, ARM announced the Built on ARM Cortex Technology licence, often shortened to Built on Cortex (BoC) licence. This licence allows companies to partner with ARM and make modifications to ARM Cortex designs. These design modifications will not be shared with other companies. These semi-custom core designs also have brand freedom, for example.

Companies that are current licensees of Built on ARM Cortex Technology include.

Architectural licence
Companies can also obtain an ARM architectural licence for designing their own CPU cores using the ARM instruction sets. These cores must comply fully with the ARM architecture. Companies that have designed cores that implement an ARM architecture include Apple, AppliedMicro, Broadcom, (now: Marvell), Nvidia, Qualcomm, and Samsung Electronics.

Cores
Arm Holdings provides a list of vendors who implement ARM cores in their design (application specific standard products (ASSP), microprocessor and microcontrollers).

Example applications of ARM cores
ARM cores are used in a number of products, particularly s and s. Some examples are 's  and, 's s and 's  s, and several  laptops. Others include Apple's  and ,  s,  hybrid and  s, and  turn-by-turn.

In 2005, Arm Holdings took part in the development of 's computer, which used ARM cores to simulate the.

ARM chips are also used in, , , and other s, because they are very small, inexpensive and consume very little power.

32-bit architecture
The 32-bit ARM architecture, such as ARMv7-A (implementing ; see section on for more on it), was the most widely used architecture in mobile devices.

Since 1995, the ARM Architecture Reference Manual has been the primary source of documentation on the ARM processor architecture and instruction set, distinguishing interfaces that all ARM processors are required to support (such as instruction semantics) from implementation details that may vary. The architecture has evolved over time, and version seven of the architecture, ARMv7, defines three architecture "profiles":


 * A-profile, the "Application" profile, implemented by 32-bit cores in the series and by some non-ARM cores
 * R-profile, the "Real-time" profile, implemented by cores in the series
 * M-profile, the "Microcontroller" profile, implemented by most cores in the series

Although the architecture profiles were first defined for ARMv7, ARM subsequently defined the ARMv6-M architecture (used by the Cortex //) as a subset of the ARMv7-M profile with fewer instructions.

CPU modes
Except in the M-profile, the 32-bit ARM architecture specifies several CPU modes, depending on the implemented architecture features. At any moment in time, the CPU can be in only one mode, but it can switch modes due to external events (interrupts) or programmatically.


 * User mode: The only non-privileged mode.
 * FIQ mode: A privileged mode that is entered whenever the processor accepts a.
 * IRQ mode: A privileged mode that is entered whenever the processor accepts an interrupt.
 * Supervisor (svc) mode: A privileged mode entered whenever the CPU is reset or when an SVC instruction is executed.
 * Abort mode: A privileged mode that is entered whenever a prefetch abort or data abort exception occurs.
 * Undefined mode: A privileged mode that is entered whenever an undefined instruction exception occurs.
 * System mode (ARMv4 and above): The only privileged mode that is not entered by an exception. It can only be entered by executing an instruction that explicitly writes to the mode bits of the Current Program Status Register (CPSR) from another privileged mode (not from user mode).
 * Monitor mode (ARMv6 and ARMv7 Security Extensions, ARMv8 EL3): A monitor mode is introduced to support TrustZone extension in ARM cores.
 * Hyp mode (ARMv7 Virtualization Extensions, ARMv8 EL2): A hypervisor mode that supports for the non-secure operation of the CPU.
 * Thread mode (ARMv6-M, ARMv7-M, ARMv8-M): A mode which can be specified as either privileged or unprivileged. Whether the Main Stack Pointer (MSP) or Process Stack Pointer (PSP) is used can also be specified in CONTROL register with privileged access. This mode is designed for user tasks in RTOS environment but it's typically used in bare-metal for super-loop.
 * Handler mode (ARMv6-M, ARMv7-M, ARMv8-M): A mode dedicated for exception handling (except the RESET which are handled in Thread mode). Handler mode always uses MSP and works in privileged level.

Instruction set
The original (and subsequent) ARM implementation was hardwired without, like the much simpler  processor used in prior Acorn microcomputers.

