Not Quite a Pentium, Not Quite an Atom: The Larrabee Core

Intel gave us enough information about Larrabee to begin a discussion of specifications, but not enough to even begin making any conclusions. We'll start with what we pretty much already know.

Intel's Larrabee is built out of a number of x86 cores that look, at a very high level, like this:

Each core is a dual-issue, in-order architecture loosely derived from the original Pentium microprocessor. The Pentium core was modified to include support for 64-bit operations, the updates to the x86 instruction set, larger caches, 4-way SMT/Hyper Threading and a 16-wide vector ALU.

While the team that ended up working on Atom may have originally worked on the Larrabee cores, there are some significant differences between Larrabee and Atom. Atom is geared towards much higher single threaded performance, with a deeper pipeline, a larger L2 cache and additional microarchitectural tweaks to improve general desktop performance.

  Intel Larrabee Core Intel Pentium Core (P54C) Intel Atom Core
Manufacturing Process 45nm 0.60µm 45nm
Simultaneous Multi-Threading 4-way 1-way 2-way
Issue Width dual-issue dual-issue dual-issue
Pipeline Depth 5-stages (?) 5-stages 16-stages
Scalar Execution Resources 2 x Integer ALUs (?)
1 x FPU (?)
2 x Integer ALUs
1 x FPU
2 x Integer ALUs
1 x FPU
Vector Execution Resources 16-wide Vector ALU None 1 x SIMD SSE
L1 Cache (I/D) 32KB/32KB 8KB/8KB 32KB/24KB
L2 Cache 256KB None (External) 512KB
ISA 64-bit x86
SSEn support?
Parallel/Graphics?
32-bit x86 64-bit x86
Full Merom ISA compatibility

 

Larrabee on the other hand is more Pentium-like to begin with; Intel states that Larrabee's execution pipeline is "short" and followed up with us by saying that it's closer to the 5-stage pipeline of the original Pentium than the 16-stage pipeline of Atom. While both Atom and Larrabee support SMT (Simultaneous Multi-Threading), Larrabee can work on four threads concurrently compared to two on Atom and one on the original Pentium.

L1 cache sizes are similar between Larrabee and Atom, but Larrabee gets a full 32KB data cache compared to 24KB on Atom. If you remember back to our architectural discussion of Atom, the smaller L1 D-cache was a side effect of going to a register file instead of a small signal array for the cache. Die size increased but operating voltage decreased, forcing Atom to have a smaller L1 D-cache but enabling it to reach lower power targets. Larrabee is a little less constrained and thus we have conventional balanced L1 caches, at 4x the size of that in the original Pentium.

The Pentium had no on-die L2 cache, it relied on external SRAM to be installed on the motherboard. In order to maintain good desktop performance Atom came equipped with a 512KB L2 cache, while each Larrabee core will feature a 256KB L2 cache. Larrabee's architecture does stress the importance of large, fast caches as you'll soon see, but 256KB is the right size for Intel's architecture at this point. Larrabee's default OpenGL/DirectX renderer is tile based and it turns out that most 64x64 or 128x128 tiles with 32-bit color/32-bit Z can fit in a 128KB space, leaving an additional 128KB left over for caching additional data. And remember, this is just on one Larrabee core - the whole GPU will be built out of many more.

The big difference between Larrabee, Pentium and Atom is in the vector execution side. The original Pentium had no SIMD units, Atom added support for SSE and Larrabee takes a giant leap with a massive 16-wide vector ALU. This unit is able to work on up to 16 32-bit floating point operations simultaneously, making it far wider than any of the aforementioned cores. Given the nature of the applications that Larrabee will be targeting, such a wide vector unit makes total sense.

Other changes to the Pentium core that made it into Larrabee are things like 64-bit x86 support and hardware prefetchers, although it is unknown as to how these compare to Atom's prefetchers. It is a fair guess to say that prefetching will include optimizations for data parallel situations, but whether this is in addition to other prefetch technology or a replacement for it is something we'll have to wait to find out.

The Design Experiment: Could Intel Build a GPU? Drilling Deeper and Making the AMD/NVIDIA Comparison
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  • del - Friday, August 15, 2008 - link

    Don't be a hater. :P Intel has got it goin' on right now. Believe in the POWAH of Larrabee... unless it proves to be a failure upon release.

