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It also designs cores that implement this instruction set 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 transistors than those with a complex instruction set computing CISC architecture such as the x86 processors found in most personal computers , which improves cost, power consumption, and heat dissipation.

For supercomputers , which consume large amounts of electricity, ARM is also a power-efficient solution. Arm Holdings periodically releases updates to the architecture. Architecture versions ARMv3 to ARMv7 support bit address space pre-ARMv3 chips, made before Arm Holdings was formed, as used in the Acorn Archimedes , had bit address space and bit arithmetic; most architectures have bit fixed-length instructions.

The Thumb version supports a variable-length instruction set that provides both and bit instructions for improved code density. Some older cores can also provide hardware execution of Java bytecodes ; and newer ones have one instruction for JavaScript. Released in , the ARMv8-A architecture added support for a bit address space and bit arithmetic with its new bit fixed-length instruction set. Arm Neoverse E1 being able to execute two threads concurrently for improved aggregate throughput performance.

The Neoverse N1 is designed for "as few as 8 cores" or "designs that scale from 64 to N1 cores within a single coherent system". After testing all available processors and finding them lacking, Acorn decided it needed a new architecture. Hauser gave his approval and assembled a small team to implement Wilson's model in hardware. Wilson and Furber led the design. They implemented it with efficiency principles similar to the The 's memory access architecture had let developers produce fast machines without costly direct memory access DMA hardware.

The first samples of ARM silicon worked properly when first received and tested on 26 April The original aim of a principally ARM-based computer was achieved in with the release of the Acorn Archimedes. This simplicity enabled low power consumption, yet better performance than the Intel This work was later passed to Intel as part of a lawsuit settlement, and Intel took the opportunity to supplement their i line with the StrongARM.

Intel later developed its own high performance implementation named XScale, which it has since sold to Marvell. In , the bit ARM architecture was the most widely used architecture in mobile devices and the most popular bit one in embedded systems.

The original design manufacturer combines the ARM core with other parts to produce a complete device, typically one that can be built in existing semiconductor fabrication plants fabs at low cost and still deliver substantial performance. 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 compiler , debugger , software development kit and the right to sell manufactured silicon containing the ARM CPU.

Fabless licensees, who wish to integrate an ARM core into their own chip design, are usually only interested in acquiring a ready-to-manufacture verified semiconductor intellectual property core. For these customers, Arm Holdings delivers a gate netlist description of the chosen ARM core, along with an abstracted simulation model and test programs to aid design integration and verification. 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 high clock speed , 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. Merchant foundries 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. 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 developed chips with cores designed by Arm Holdings include Amazon. These design modifications will not be shared with other companies.

These semi-custom core designs also have brand freedom, for example Kryo These cores must comply fully with the ARM architecture. Arm Flexible Access provides unlimited access to included Arm intellectual property IP for development. Per product licence fees are required once customers reaches foundry tapeout or prototyping. As of October Marvell ThunderX3 v8.

Arm Holdings provides a list of vendors who implement ARM cores in their design application specific standard products ASSP , microprocessor and microcontrollers. ARM chips are also used in Raspberry Pi , BeagleBoard , BeagleBone , PandaBoard and other single-board computers , because they are very small, inexpensive and consume very little power.

Since , the ARM Architecture Reference Manual [79] 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":. 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. The original and subsequent ARM implementation was hardwired without microcode , like the much simpler 8-bit processor used in prior Acorn microcomputers. To compensate for the simpler design, compared with processors like the Intel and Motorola , some additional design features were used:.

ARM includes integer arithmetic operations for add, subtract, and multiply; some versions of the architecture also support divide operations. 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 R These registers generally contain the stack pointer and the return address from function calls, respectively.

Almost every ARM instruction has a conditional execution feature called predication , 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. The standard example of conditional execution is the subtraction-based Euclidean algorithm :. In the C programming language , the function is:.

For ARM assembly , the function can be effectively transformed into:. If r0 and r1 are equal then neither of the SUB instructions will be executed, eliminating the need for a conditional branch to implement the while check at the top of the loop, for example had SUBLE 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.

Another feature of the instruction set 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.

This results in the typical ARM program being denser than expected with fewer memory accesses; thus the pipeline is used more efficiently. The ARM instruction set has increased over time. The ARM7 and earlier implementations have a three-stage pipeline ; the stages being fetch, decode and execute. Additional implementation changes for higher performance include a faster adder and more extensive branch prediction logic. 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. 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. 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. To improve the ARM architecture for digital signal processing and multimedia applications, DSP instructions were added to the set. E-variants also imply T, D, M, and I. The new instructions are common in digital signal processor DSP architectures.

They include variations on signed multiply—accumulate , saturated add and subtract, and count leading zeros. 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.

To improve compiled code-density, processors since the ARM7TDMI released in [99] have featured the Thumb instruction set, which have their own state. When in this state, the processor executes the Thumb instruction set, a compact bit encoding for a subset of the ARM instruction set. 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 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.

Unlike processor architectures with variable length or bit instructions, such as the Cray-1 and Hitachi SuperH , the ARM and Thumb instruction sets exist independently of each other. Embedded hardware, such as the Game Boy Advance , typically have a small amount of RAM accessible with a full bit datapath; the majority is accessed via a 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 bit ARM instructions, placing these wider instructions into the bit bus accessible memory. Thumb-2 extends the limited bit instruction set of Thumb with additional 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 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. 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.


ARM architecture

What processors are supported by Linux, and when was support added for each one? The ARM processor is popular in the embedded space because it has the best power consumption to performance ratio, meaning it has the longest battery life and smallest amount of heat generated for a given computing task. It's the standard processor of smartphones. The 64 bit version ARMv8 was announced in with a ship date for volume silicon. Although ARM hardware has many different processor designs with varying clock speeds, cache sizes, and integrated peripherals, from a software perspective what matters is ARM architectures , which are the different instruction sets a compiler can produce.


ARMv5 Architecture Reference Manual



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