What AMD Taught Me: x86 Everywhere

Back when AMD first announced its intentions to extend the x86 ISA to 64-bits I asked Fred Weber, AMD's old CTO, whether it really made sense to extend x86 or if Intel made the right move with Itanium and its brand new ISA. His response made sense at the time, but I didn't quite understand the magnitude of what he was saying.

Fred said that the overhead of maintaining x86 compatibility was negligible, at the time around 10% of the die was the x86 decoder and that percentage would only shrink over time. We're now at around 8x the transistor count of the K8 processor that Fred was talking about back then and the cost of maintaining x86 backwards compatibility has shrunk to a very small number. But the benefit of backwards compatibility is huge.

These days just about everything is written for x86, everything in the PC space that is. The consumer electronics world isn't quite as blessed. We're starting to see an increasing number of PC applications asked to run on CE devices, things like web browsers, email clients, or simple network media players. Unfortunately, these CE devices don't run x86 platforms thus the manufacturers are either forced to port open source applications to their platform or try to develop something comparable in house.

The problem is that, generally speaking, the best applications currently exist on the PC. The last thing we want is for a company like Sony to enter the web browser market, I'd much rather have Firefox or IE on my internet enabled TV, or on my touch screen in my kitchen. Sure it can be ported to any architecture, but software developers don't exactly like supporting multiple platforms - it takes a lot of time to debug and maintain, and ends up costing a great deal of money.

The concept Fred was trying to get me to understand back in 2002 was this idea of having x86 everywhere. The instruction set didn't matter, what mattered was being able to run the same code on virtually any device. I've always pointed out that Apple must have hated making the iPhone because it became the only computer-like device in its product lineup that didn't run x86. It meant Apple had to maintain a completely separate software stack, specifically for the iPhone.

Fred was right. As computers infiltrate our homes in less conventional ways, being able to run the same applications on all devices will become increasingly more important.

What's ironic is that while Fred Weber first illuminated this issue for me, it would be Intel that was first to act on it.

Intel Aims at the Mainstream A Prelude to Success


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  • highlandsun - Thursday, April 03, 2008 - link

    With all due respect to Fred Weber, with Atom at 47 million transistors, it's pretty obvious that the 10% figure for X86 ISA compatibility is not negligible, particularly in this performance-at-absolute-minimum-power space. Anybody using X86 in tiny embedded systems is automatically giving up a chunk of their power budget that someone using a cleaner instruction set encoding can apply directly to useful work. And as the previous poster already pointed out - source code portability is the only thing that matters to application developers, and that's a non-problem these days. Using the X86 instruction set encoding is stupid. Using it on a low-power-budget device is suicide. Reply
  • Jovec - Thursday, April 03, 2008 - link

    I don't think the 10% reference meant 10% of all chips, but rather 10% of the current chip at the time the statement was made. In other words, x86 instruction decoding requires (roughly) a fixed amount of transistors for any chip, so the smaller the die size and larger the transistor count, less and less space is devoted to it. Reply
  • highlandsun - Thursday, April 03, 2008 - link

    Yes, that's obvious. And it's also obvious that Atom at 47 million transistors is paying a greater proportionate cost than Core2 Duo at 410 million transistors. In 2002 when Fred made that statement, AMD's current chip was the AthlonXP Thoroughbred, with about 37 million transistors. At the same time the Pentium 4 had 55 million. Put in context, I'd guess that the Atom at 47M vs P4 at 55M has more than 10% of its resources devoted to X86 decoding.

    Also, Fred's statement in 2002 didn't take into account the additional complexity introduced by the AMD64 instruction extensions, where now a single instruction may be anywhere from 1 to 16 bytes long. Given that you're doing a completely clean ground-up chip design in the first place, it would have made more sense (from both a power budget and real estate perspective) to design a clean, orthogonal, uniform-length encoding at the same time.

    Cross-platform ABI compatibility is stupid in the context they're aiming for; nobody is going to run their PC version of Crysis or MSWord on their cellphone. All that matters is API compatibility. With a consistent API, you can still run a separate binary translator if you really really want to move a desktop app to your mobile device but in most cases it would be a bad idea because a desktop app is unlikely to take advantage of power-saving APIs that would be important on a mobile. I.e., most of the time you're going to want purpose-built mobile apps anyway.
  • floxem - Tuesday, April 15, 2008 - link

    I agree. But it's Intel. What do you expect? Reply
  • maree - Thursday, April 03, 2008 - link

    I dont think MS will be ready before Windows 7 is released, which is another 3-5 years... and might coincide with Moorestown. Microsoft started work on WindowsLite only after releasing Vista. Vista is bloatware as of now. As of now MS has to rely on crippled versions of XP and Vista like starter and home, which is not very ideal.

    Apple and Linux are going to have a free run till then...
  • TA152H - Wednesday, April 02, 2008 - link

    Bringing up the Pentium is a little strange, because the whole market is completely different.

    The Pentium wasn't supposed to be for everyone when it came out. The processor market was different back then where previous generations lasted a long, long time. The Pentium wasn't supposed to replace the 486 right away, or even quickly, and being huge and a terrible power hog was acceptable because the initial iteration was just for a very small group of people who absolutely needed it. The original Pentium had a lot of problems, and struggled badly to reach 66 MHz, so they sold most of their processors at 60 MHz. The second generation was intended more for mainstream.

    Nowadays the latest generation replaces the earlier much more quickly, and has to cover more market segments more quickly. I still remember IBM releasing new machines for the 8086 in 1987. That's 9 years after the chip was made. It's just a different market.

    The Pentium is nothing like the Silverthorne though, and it's a strange comparison. The Pentium executed x86 instructions, it wasn't decoupled. It also had both pipes, the U and V, lockstepped, which is limitation the Silverthorne doesn't have.

    Saying the Pentium Pro was the first processor that allowed out of order processing is strange indeed. The only other processor this would have made sense with was the Pentium, since it was the only previous processor that was superscalar. So, they only made one in order processor, and then went to out of order with the next. It's difficult to see the extrapolation from this that it will be five years or more before Silverthorne goes out of order. It might be that long, but the backwards reference shouldn't be used to back that; it does more to contradict it.

    The Pentium reference was merely to show that what was once a huge, 300mm^2 design could now be built on a much, much smaller scale. And starting from scratch it's now possible to build something in-order that's significantly faster.

    The Pentium was an obvious comparison given that it was Intel's last two-issue in-order design, but I didn't mean to imply anything beyond that.

    It won't be too long before we'll be able to have something the speed of a Core 2 in a similarly small/cool running package as well :)

    Take care,
  • fitten - Wednesday, April 02, 2008 - link

    I remember back in the days of the Mac FX we talked about 'what ifs' like making a 6502 with the (then) modern process technologies and how fast would it run. I wonder what about now :) Reply
  • crimson117 - Wednesday, April 02, 2008 - link



    I am SO going to hold you to that! But I can only hope "won't be long" will mean within 12 months rather than within 12 years :P

    Especially after my fiasco mounting a Freezer 7 Pro on an Abit IP35-E, I'd love if a heatsink weren't even necessary.

    12 months won't be a reality unfortunately :) But look at it this way, the first Pentium M came out in 2003? And 5 years later we're able to have somewhat comparable performance with the Atom processor.

    I'm really curious to see what happens with Atom on 32nm...

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