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ARM architecture

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The ARM architecture (originally the Acorn RISC Machine) is a 32-bit RISC processor architecture that is widely used in a number of embedded designs. Due to their power saving features, ARM CPUs are dominant in the mobile electronics market, where low power consumption is a critical design goal.

Today, the ARM family accounts for over 75% of all 32-bit embedded CPUs^[1], making it one of the most prolific 32-bit architectures in the world. ARM CPUs are found in all corners of consumer electronics, from portable devices (PDAs, mobile phones, media players, handheld gaming units, and calculators) to computer peripherals (hard drives, desktop routers.) The most noticeable branch in this family nowadays is Intel's XScale.

ARM's headquarters in Cambridge, UK

1 History
2 The cores
3 Design notes
4 ARM licensees
5 See also
6 External links

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History

A Conexant ARM processor used mainly in routers

The ARM design was started in 1983 as a development project at Acorn Computers Ltd.

The team, led by Roger Wilson and Steve Furber, started development of what in some ways resembles an advanced MOS Technology 6502. Acorn had a long line of computers based on the 6502, so a chip that was similar to program could represent a significant advantage for the company.

The team completed development samples called ARM1 by 1985, and the first "real" production systems as ARM2 the following year. The ARM2 featured a 32-bit data bus, a 26-bit address space giving a 64 Mbyte address range and 16 32-bit registers. One of these registers served as the (word aligned) program counter with its top 6 bits and lowest 2 bits holding the processor status flags. The ARM2 was possibly the simplest useful 32-bit microprocessor in the world, with only 30,000 transistors (compare with Motorola's six-year older 68000 with around 68,000). Much of this simplicity comes from not having microcode (which represents about one-fourth to one-third of the 68000) and, like most CPUs of the day, not including any cache. This simplicity led to its low power usage, while performing better than the 286. A successor, ARM3, was produced with a 4KB cache which further improved performance.

In the late 1980s Apple Computer started working with Acorn on newer versions of the ARM core. The work was so important that Acorn spun off the design team in 1990 into a new company called Advanced RISC Machines Ltd.. For this reason, ARM is sometimes expanded as Advanced RISC Machine instead of Acorn RISC Machine. Advanced RISC Machines became ARM Ltd when its parent company, ARM Holdings plc, floated on the London Stock Exchange and NASDAQ in 1998.[1]

This work would eventually turn into the ARM6. The first models were released in 1991, and Apple used the ARM6-based ARM 610 as the basis for their Apple Newton PDA. In 1994, Acorn used the ARM 610 as the main CPU in their RiscPC computers.

The core has remained largely the same size throughout these changes. ARM2 had 30,000 transistors, while the ARM6 grew to only 35,000. The idea is that the Original Design Manufacturer combines the ARM core with a number of optional parts to produce a complete CPU, one that can be built on old semiconductor fabs and still deliver lots of performance at a low cost.

The most successful implementation has been the ARM7TDMI with hundreds of millions sold in mobile phones, handheld video game systems, and Sega Dreamcasts. While ARM's business has always been to sell IP cores, some of the licensees generated microcontrollers based on this core.

The Dreamcast features a SH4 processor which only borrows concepts from ARM (low power consumption, optional compact instruction set etc.), but is otherwise different from an ARM. The Dreamcast also features a sound chip designed by Yamaha with an ARM7 core. Nintendo's Gameboy Advance, however, uses the ARM7TDMI at 16.78MHz.

DEC licensed the architecture (which caused some confusion because they also produced the DEC Alpha) and produced the StrongARM. At 233 MHz this CPU drew only 1 watt of power (more recent versions draw far less). This work was later passed to Intel as a part of a lawsuit settlement, and Intel took the opportunity to supplement their aging i960 line with the StrongARM. Intel later developed its own high performance implementation known as XScale which it has since sold.

The common architecture supported on Windows Mobile smartphones, Personal Digital Assistants and other handheld devices is ARM4.

