Intel's Atom Architecture: The Journey Begins
by Anand Lal Shimpi on April 2, 2008 12:05 AM EST- Posted in
- CPUs
Building by FUBs
Below we have a conventional block diagram of the Intel Atom processor:
You'll see nine distinct blocks and in conventional microprocessor design at Intel, this is how the chip would be divided and conquered. You'd have one team working on the L2 cache, one team working on the IO interfaces, etc... Unfortunately the team at Austin was significantly smaller than your conventional microprocessor design group, so things had to be done a little differently.
Intel calls the Atom approach a Sea-of-FUBs chip layout, a more granular approach to chip layout and design. A FUB (Functional Unit Block) is a floor-plannable object, an individual adder is a FUB, a decoder is a FUB, a cache is a FUB, etc... Because the size of each FUB made them more manageable, a single designer could handle multiple FUBs. The FUBs were also treated more modularly so that once a FUB was defined, designers can work on the FUB, chip integraters can give it to the layout guys and the timing guys can work in parallel as well. It allows for much more concurrent design than the conventional, more serial approach to chip design.
Approximately 90% of the fubs are built using standard Intel logic cells, minimizing the amount of custom work that needs to be done and decreasing the time to market on a processor like Atom. Minimizing the amount of custom Intel logic also means that it's easier to produce variants of Atom that may incorporate other important features depending on the target market.
The Sea-of-FUBs approach was also driven by a desire to keep power consumption and feature creep to a minimum. Area budgets were developed for each FUB and cluster of FUBs and Intel forced the designers to stick to these budgets. If a designer needed more die space for their FUB they couldn't just take it. A review board was setup where designers would come in and ask to grow their FUB. They were allowed to grow the FUB only if they were able to find another FUB that would shrink to accommodate it. The same thing applied for power.
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highlandsun - Thursday, April 3, 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.Jovec - Thursday, April 3, 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.highlandsun - Thursday, April 3, 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?maree - Thursday, April 3, 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 2, 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.
Anand Lal Shimpi - Wednesday, April 2, 2008 - link
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,
Anand
fitten - Wednesday, April 2, 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 :)crimson117 - Wednesday, April 2, 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.
Anand Lal Shimpi - Wednesday, April 2, 2008 - link
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...