The 32-bit ARM architecture (and the 64-bit architecture for the most part) includes the following RISC features:


 * No support for in the original version of the architecture. ARMv6 and later, except some microcontroller versions, support unaligned accesses for half-word and single-word load/store instructions with some limitations, such as no guaranteed.
 * Uniform 16× 32-bit (including the program counter, stack pointer and the link register).
 * Fixed instruction width of 32 bits to ease decoding and, at the cost of decreased . Later, the added 16-bit instructions and increased code density.
 * Mostly single clock-cycle execution.
 * Mostly single clock-cycle execution.

To compensate for the simpler design, compared with processors like the Intel 80286 and, some additional design features were used:
 * Conditional execution of most instructions reduces branch overhead and compensates for the lack of a in early chips.
 * Arithmetic instructions alter s only when desired.
 * 32-bit can be used without performance penalty with most arithmetic instructions and address calculations.
 * Has powerful indexed s.
 * A supports fast leaf function calls.
 * A simple, but fast, 2-priority-level subsystem has switched register banks.

Arithmetic instructions
ARM includes integer arithmetic operations for add, subtract, and multiply; some versions of the architecture also support divide operations.

ARM supports 32-bit × 32-bit multiplies with either a 32-bit result or 64-bit result, though Cortex-M0 / M0+ / M1 cores don't support 64-bit results. Some ARM cores also support 16-bit × 16-bit and 32-bit × 16-bit multiplies.

The divide instructions are only included in the following ARM architectures:


 * ARMv7-M and ARMv7E-M architectures always include divide instructions.
 * ARMv7-R architecture always includes divide instructions in the Thumb instruction set, but optionally in its 32-bit instruction set.
 * ARMv7-A architecture optionally includes the divide instructions. The instructions might not be implemented, or implemented only in the Thumb instruction set, or implemented in both the Thumb and ARM instruction sets, or implemented if the Virtualization Extensions are included.

Registers
Registers R0 through R7 are the same across all CPU modes; they are never banked.

Registers R8 through R12 are the same across all CPU modes except FIQ mode. FIQ mode has its own distinct R8 through R12 registers.

R13 and R14 are banked across all privileged CPU modes except system mode. That is, each mode that can be entered because of an exception has its own R13 and R14. These registers generally contain the stack pointer and the return address from function calls, respectively.

Aliases:


 * R13 is also referred to as SP, the.
 * R14 is also referred to as LR, the.
 * R15 is also referred to as PC, the.

The Current Program Status Register (CPSR) has the following 32 bits.


 * M (bits 0–4) is the processor mode bits.
 * T (bit 5) is the Thumb state bit.
 * F (bit 6) is the FIQ disable bit.
 * I (bit 7) is the IRQ disable bit.
 * A (bit 8) is the imprecise data abort disable bit.
 * E (bit 9) is the data endianness bit.
 * IT (bits 10–15 and 25–26) is the if-then state bits.
 * GE (bits 16–19) is the greater-than-or-equal-to bits.
 * DNM (bits 20–23) is the do not modify bits.
 * J (bit 24) is the Java state bit.
 * Q (bit 27) is the sticky overflow bit.
 * V (bit 28) is the overflow bit.
 * C (bit 29) is the carry/borrow/extend bit.
 * Z (bit 30) is the zero bit.
 * N (bit 31) is the negative/less than bit.

Conditional execution
Almost every ARM instruction has a conditional execution feature called, which is implemented with a 4-bit condition code selector (the predicate). To allow for unconditional execution, one of the four-bit codes causes the instruction to be always executed. Most other CPU architectures only have condition codes on branch instructions.

Though the predicate takes up four of the 32 bits in an instruction code, and thus cuts down significantly on the encoding bits available for displacements in memory access instructions, it avoids branch instructions when generating code for small. Apart from eliminating the branch instructions themselves, this preserves the fetch/decode/execute pipeline at the cost of only one cycle per skipped instruction.

The standard example of conditional execution is the subtraction-based :

In the, the loop is: For ARM, the loop can be effectively transformed into: and coded as:

which avoids the branches around the  and   clauses. If  and   are equal then neither of the   instructions will be executed, eliminating the need for a conditional branch to implement the   check at the top of the loop, for example had   (less than or equal) been used.