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The cores

Family	Core	Feature	Cache (I/D)/MMU	typical MIPS @ MHz	in Application
ARM7TDMI	ARM7TDMI(-S)	3-stage pipeline	none	15 MIPS @ 16.8 MHz	Game Boy Advance, Nintendo DS, iPod
	ARM710T		MMU	36 MIPS @ 40 MHz	Psion 5 series, eMate 300
	ARM720T		8KB unified, MMU	60 MIPS @ 59.8 MHz
	ARM740T		MPU
	ARM7EJ-S	Jazelle DBX	none
ARM9TDMI	ARM9TDMI	5-stage pipeline	none
	ARM920T		16KB/16KB, MMU	200 MIPS @ 180 MHz	Armadillo, GP32,GP2X (first core), Tapwave Zodiac (Motorola i. MX1)
	ARM922T		8KB/8KB, MMU
	ARM940T		4KB/4KB, MPU		GP2X (second core)
ARM9E	ARM946E-S		variable, tightly coupled memories, MPU		Nintendo DS, Nokia N-Gage, Conexant 802.11 chips
	ARM966E-S		no cache, TCMs		ST Micro STR91xF, includes Ethernet [2]
	ARM968E-S		no cache, TCMs
	ARM926EJ-S	Jazelle DBX	variable, TCMs, MMU	220 MIPS @ 200 MHz	Mobile phones: Sony Ericsson (K, W series),Siemens and Benq (x65 series and newer)
	ARM996HS	Clockless processor	no caches, TCMs, MPU
ARM10E	ARM1020E	(VFP)	32KB/32KB, MMU
	ARM1022E	(VFP)	16KB/16KB, MMU
	ARM1026EJ-S	Jazelle DBX	variable, MMU or MPU
ARM11	ARM1136J(F)-S	SIMD, Jazelle DBX, (VFP)	variable, MMU		Omap2 Nokia N93(?)
	ARM1156T2(F)-S	SIMD, Thumb-2, (VFP)	variable, MPU
	ARM1176JZ(F)-S	SIMD, Jazelle DBX, (VFP)	variable, MMU+TrustZone
	ARM11 MPCore	1-4 core SMP, SIMD, Jazelle DBX, (VFP)	variable, MMU
Cortex	Cortex-A8	Application profile, NEON, Jazelle RCT, Thumb-2	variable (L1+L2), MMU+TrustZone	up to 2000 (2.0 DMIPS/MHz in speed from 600 MHz to greater than 1 GHz)	Texas Instruments OMAP3
	Cortex-R4	Embedded profile	variable cache, MMU optional	600 DMIPS	Broadcom is a user
	Cortex-M3	Microcontroller profile	no cache, (MPU)	120 DMIPS @ 100MHz	Luminary Micro[3] microcontroller family
XScale	80200/IOP310/IOP315	I/O Processor
	80219
	IOP321				Iyonix
	IOP33x
	PXA210/PXA250	Applications processor			Zaurus SL-5600
	PXA255		32KB/32KB, MMU	400 BogoMips @400 MHz	Gumstix
	PXA26x
	PXA27x			800 MIPS @ 624 MHz	HTC Universal, Zaurus SL-C1000
	PXA800(E)F
	Monahans			1000 MIPS @ 1.25 GHz
	PXA900				Blackberry 8700
	IXC1100	Control Plane Processor
	IXP2400/IXP2800
	IXP2850
	IXP2325/IXP2350
	IXP42x				NSLU2
	IXP460/IXP465

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Design notes

To keep the design clean, simple and fast, it was hardwired without microcode, like the much simpler 8-bit 6502 processor used in prior Acorn microcomputers.