    :)
    Reply
  • atlmann10 - Saturday, August 09, 2008 - link

    Think about this ok AMD originally was a private IBM cpu manufacturer. Then bought out and run as a side unit of INTEL, that was dropped after they were done with them. So in a way the were partners and I'm sure there was some friendliness. As it's always been said keep your friends close but your enemies closer. There have been some things especially in these past two years that struck me kind of odd. Such as AMD's graphics chips running fine on a x38/48 chipset and the physics collaboration things as well as a few other rumors. Then Nvidia starts spouting off about how they could kick INTELS A77 etc. Now AMD has a definite GPU coprocessor in ATI and they wanna break into the market of GPU's etc. They know that there will be graphics competition with Nvidia being there largest competitior because there dedicated to GPU's solely and have a reputation. However now AMD has some chips that compete straight on weakening Nvidia to a point. Then AMD is getting more and more out of there cpu's gpu's and chipsets so INTEl jumps in the CPU GPU market just like AMD. Either way it turns out more are going to go with INTEL cpu's and many other products where AMD is kind of a fringe player. Who would you rather compete against full on 2 major GPU manufacturers or attempt to kind of co-align yourself with there competetitor while the somewhat down. Then throw out a whole new way to do graphics that performs well Nvidia is already loosing market share. So more people try it and the same number of people go with ATI. That leaves a much lower market for Nvidia plus there paying back what some 200 million dollars in bad GPU's right now as well and a few other problems they been having. Now this is not anything I know but knowing INTEL loves to stick it to competitors when there weak think about it. Reply
  • benkantor - Wednesday, August 06, 2008 - link

    if you could fit 10 Larrabees on 143 mm^2, you could fit 40 Larrabees on 286 mm^2, not 20... :P Reply
  • MamiyaOtaru - Saturday, August 09, 2008 - link

    For the love of education. We've already been through this. See the end of page 6 through page 7 in the comments section.

    143mm^2 doesn't mean 143*143. It means 143 square millimeters. 286 square millimeters is twice as many, allowing twice as many cores.
    http://img379.imageshack.us/my.php?image=squaremmh...">http://img379.imageshack.us/my.php?image=squaremmh...

    The article is right and you are so very wrong.
    Reply
  • Barack Obama - Wednesday, August 06, 2008 - link

    Derek and Anand deliver again! Reply
  • KGR - Wednesday, August 06, 2008 - link

    I am not a profeesional about software and hardware that is why maybe this question can sound nonsense .
    If larrabee will have a software renderer and programmed by C++ is it possible that it is not depended on windows? I mean if it doesnt need direct X can we run the games on Linux also??
    Reply
  • npoe1 - Tuesday, August 05, 2008 - link

    I enjoyed reading this so much. I think that this kind of articles is what Anandtech needs; I usually go to Arstechnica to read things like this one.

    Again, thanks!
    Reply
  • TrEmEnDo - Tuesday, August 05, 2008 - link

    I am definitely impressed with this new development and I expect that this technology will be disruptive down the road, however I feel that somehow they are about to commit another of their megalomaniac mistakes.
    Has anyone stopped for a sec and look where all gaming industry is heading into? Are PCs the future gaming platform? Maybe I am missing something but aren't the big guys already struggling to retain a 'decent' percentage of the multibillion gaming pie (PC gaming alliance anyone...)? I believe that whether us, tech enthusiast, hardcore pc gamers like it or not, it is the console arena where the big guns are going to be playing in a few years from now.
    Guys, we are seeing this happening everyday, we see tittles appearing and disappearing everyday b/c companies don't want to commit the resources to develop games for more than one or two platforms (normally doing a sloppy work BTW). Now that the grandpas of graphic hardware had manage to get DX/D3D derived engines into the last gen consoles (xenos, RSX) and a terribly inertial and rigid developer community avoids and whines about how difficult is to program for the few hardware 'jewels' that we have already in our hands (Cell/RV770/G200) do you think anyone except Intel is in the mood for yet another graphics industry spin?

    I have no doubt that this new development will have its own niche application or someone will definitely find something appropriate for it, but to say that Larrabee CAN do graphics and to say larrabee will kick ass so bad that in 3 years from now we all will be gaming from a Larrabee containing computer are two very different things.
    Congrats to Intel as the fathers of the creature, and congrats to us to see the tech world moving on....but just don't think this will change the world as we know it.
    Reply
  • hooflung - Tuesday, August 05, 2008 - link

    They are doing something very AMD like and taking it a step further and tossing in a few Power ideals in. I just wonder what the power profile will look like and who will partner up with Intel for it.

    I am sure they will have 4+ of these cores built into integrated chip sets for OEMs and laptops to really boost those areas. And people who buy laptops will see that they can get a desktop with 'bigger larrabee' and play their games faster than their budget/laptop computer.

    So it does make sense. However, it is an empire made on a lot of ifs. It will be fun to watch. Thanks anandtech for the informative article.
    Reply
  • christophergorge - Tuesday, August 05, 2008 - link

    is it just me or does it look like another transmeta crusoe in the making? Reply

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