One of the ways that Thumb code provides a more dense encoding is to remove the four-bit selector from non-branch instructions.

Other features
Another feature of the is the ability to fold shifts and rotates into the "data processing" (arithmetic, logical, and register-register move) instructions, so that, for example, the C statement

could be rendered as a single-word, single-cycle instruction:

This results in the typical ARM program being denser than expected with fewer memory accesses; thus the pipeline is used more efficiently.

The ARM processor also has features rarely seen in other RISC architectures, such as -relative addressing (indeed, on the 32-bit ARM the is one of its 16 registers) and pre- and post-increment addressing modes.

The ARM instruction set has increased over time. Some early ARM processors (before ARM7TDMI), for example, have no instruction to store a two-byte quantity.

Pipelines and other implementation issues
The ARM7 and earlier implementations have a three-stage ; the stages being fetch, decode and execute. Higher-performance designs, such as the ARM9, have deeper pipelines: Cortex-A8 has thirteen stages. Additional implementation changes for higher performance include a faster and more extensive  logic. The difference between the ARM7DI and ARM7DMI cores, for example, was an improved multiplier; hence the added "M".

Coprocessors
The ARM architecture (pre-ARMv8) provides a non-intrusive way of extending the instruction set using "coprocessors" that can be addressed using MCR, MRC, MRRC, MCRR and similar instructions. The coprocessor space is divided logically into 16 coprocessors with numbers from 0 to 15, coprocessor 15 (cp15) being reserved for some typical control functions like managing the caches and operation on processors that have one.

In ARM-based machines, peripheral devices are usually attached to the processor by mapping their physical registers into ARM memory space, into the coprocessor space, or by connecting to another device (a bus) that in turn attaches to the processor. Coprocessor accesses have lower latency, so some peripherals—for example, an XScale interrupt controller—are accessible in both ways: through memory and through coprocessors.

In other cases, chip designers only integrate hardware using the coprocessor mechanism. For example, an image processing engine might be a small ARM7TDMI core combined with a coprocessor that has specialised operations to support a specific set of HDTV transcoding primitives.

Debugging
All modern ARM processors include hardware debugging facilities, allowing software debuggers to perform operations such as halting, stepping, and breakpointing of code starting from reset. These facilities are built using support, though some newer cores optionally support ARM's own two-wire "SWD" protocol. In ARM7TDMI cores, the "D" represented JTAG debug support, and the "I" represented presence of an "EmbeddedICE" debug module. For ARM7 and ARM9 core generations, EmbeddedICE over JTAG was a de facto debug standard, though not architecturally guaranteed.

The ARMv7 architecture defines basic debug facilities at an architectural level. These include breakpoints, watchpoints and instruction execution in a "Debug Mode"; similar facilities were also available with EmbeddedICE. Both "halt mode" and "monitor" mode debugging are supported. The actual transport mechanism used to access the debug facilities is not architecturally specified, but implementations generally include JTAG support.

There is a separate ARM "CoreSight" debug architecture, which is not architecturally required by ARMv7 processors.

DSP enhancement instructions
To improve the ARM architecture for and multimedia applications, DSP instructions were added to the set. These are signified by an "E" in the name of the ARMv5TE and ARMv5TEJ architectures. E-variants also imply T, D, M, and I.

The new instructions are common in (DSP) architectures. They include variations on signed, saturated add and subtract, and count leading zeros.

SIMD extensions for multimedia
Introduced in the ARMv6 architecture, this was a precursor to Advanced SIMD, also known as.

Jazelle
Jazelle DBX (Direct Bytecode eXecution) is a technique that allows to be executed directly in the ARM architecture as a third execution state (and instruction set) alongside the existing ARM and Thumb-mode. Support for this state is signified by the "J" in the ARMv5TEJ architecture, and in ARM9EJ-S and ARM7EJ-S core names. Support for this state is required starting in ARMv6 (except for the ARMv7-M profile), though newer cores only include a trivial implementation that provides no hardware acceleration.

Thumb
To improve compiled code-density, processors since the ARM7TDMI (released in 1994) have featured the Thumb instruction set, which have their own state. (The "T" in "TDMI" indicates the Thumb feature.) When in this state, the processor executes the Thumb instruction set, a compact 16-bit encoding for a subset of the ARM instruction set. Most of the Thumb instructions are directly mapped to normal ARM instructions. The space-saving comes from making some of the instruction operands implicit and limiting the number of possibilities compared to the ARM instructions executed in the ARM instruction set state.