The ARM architecture includes the following RISC features:

Load/store architecture
No support for misaligned memory accesses (now supported in v6 Arm cores)
Orthogonal instruction set
Large 16 × 32-bit register file
Fixed opcode width of 32 bits to ease decoding and pipelining, at the cost of decreased code density
Mostly single-cycle execution

To compensate for the simpler design, compared to contemporary processors like the Intel 80286 and Motorola 68020, some unique design features were used:

Conditional execution of most instructions, reducing branch overhead and compensating for the lack of a branch predictor
Arithmetic instructions only alter condition codes when desired
32-bit barrel shifter which can be used without performance penalty with most arithmetic instructions and address calculations
Powerful indexed addressing modes
Simple, but fast, 2-priority-level interrupt subsystem with switched register banks

An interesting addition to the ARM design is the use of a 4-bit condition code on the front of every instruction, meaning that execution of every instruction is optionally conditional.

This cuts down significantly on the encoding bits available for displacements in memory access instructions, but on the other hand it avoids branch instructions when generating code for small if statements. The standard example of this is Euclid's GCD algorithm:

In the C programming language, the loop is:

int gcd (int i, int j) 
{
   while (i != j) 
      if (i > j) 
          i -= j;
      else 
          j -= i;
   return i;
}

In ARM assembly, the loop is:

loop   CMP    Ri, Rj       ; set condition "NE" if (i != j)
                           ;               "GT" if (i > j), 
                           ;           or  "LT" if (i < j)           
       SUBGT  Ri, Ri, Rj   ; if "GT", i = i-j;  
       SUBLT  Rj, Rj, Ri   ; if "LT", j = j-i; 
       BNE    loop         ; if "NE", then loop

which avoids the branches around the then and else clauses.

Another unique 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

a += (j << 2);

could be rendered as a single word, single cycle instruction on the ARM.

ADD Ra, Ra, Rj, LSL #2

This results in the typical ARM program being denser than expected with less memory access so the pipeline is used more efficiently. Even though the ARM runs at what many would consider to be low speeds, it nevertheless competes quite well with much more complex CPU designs.

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

Another item of note is that the ARM has been around for a while, with the instruction set increasing somewhat over time. Some early ARM processors (prior to ARM7TDMI), for example, have no instruction to load a two-byte quantity, so that, strictly speaking, for them it's not possible to generate code that would behave the way one would expect for C objects of type "volatile short".

The ARM7 and most earlier designs have a three stage pipeline; the stages being fetch, decode, and execute. Higher performance designs, such as the ARM9, have a five stage pipeline. Additional changes for higher performance include a faster adder, and more extensive branch prediction logic.

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Thumb

Newer ARM processors have a 16-bit instruction mode, called Thumb, perhaps related to the conditional execution facility using four bits of every instruction. In Thumb, the smaller opcodes have less functionality. For example, only branches can be conditional, and many opcodes cannot access all of the CPU's registers. However, the shorter opcodes give improved code density overall, even though some operations require more instructions. Particularly in situations where the memory port or bus width is constrained to less than 32 bits, the shorter Thumb opcodes allows greater performance than with 32-bit code because of the more efficient use of the limited memory bandwidth. Typically embedded hardware has a small range of addresses of 32-bit datapath and the rest are 16 bits or narrower (e.g. the Game Boy Advance). In this situation, it usually makes sense to compile Thumb code and hand-optimise a few of the most CPU-intensive sections using the (non-Thumb) 32-bit instruction set, placing them in the limited 32-bit bus width memory.

The first processor with Thumb technology was the ARM7TDMI. All ARM9 and later families, including XScale have included Thumb technology.

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Jazelle

ARM has implemented a technology that allows certain of their architectures to execute Java bytecode natively in hardware, as another execution mode. It interoperates alongside the existing ARM and Thumb modes.

The first processor with Jazelle technology was the ARM926EJ-S: Jazelle being denoted by the 'J' in the CPU name. It is used by mobile phone manufacturers to speed up execution of Java ME games and applications, which is probably what drove development of the technology.

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Thumb-2

Thumb-2 technology made its debut 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. As a result the stated aim for Thumb-2 is to achieve code density that is similar to Thumb with performance similar to the ARM instruction set on 32-bit memory.

Thumb-2 also extends both the ARM and Thumb instruction set with yet more instructions, including bit-field manipulation, table branches, and conditional execution.

