How does the ARM architecture differ from x86

Question

Is the x86 Architecture specially designed to work with a keyboard while ARM expects to be mobile  What are the key differences between the two

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Neither has anything specific to keyboard or mobile, other than the fact that for years ARM has had a pretty substantial advantage in terms of power consumption, which made it attractive for all sorts of battery operated devices.

As far as the actual differences: ARM has more registers, supported predication for most instructions long before Intel added it, and has long incorporated all sorts of techniques (call them "tricks", if you prefer) to save power almost everywhere it could.

There's also a considerable difference in how the two encode instructions. Intel uses a fairly complex variable-length encoding in which an instruction can occupy anywhere from 1 up to 15 byte. This allows programs to be quite small, but makes instruction decoding relatively difficult (as in: decoding instructions fast in parallel is more like a complete nightmare).

ARM has two different instruction encoding modes: ARM and THUMB. In ARM mode, you get access to all instructions, and the encoding is extremely simple and fast to decode. Unfortunately, ARM mode code tends to be fairly large, so it's fairly common for a program to occupy around twice as much memory as Intel code would. Thumb mode attempts to mitigate that. It still uses quite a regular instruction encoding, but reduces most instructions from 32 bits to 16 bits, such as by reducing the number of registers, eliminating predication from most instructions, and reducing the range of branches. At least in my experience, this still doesn't usually give quite as dense of coding as x86 code can get, but it's fairly close, and decoding is still fairly simple and straightforward. Lower code density means you generally need at least a little more memory and (generally more seriously) a larger cache to get equivalent performance.

At one time Intel put a lot more emphasis on speed than power consumption. They started emphasizing power consumption primarily on the context of laptops. For laptops their typical power goal was on the order of 6 watts for a fairly small laptop. More recently (much more recently) they've started to target mobile devices (phones, tablets, etc.) For this market, they're looking at a couple of watts or so at most. They seem to be doing pretty well at that, though their approach has been substantially different from ARM's, emphasizing fabrication technology where ARM has mostly emphasized micro-architecture (not surprising, considering that ARM sells designs, and leaves fabrication to others).

Depending on the situation, a CPU's energy consumption is often more important than its power consumption though. At least as I'm using the terms, power consumption refers to power usage on a (more or less) instantaneous basis. Energy consumption, however, normalizes for speed, so if (for example) CPU A consumes 1 watt for 2 seconds to do a job, and CPU B consumes 2 watts for 1 second to do the same job, both CPUs consume the same total amount of energy (two watt seconds) to do that job--but with CPU B, you get results twice as fast.

ARM processors tend to do very well in terms of power consumption. So if you need something that needs a processor's "presence" almost constantly, but isn't really doing much work, they can work out pretty well. For example, if you're doing video conferencing, you gather a few milliseconds of data, compress it, send it, receive data from others, decompress it, play it back, and repeat. Even a really fast processor can't spend much time sleeping, so for tasks like this, ARM does really well.

Intel's processors (especially their Atom processors, which are actually intended for low power applications) are extremely competitive in terms of energy consumption. While they're running close to their full speed, they will consume more power than most ARM processors--but they also finish work quickly, so they can go back to sleep sooner. As a result, they can combine good battery life with good performance.

So, when comparing the two, you have to be careful about what you measure, to be sure that it reflects what you honestly care about. ARM does very well at power consumption, but depending on the situation you may easily care more about energy consumption than instantaneous power consumption.

User · Answer

The ARM architecture was originally designed for Acorn personal computers  See Acorn Archimedes  circa 1987  and RiscPC   which were just as much keyboard-based personal computers as were x86 based IBM PC models   Only later ARM implementations were primarily targeted at the mobile and embedded market segment   Originally  simple RISC CPUs of roughly equivalent performance could be designed by much smaller engineering teams  see Berkeley RISC  than those working on the x86 development at Intel   But  nowadays  the fastest ARM chips have very complex multi-issue out-of-order instruction dispatch units designed by large engineering teams  and x86 cores may have something like a RISC core fed by an instruction translation unit     So  any current differences between the two architectures are more related to the specific market needs of the product niches that the development teams are targeting    Random opinion  ARM probably makes more in license fees from embedded applications that tend to be far more power and cost constrained   And Intel needs to maintain a performance edge in PCs and servers for their profit margins   Thus you see differing implementation optimizations