In Thumb, the 16-bit opcodes have less functionality. For example, only branches can be conditional, and many opcodes are restricted to accessing only half of all of the CPU's general-purpose registers. The shorter opcodes give improved code density overall, even though some operations require extra instructions. In situations where the memory port or bus width is constrained to less than 32 bits, the shorter Thumb opcodes allow increased performance compared with 32-bit ARM code, as less program code may need to be loaded into the processor over the constrained memory bandwidth.

Unlike processor architectures with variable length (16- or 32-bit) instructions, such as the Cray-1 and, both the ARM and Thumb instruction sets exist independently of each other. Embedded hardware, such as the, typically have a small amount of RAM accessible with a full 32-bit datapath; the majority is accessed via a 16-bit or narrower secondary datapath. In this situation, it usually makes sense to compile Thumb code and hand-optimise a few of the most CPU-intensive sections using full 32-bit ARM instructions, placing these wider instructions into the 32-bit bus accessible memory.

The first processor with a Thumb was the ARM7TDMI. All ARM9 and later families, including XScale, have included a Thumb instruction decoder. It includes instructions adopted from the Hitachi (1992), which was licensed by ARM. ARM's smallest processor families (Cortex M0 and M1) implement only the 16-bit Thumb instruction set for maximum performance in lowest cost applications.

Thumb-2
Thumb-2 technology was introduced in the ARM1156 core, announced in 2003. Thumb-2 extends the limited 16-bit instruction set of Thumb with additional 32-bit instructions to give the instruction set more breadth, thus producing a variable-length instruction set. A stated aim for Thumb-2 was to achieve code density similar to Thumb with performance similar to the ARM instruction set on 32-bit memory.

Thumb-2 extends the Thumb instruction set with bit-field manipulation, table branches and conditional execution. At the same time, the ARM instruction set was extended to maintain equivalent functionality in both instruction sets. A new "Unified Assembly Language" (UAL) supports generation of either Thumb or ARM instructions from the same source code; versions of Thumb seen on ARMv7 processors are essentially as capable as ARM code (including the ability to write interrupt handlers). This requires a bit of care, and use of a new "IT" (if-then) instruction, which permits up to four successive instructions to execute based on a tested condition, or on its inverse. When compiling into ARM code, this is ignored, but when compiling into Thumb it generates an actual instruction. For example:

All ARMv7 chips support the Thumb instruction set. All chips in the Cortex-A series, Cortex-R series, and ARM11 series support both "ARM instruction set state" and "Thumb instruction set state", while chips in the series support only the Thumb instruction set.

Thumb Execution Environment (ThumbEE)
ThumbEE (erroneously called Thumb-2EE in some ARM documentation), which was marketed as Jazelle RCT (Runtime Compilation Target), was announced in 2005, first appearing in the Cortex-A8 processor. ThumbEE is a fourth instruction set state, making small changes to the Thumb-2 extended instruction set. These changes make the instruction set particularly suited to code generated at runtime (e.g. by ) in managed Execution Environments. ThumbEE is a target for languages such as, , , and , and allows s to output smaller compiled code without impacting performance.

New features provided by ThumbEE include automatic null pointer checks on every load and store instruction, an instruction to perform an array bounds check, and special instructions that call a handler. In addition, because it utilises Thumb-2 technology, ThumbEE provides access to registers r8-r15 (where the Jazelle/DBX Java VM state is held). Handlers are small sections of frequently called code, commonly used to implement high level languages, such as allocating memory for a new object. These changes come from repurposing a handful of opcodes, and knowing the core is in the new ThumbEE state.

On 23 November 2011, Arm Holdings deprecated any use of the ThumbEE instruction set, and ARMv8 removes support for ThumbEE.