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Thumb-2EE

Thumb-2EE, marketed as Jazelle RCT, was announced in 2005, first appearing in the Cortex-A8 processor. Thumb-2EE provides a small extension to Thumb-2, making the instruction set particularly suited to code generated at runtime (e.g. by JIT compilation) in managed Execution Environments. Thumb-2EE is a target for languages such as Limbo, Java, C#, Perl and Python, and allows JIT compilers to output smaller compiled code without impacting performance.

New features provided by Thumb-2EE include automatic null pointer checks on every load and store instruction, an instruction to perform an array bounds check, and the ability to branch to handlers, which are small sections of frequently called code, commonly used to implement a feature of a high level language, such as allocating memory for a new object.

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NEON

NEON technology is a combined 64 and 128 bit SIMD (Single Instruction Multiple Data) instruction set that provides standardized acceleration for media and signal processing applications. NEON can execute MP3 audio decoding on CPUs running at 10 MHz and can run the GSM AMR (Adaptive Multi-Rate) speech codec at no more than 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 floating-point data and operates in SIMD operations for handling audio/video processing as well as graphics and gaming processing. SIMD is a crucial element in vector supercomputers which feature simultaneous multiple operations. In NEON, the SIMD supports up to 16 operations at the same time.

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VFP

VFP technology is a coprocessor extension to the ARM architecture. It provides low-cost single-precision and double-precision floating-point computation fully compliant with the ANSI/IEEE Std 754-1985 Standard for Binary Floating-Point Arithmetic. 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 also supports execution of short vector instructions allowing SIMD (Single Instruction Multiple Data) parallelism. This is useful in graphics and signal-processing applications by reducing code size and increasing throughput.

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ARM licensees

ARM Ltd does not manufacture and sell CPU devices based on their own designs, but rather, licenses the processor architecture to interested parties. ARM offers a variety of licensing terms, varying in cost and deliverables. To all licensees, ARM provides an integratable hardware description of the ARM core, as well as complete software development toolset (compiler, debugger, SDK), 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 IP core. For these customers, ARM 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. More ambitious customers, including integrated device manufacturers (IDM) and foundry operators, choose to acquire the processor IP in synthesizable RTL (Verilog) form. With the synthesizable RTL, the customer has the ability to perform architectural level optimizations 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 does not grant the licensee the right to resell the ARM architecture itself, licensees may freely sell manufactured product (chip devices, evaluation boards, complete systems, etc.) 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 remanufacture ARM cores for other customers.

Like most IP vendors, ARM prices its IP based on perceived value. In architectural terms, the lower performance ARM cores command a lower license cost than the higher performance cores. In terms of silicon implementation, a synthesizable core is more expensive than a hard macro (blackbox) core. Complicating price matters, merchant foundries who hold an ARM license (such as Samsung and Fujitsu) can offer reduced licensing costs to its fab customers. 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 license fee. Compared to dedicated semicounductor foundries (such as TSMC and UMC) without in-house design services, Fujitsu/Samsung charge 2 to 3 times more per manufactured wafer. For low to mid volume applications, a design service foundry offers lower overall pricing (through subsidization of the license fee.) For high volume mass produced parts, the long term cost reduction achievable through lower wafer pricing reduces the impact of ARM's NRE cost, making the dedicated foundry a better choice.

Many hightech semiconductor firms hold ARM licenses: Atmel, Broadcom, Cirrus Logic, Freescale (spun off from Motorola in 2004), Fujitsu, Intel (through its settlement with DEC), IBM, Infineon Technologies, Nintendo, OKI, Philips, Samsung, Sharp, STMicroelectronics, Texas Instruments and VLSI are some of the many companies who have licensed the ARM in one form or another. Although ARM's license terms are covered by NDA, within the IP industry, ARM is widely known to be among the most expensive CPU cores. A single customer product containing a basic ARM core can incur a one time license fee in excess of (USD) $200,000. Where significant quantity and architectural modification are involved, the license fee can exceed $10M.