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?

Answer 1

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.

Answer 2

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.

Answer 3

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,

1. Lower board cost
2. Lower power (note1)
3. Easier manufacture
4. 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.

Answer 4

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).

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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 & 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).

Answer 5

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.)