Floating-point (VFP)
VFP (Vector Floating Point) technology is an FPU coprocessor extension to the ARM architecture (implemented differently in ARMv8 – coprocessors not defined there). It provides low-cost and  floating-point computation fully compliant with the . VFP provides floating-point computation suitable for a wide spectrum of applications such as PDAs, smartphones, voice compression and decompression, three-dimensional graphics and digital audio, printers, set-top boxes, and automotive applications. The VFP architecture was intended to support execution of short "vector mode" instructions but these operated on each vector element sequentially and thus did not offer the performance of true (SIMD) vector parallelism. This vector mode was therefore removed shortly after its introduction, to be replaced with the much more powerful NEON Advanced SIMD unit.

Some devices such as the ARM Cortex-A8 have a cut-down VFPLite module instead of a full VFP module, and require roughly ten times more clock cycles per float operation. Pre-ARMv8 architecture implemented floating-point/SIMD with the coprocessor interface. Other floating-point and/or SIMD units found in ARM-based processors using the coprocessor interface include, FPE, , some of which were implemented in software by trapping but could have been implemented in hardware. They provide some of the same functionality as VFP but are not -compatible with it.


 * VFPv1: Obsolete
 * VFPv2: An optional extension to the ARM instruction set in the ARMv5TE, ARMv5TEJ and ARMv6 architectures. VFPv2 has 16 64-bit FPU registers.
 * VFPv3 or VFPv3-D32: Implemented on most Cortex-A8 and A9 ARMv7 processors. It is backwards compatible with VFPv2, except that it cannot trap floating-point exceptions. VFPv3 has 32 64-bit FPU registers as standard, adds VCVT instructions to convert between scalar, float and double, adds immediate mode to VMOV such that constants can be loaded into FPU registers.
 * VFPv3-D16: As above, but with only 16 64-bit FPU registers. Implemented on Cortex-R4 and R5 processors and the (Cortex-A9).
 * VFPv3-F16: Uncommon; it supports as a storage format.
 * VFPv4 or VFPv4-D32: Implemented on the Cortex-A12 and A15 ARMv7 processors, Cortex-A7 optionally has VFPv4-D32 in the case of an FPU with NEON. VFPv4 has 32 64-bit FPU registers as standard, adds both half-precision support as a storage format and instructions to the features of VFPv3.
 * VFPv4-D16: As above, but it has only 16 64-bit FPU registers. Implemented on Cortex-A5 and A7 processors (in case of an FPU without NEON).
 * VFPv5-D16-M: Implemented on Cortex-M7 when single and double-precision floating-point core option exists.

In GNU/Linux, and derivatives such as, armhf (ARM hard float) refers to the ARMv7 architecture including the additional VFP3-D16 floating-point hardware extension (and Thumb-2) above. Software packages and cross-compiler tools use the armhf vs. arm/armel suffixes to differentiate.

Advanced SIMD (NEON)
The Advanced SIMD extension (aka NEON or "MPE" Media Processing Engine) is a combined 64- and SIMD instruction set that provides standardized acceleration for media and signal processing applications. NEON is included in all Cortex-A8 devices, but is optional in Cortex-A9 devices. NEON can execute MP3 audio decoding on CPUs running at 10 MHz, and can run the  (AMR) speech codec at 13 MHz. It features a comprehensive instruction set, separate register files, and independent execution hardware. NEON supports 8-, 16-, 32-, and 64-bit integer and single-precision (32-bit) floating-point data and SIMD operations for handling audio and video processing as well as graphics and gaming processing. In NEON, the SIMD supports up to 16 operations at the same time. The NEON hardware shares the same floating-point registers as used in VFP. Devices such as the ARM Cortex-A8 and Cortex-A9 support 128-bit vectors, but will execute with 64 bits at a time, whereas newer Cortex-A15 devices can execute 128 bits at a time.

ProjectNe10 is ARM's first open-source project (from its inception; while they acquired an older project, now known as ). The Ne10 library is a set of common, useful functions written in both NEON and C (for compatibility). The library was created to allow developers to use NEON optimisations without learning NEON, but it also serves as a set of highly optimised NEON intrinsic and assembly code examples for common DSP, arithmetic, and image processing routines. The source code is available on GitHub.