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ARM is a RISC  Reduced Instruction Set Computing  architecture while x86 is a CISC  Complex Instruction Set Computing  one   The core difference between those in this aspect is that ARM instructions operate only on registers with a few instructions for loading and saving data from   to memory while x86 can operate directly on memory as well  Up until v8 ARM was a native 32 bit architecture  favoring four byte operations over others   So ARM is a simpler architecture  leading to small silicon area and lots of power save features while x86 becoming a power beast in terms of both power consumption and production   About question on  Is the x86 Architecture specially designed to work with a keyboard while ARM expects to be mobile    x86 isn t specially designed to work with a keyboard neither ARM for mobile  However again because of the core architectural choices actually x86 also has instructions to work directly with IO while ARM has not  However with specialized IO buses like USBs  need for such features are also disappearing    If you need a document to quote  this is what Cortex-A Series Programmers Guide  4 0  tells about differences between RISC and CISC architectures      An ARM processor is a Reduced Instruction Set Computer  RISC    processor       Complex Instruction Set Computer  CISC  processors  like   the x86  have a rich instruction set capable of doing complex things   with a single instruction  Such processors often have significant   amounts of internal logic that decode machine instructions to   sequences of internal operations  microcode        RISC architectures  in   contrast  have a smaller number of more general purpose instructions    that might be executed with significantly fewer transistors  making   the silicon cheaper and more power efficient  Like other RISC   architectures  ARM cores have a large number of general-purpose   registers and many instructions execute in a single cycle  It has   simple addressing modes  where all load store addresses can be   determined from register contents and instruction fields    ARM company also provides a paper titled Architectures  Processors  and Devices Development Article describing how those terms apply to their bussiness   An example comparing instruction set architecture   For example if you would need some sort of bytewise memory comparison block in your application  generated by compiler  skipping details   this is how it might look like on x86  repe cmpsb            repeat while equal compare string bytewise      while on ARM shortest form might look like  without error checking etc    top  ldrb r2   r0   1      load a byte from address in r0 into r2  increment r0 after    ldrb r3   r1   1      load a byte from address in r1 into r3  increment r1 after    subs r2  r3  r2       subtract r2 from r3 and put result into r2         beq  top              branch  jump  if result is zero                      which should give you a hint on how RISC and CISC instruction sets differ in complexity

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Additional to Jerry Coffin s first paragraph   Ie  ARM design gives lower power consumption   The company ARM  only licenses the CPU technology   They don t make physical chips   This allows other companies to add various peripheral technologies  typically called SOC or system-on-chip   Whether the device is a tablet  a cell phone  or an in-car entertainment system   This allows chip vendors to tailor the rest of the chip to a particular application   This has additional benefits    Lower board cost Lower power  note1  Easier manufacture Smaller form factor   ARM supports SOC vendors with AMBA  allowing SOC implementers to purchase off the shelf 3rd party modules  like an Ethernet  memory and interrupt controllers   Some other CPU platforms support this  like MIPS  but MIPS is not as power conscious   All of these are beneficial to a handheld battery operated design   Some are just good all around   As well  ARM has a history of battery operated devices  Apple Newton  Psion Organizers   The PDA software infra-structure was leveraged by some companies to create smart phone type devices   Although  more success was had by those who re-invented the GUI for use with a smart phone   The rise of Open source tool sets and operating systems also facilitated the various SOC chips   A closed organization would have issues trying to support all the various devices available for the ARM   The two most popular cellular platforms  Andriod and OSx IOS  are based up Linux and FreeBSD  Mach and NetBSD os s   Open Source helps SOC vendors provide software support for their chip sets   Hopefully  why x86 is used for the keyboard is self-evident   It has the software  and more importantly people trained to use that software   Netwinder is one ARM system that was originally designed for the keyboard   Also  manufacturer s are currently looking at ARM64 for the server market   Power heat is a concern at 24 7 data centers   So I would say that the ecosystem that grows around these chips is as important as features like low power consumption   ARM has been striving for low power  higher performance computing for some time  mid to late 1980 s  and they have a lot of people on board   Note1  Multiple chips need bus drivers to inter-communicate at known voltages and drive   Also  typically separate chips need support capacitors and other power components which can be shared in an SOC system

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The ARM is like an Italian sports car     Well balanced  well tuned  engine  Gives good acceleration  and top speed  Excellent chases  brakes and suspension  Can stop quickly  can corner without slowing down    The x86 is like an American muscle car    Big engine  big fuel pump  Gives excellent top speed  and acceleration  but uses a lot of fuel  Dreadful brakes  you need to put an appointment in your diary  if you want to slowdown  Terrible steering  you have to slow down to corner    In summary  the x86 is based on a design from 1974 and is good in a straight line  but uses a lot of fuel   The arm uses little fuel  does not slowdown for corners  branches      Metaphor over  here are some real differences    Arm has more registers  Arm has few special purpose registers  x86 is all special purpose registers  so less moving stuff around   Arm has few memory access commands  only load store register  Arm is internally Harvard architecture my design  Arm is simple and fast  Arm instructions are architecturally single cycle  except load store multiple   Arm instructions often do more than one thing  in a single cycle   Where more that one Arm instruction is needed  such as the x86 s looping store  amp  auto-increment  the Arm still does it in less clock cycles  Arm has more conditional instructions  Arm s branch predictor is trivially simple  if unconditional or backwards then assume branch  else assume not-branch   and performs better that the very very very complex one in the x86  there is not enough space here to explain it  not that I could   Arm has a simple consistent instruction set  you could compile by hand  and learn the instruction set quickly

[x86] How does the ARM architecture differ from x86?

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