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?
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


View All Comments

  • The Preacher - Tuesday, August 5, 2008 - link

    Man, you must have OC'ed yourself way too high! :D Reply
  • iop3u2 - Monday, August 4, 2008 - link

    First of all it's called d3d not directx.

    Secondly you seem to imply that direct3d/opengl will cease to exist at some point if larrabee succeeds. I thinks you don't quite get what they are. They are APIs. Larrabee won't make programming APIless. Are you serious anand or what?
  • The Preacher - Tuesday, August 5, 2008 - link

    It could make programming D3D/OpenGL-less for programs/PCs that exploit Larrabee. And if the share of such programs/PCs increases, the share of competing solutions logically decreases and might eventually vanish (although not anytime soon). Reply
  • iop3u2 - Tuesday, August 5, 2008 - link

    Just because you can for example write a c program without the c lib it doesn't mean that people follow that road. It's all about what programmers will choose to do.

    Also, even if they do vanish there will still be a need for an api. So there will either be a new api or they won't vanish. Both situations make no difference whatsoever to the fact that larrabee will always need api implementations.
  • ZootyGray - Tuesday, August 5, 2008 - link

    right - and I will put hotels on boardwalk and park place :)

    I used to own an 815chipset - it was like version 14 or whatever so it didn't suk as bad as some of the earlier ones - but it did blow up - I think pixelated FarCry and Doom3 really killed it. But o sure, the software fixes and bubblegum patches made it good, for a while. I really do think I am going to wait for this just so I can watch the lineups of returns - or read the funny forums posts of sheep seeking help - baaaahaha :) The best part is that it doesn't exist - delay, postpone - kinda like the 64bit chip also. Maybe later, maybe. But the ads invade the livingroom.
    Make sure you keep yer getouttajailfree card - receipt.
    Ummm let's see: I think I will buy this one!

    Reality is that 4870x2 is on deck. Not 'rumour and sigh'. I just know there will be a 16page article on that - not!
  • Pok3R - Monday, August 4, 2008 - link

    Larrabee means good news for consumers, and definitely bad news for nvidia. Maybe the worst in decades...with AMD and Ati having enough human resources now to face it, and Nvidia having nothing but bad policies and falling stocks despite good $elling numbers...

    The future, today, is definitely Intel vs AMD/Ati.
  • initialised - Monday, August 4, 2008 - link

    a miniature render farm (you know like they use to make films like Hulk and WALL-E) on a chip. Lets hope AMD and nVidia can keep up. Reply
  • ZootyGray - Monday, August 4, 2008 - link

    Really? Guess again. There is NOT anything to keep up to.

    I do not accept that the grafx loser in the industry is going to simply become numero uno overnight.

    You really think that nvidia and ati have been sleeping for decades?

    Supporting the destruction of ntel's only competitors leaves us at the mercy of a group that's already been busted for monop and antitrst.

    Well written article? Of course, but I think it's like you are all fished in on many fronts. Nothing is really known except spin. This is beachfront property in the desert.

    There's nothing to watch except what we usually watch - released hardware benchmarks.

    I tell you AMD is going to be the cpu of choice in a few months when the truth about the bias in the benchies is revealed. And try - try real hard - to imagine ati+amd creating the ultimate cpu+gpu powerhouse. ntel needs this hype because I am not the only one with vision here. they are rich and scared, for now.

    but such talk seems to be frowned upon - so let's all cheer for the best grafx manufacturer - ntel = kkaakk! sorry to offend, so many of you just might be lost in the paid mob. so just watch and you will see for yourself- no need to believe me. I really know almost nothing - but I am free to see for myself. sorry to offend - I just can't cosign bs. but that's just me and a very few other posters here who have also been criticized. watch and see for yourself. watch...
  • Mr Roboto - Monday, August 4, 2008 - link

    I'd have to agree with the skeptics here. While the article is well written and informative (What AnandTech articles aren't?) it's purely speculation that Intel can get all of the variables right. How does a company that hasn't made a competitive GPU since the days of the 486 suddenly jump to Nvidia and ATI GPU type levels on their first try, never mind surpassing them. It's absolutely absurd to think that these chips are going to replace GPU's in terms of performance. I believe Larrabee will kick the shit out of Intel's own IGP but then again that's not much of a feat.

    Again I have to agree with previous posters that Intel just isn't that innovative. Even as I speak their are many lawsuits pending against Intel, most of them having to do with accusations of stolen IP that were used to design the Core2Duo. Antitrust suits aside, it's clear that Intel is similar to MS in that they just bully, bribe or outright steal to get ahead then pay whatever fines are levied because in the end they can never fine them enough to not make it worthwhile for Intel or MS to break the law.

    The 65nm Core2Duo is amazing. The 45nm E8400 I just bought is even more so. However the more I think about Intel's past failures as well as how they operate as a company the more far fetched this whole thing becomes.

    IMO they should have tried to compete in the dedicated GPU market before trying something like this. From a purely marketing standpoint Intel and graphics just don't go together. To come in to a new field in which they are unproven (I would bet Intel executives believe that building IGP's have somehow given them experience) and make outrageous claims such as the GPU is dead and Intel will now be the leader, is absurd.
  • JarredWalton - Tuesday, August 5, 2008 - link

    I think a lot of you are missing the point that we fully understand this is all on paper and what remains to be seen is how it actually pans out in practice. Without the necessary drivers to run DirectX and OpenGL at high performance, this will fail. How many times was that mentioned? At least two or three.

    Now, the other thing to consider is that in terms of complexity, a modern Core 2 core is far more complex to design than any of the GPUs out there. You have all sorts of general functions that need to be coded. A GPU core these days consists of a relatively simple core that you then repeat 4, 8, 16, 32, etc. times. Intel is doing exactly that with Larrabee. They went back to a simple x86 core and tacked on some serious vector processing power. Sounds a lot like NVIDIA's SP or ATI's SPU really.

    Fundamentally, they have what is necessary to make this work, and all that remains is to see if they can pull off the software side. That's a big IF, but then Intel is a big company. We have reached the point where GPUs and CPUs are merging - CUDA and GPGPU aim to do just that in some ways - so for Intel to start at the CPU side and move towards a GPU is no less valid an approach than NVIDIA/ATI starting at GPUs and moving towards general purpose CPUs.

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