TrustZone (for Cortex-A profile)
The Security Extensions, marketed as TrustZone Technology, is in ARMv6KZ and later application profile architectures. It provides a low-cost alternative to adding another dedicated security core to an SoC, by providing two virtual processors backed by hardware based access control. This lets the application core switch between two states, referred to as worlds (to reduce confusion with other names for capability domains), in order to prevent information from leaking from the more trusted world to the less trusted world. This world switch is generally orthogonal to all other capabilities of the processor, thus each world can operate independently of the other while using the same core. Memory and peripherals are then made aware of the operating world of the core and may use this to provide access control to secrets and code on the device.

Typically, a rich operating system is run in the less trusted world, with smaller security-specialized code in the more trusted world, aiming to reduce the. Typical applications include functionality for controlling the use of media on ARM-based devices, and preventing any unapproved use of the device.

In practice, since the specific implementation details of proprietary TrustZone implementations have not been publicly disclosed for review, it is unclear what level of assurance is provided for a given, but they are not immune from attack.

Open Virtualization is an open source implementation of the trusted world architecture for TrustZone.

has licensed and incorporated TrustZone technology into its Secure Processor Technology. Enabled in some but not all products, AMD's s include a Cortex-A5 processor for handling secure processing. In fact, the Cortex-A5 TrustZone core had been included in earlier AMD products, but was not enabled due to time constraints.

uses TrustZone for purposes such as detecting modifications to the kernel.

TrustZone for ARMv8-M (for Cortex-M profile)
The Security Extension, marketed as TrustZone for ARMv8-M Technology, was introduced in the ARMv8-M architecture.

No-execute page protection
As of ARMv6, the ARM architecture supports, which is referred to as XN, for eXecute Never.

Large Physical Address Extension (LPAE)
The Large Physical Address Extension (LPAE), which extends the physical address size from 32 bits to 40 bits, was added to the ARMv7-A architecture in 2011. Physical address size is larger, 44 bits, in Cortex-A75 and Cortex-A65AE.

ARMv8-R and ARMv8-M
The ARMv8-R and ARMv8-M sub-architectures, announced after the ARMv8-A sub-architecture, share some features with ARMv8-A, but don't include any 64-bit AArch64 instructions.

ARMv8-A
Announced in October 2011, ARMv8-A (often called ARMv8 while the ARMv8-R is also available) represents a fundamental change to the ARM architecture. It adds an optional 64-bit architecture (e.g. Cortex-A32 is a 32-bit ARMv8-A CPU while most ARMv8-A CPUs support 64-bit, unlike all ARMv8-R), named "AArch64", and the associated new "A64" instruction set. AArch64 provides compatibility with ARMv7-A, the 32-bit architecture, therein referred to as "AArch32" and the old 32-bit instruction set, now named "A32". The Thumb instruction set is referred to as "T32" and has no 64-bit counterpart. ARMv8-A allows 32-bit applications to be executed in a 64-bit OS, and a 32-bit OS to be under the control of a 64-bit. ARM announced their Cortex-A53 and Cortex-A57 cores on 30 October 2012. Apple was the first to release an ARMv8-A compatible core in a consumer product. , using an, was the first to demo ARMv8-A. The first ARMv8-A from  is the Exynos 5433 used in the, which features two clusters of four Cortex-A57 and Cortex-A53 cores in a big.LITTLE configuration; but it will run only in AArch32 mode.

To both AArch32 and AArch64, ARMv8-A makes VFPv3/v4 and advanced SIMD (NEON) standard. It also adds cryptography instructions supporting, / and.

AArch64 features

 * New instruction set, A64
 * Has 31 general-purpose 64-bit registers.
 * Has dedicated zero or stack pointer (SP) register (depending on instruction).
 * The program counter (PC) is no longer directly accessible as a register.
 * Instructions are still 32 bits long and mostly the same as A32 (with LDM/STM instructions and most conditional execution dropped).
 * Has paired loads/stores (in place of LDM/STM).
 * No for most instructions (except branches).
 * Most instructions can take 32-bit or 64-bit arguments.
 * Addresses assumed to be 64-bit.
 * Advanced SIMD (NEON) enhanced
 * Has 32× 128-bit registers (up from 16), also accessible via VFPv4.
 * Supports double-precision floating point.
 * Fully compliant.
 * AES encrypt/decrypt and SHA-1/SHA-2 hashing instructions also use these registers.
 * A new exception system
 * Fewer banked registers and modes.
 * Memory translation from 48-bit virtual addresses based on the existing Large Physical Address Extension (LPAE), which was designed to be easily extended to 64-bit.

AArch64 was introduced in ARMv8-A and is included in subsequent versions of ARMV8-A. AArch64 is not included in ARMv8-R or ARMv8-M, because they are both 32-bit architectures.

ARMv8.1-A
In December 2014, ARMv8.1-A, an update with "incremental benefits over v8.0", was announced. The enhancements fell into two categories: changes to the instruction set, and changes to the exception model and memory translation.

Instruction set enhancements included the following:


 * A set of AArch64 atomic read-write instructions.
 * Additions to the Advanced SIMD instruction set for both AArch32 and AArch64 to enable opportunities for some library optimizations:
 * Signed Saturating Rounding Doubling Multiply Accumulate, Returning High Half.
 * Signed Saturating Rounding Doubling Multiply Subtract, Returning High Half.
 * The instructions are added in vector and scalar forms.
 * A set of AArch64 load and store instructions that can provide memory access order that is limited to configurable address regions.
 * The optional CRC instructions in v8.0 become a requirement in ARMv8.1.

Enhancements for the exception model and memory translation system included the following:


 * A new Privileged Access Never (PAN) state bit provides control that prevents privileged access to user data unless explicitly enabled.
 * An increased VMID range for virtualization; supports a larger number of virtual machines.
 * Optional support for hardware update of the page table access flag, and the standardization of an optional, hardware updated, dirty bit mechanism.
 * The Virtualization Host Extensions (VHE). These enhancements improve the performance of Type 2 hypervisors by reducing the software overhead associated when transitioning between the Host and Guest operating systems. The extensions allow the Host OS to execute at EL2, as opposed to EL1, without substantial modification.
 * A mechanism to free up some translation table bits for operating system use, where the hardware support is not needed by the OS.

ARMv8.2-A
In January 2016, ARMv8.2-A was announced. Its enhancements fell into four categories:


 * Optional data processing (half-precision was already supported, but not for processing, just as a storage format.)
 * Memory model enhancements
 * Introduction of (RAS Extension)
 * Introduction of statistical profiling

Scalable Vector Extension (SVE)
The Scalable Vector Extension (SVE) is a new extension for ARMv8 allowing "implementation choices for vector lengths that scale from 128 to 2048 bits"; a complementary extension that does not replace. (A 512-bit variant has already been implemented. A supercomputer based on an ARM CPU prototype with that SVE variant aims to be the world's highest-performing supercomputer with "the goal of beginning full operations around 2021." See at .) SVE is "an optional extension to the ARMv8.2-A architecture and newer", and is supported by the 8 compiler.

ARMv8.3-A
In October 2016, ARMv8.3-A was announced. Its enhancements fell into six categories:


 * Pointer authentication (AArch64 only); mandatory extension (based on a new block cipher, ) to the architecture (compilers need to exploit the security feature, but as the instructions are in NOP space, they are backwards compatible albeit providing no extra security on older chips).
 * Nested virtualization (AArch64 only)
 * Advanced SIMD support (AArch64 and AArch32); e.g. rotations by multiples of 90 degrees.
 * New FJCVTZS (Floating-point Convert to Signed fixed-point, rounding toward Zero) instruction.
 * A change to the memory consistency model (AArch64 only); to support the (non-default) weaker RCpc (Release Consistent processor consistent) model of / (the default C++11/C11 consistency model was already supported in previous ARMv8).
 * ID mechanism support for larger system-visible caches (AArch64 and AArch32)

ARMv8.3-A architecture is now supported by (at least) the 7 compiler.

ARMv8.4-A
ARMv8.4-A adds e.g. "SHA3 / SHA512 / SM3 / crypto extensions", improved virtualization support, Memory Partitioning and Monitoring (MPAM) capabilities, a new Secure EL2 state and Activity Monitors, and dot product (SDOT and UDOT) instructions.

ARMv8.5-A
ARMv8.5-A adds e.g. Memory Tagging Extension (MTE), Branch Target Indicators (BTI) to reduce "the ability of an attacker to execute arbitrary code" and "Random Number Generator instructions – providing Deterministic and True Random Numbers conforming to various National and International Standards."

On August 2, 2019, announced  would adopt Memory Tagging Extension (MTE).

Future ARM architecture features
In 2019, ARM announced their upcoming Scalable Vector Extension 2 (SVE2), Transactional Memory Extension (TME) and support.

Scalable Vector Extension 2 (SVE2)
SVE2 builds on SVE's scalable vectorization for increased fine-grain, to allow more work done per instruction. SVE2 aims to bring these benefits to a wider range of software including DSP and multimedia SIMD code that currently use. The / 9.0 and 10.0 development codes were updated to support SVE2.

Transactional Memory Extension (TME)
TME brings support for and. TME aims to bring scalable concurrency to increase coarse-grain, to allow more work done per thread. The / 9.0 and 10.0 development codes were updated to support TME.

Platform Security Architecture
Platform Security Architecture (PSA) is an architecture-agnostic security framework and evaluation scheme, intended to help secure Internet of Things (IoT) devices built on system-on-a-chip (SoC) processors. It was introduced by Arm in 2017 at the annual TechCon event and will be first used on Arm Cortex-M processor cores intended for microcontroller use. The PSA includes freely available threat models and security analyses that demonstrate the process for deciding on security features in common IoT products. The PSA also provides freely downloadable application programming interface (API) packages, architectural specifications, open-source firmware implementations, and related test suites. PSA Certified offers a multi-level security evaluation scheme for chip vendors, OS providers and IoT device makers.

Historical operating systems
The first 32-bit ARM-based personal computer, the, ran an interim operating system called , which evolved into , used on later ARM-based systems from Acorn and other vendors. Some Acorn machines also had a port called. (Neither is to be confused with, a contemporary Unix variant for the MIPS architecture.)

Embedded operating systems
The 32-bit ARM architecture is supported by a large number of and, including:


 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * SCIOPTA
 * SCIOPTA
 * SCIOPTA
 * SCIOPTA
 * SCIOPTA
 * SCIOPTA
 * SCIOPTA

Mobile device operating systems
The 32-bit ARM architecture is the primary hardware environment for most mobile device operating systems such as:

Previously, but now discontinued:
 * 10 and earlier

Desktop/server operating systems
The 32-bit ARM architecture is supported by RISC OS and by multiple operating systems including:
 * several distributions, such as:
 * several distributions, such as:
 * several distributions, such as:
 * several distributions, such as:
 * several distributions, such as:

Embedded operating systems

 * SCIOPTA
 * Pharos
 * SCIOPTA
 * Pharos
 * Pharos

Mobile device operating systems

 * supports ARMv8-A in and later on 64-bit s.  and later only supports 64-bit ARM processors and applications.
 * supports ARMv8-A in (5.0) and later.

Desktop/server operating systems

 * Support for ARMv8-A was merged into the version 3.7 in late 2012. ARMv8-A is supported by a number of s, such as:
 * Support for ARMv8-A was merged into in late 2014.
 * has experimental ARMv8 support as of 2017.
 * has ARMv8 support as of the begin of 2018.
 * – runs 32-bit " and 32-bit ARM applications", as well as native ARM64 desktop apps. Support for 64-bit ARM apps in the Windows Store is forthcoming.
 * Support for ARMv8-A was merged into in late 2014.
 * has experimental ARMv8 support as of 2017.
 * has ARMv8 support as of the begin of 2018.
 * – runs 32-bit " and 32-bit ARM applications", as well as native ARM64 desktop apps. Support for 64-bit ARM apps in the Windows Store is forthcoming.
 * has ARMv8 support as of the begin of 2018.
 * – runs 32-bit " and 32-bit ARM applications", as well as native ARM64 desktop apps. Support for 64-bit ARM apps in the Windows Store is forthcoming.

Porting to 32- or 64-bit ARM operating systems
Windows applications recompiled for ARM and linked with Winelib – from the  project can run on 32-bit or 64-bit ARM in Linux (or FreeBSD or other compatible enough operating systems). x86 binaries, e.g. when not specially compiled for ARM, have been demonstrated on ARM using with Wine (on Linux and more), but do not work at full speed or same capability as with Winelib.