Intel's Z68 Chipset, Thunderbolt & Display IO

Sandy Bridge integrates a GPU occupying roughly 20% of the quad-core CPU die. While what Intel calls its processor graphics is used on the MacBook Pro, outside of Quick Sync it's never used on the iMac. In fact, the processor graphics isn't even connected to the display at all.

Intel quad-core Sandy Bridge die

On the MacBook Pro Apple connects both GPUs to the display via a multiplexer and can instantaneously switch between which one is driving the panel without any frame buffer copying. With both GPUs connected and real time switching, accessing Quick Sync isn't an issue. On the iMac however, with only the discrete GPU connected, it is a problem. Apple claims this is why it used Intel's Z68 chipset: to access Quick Sync without a display output connected to the processor graphics. The Z68 based iMac actually went on sale before Intel's NDA on the Z68 chipset lifted. On the PC side we have Lucid's Virtu but for some reason Apple needs to use Z68 to achieve a similar end result. All other Z68 specific features go unused - there's no support for SSD caching and obviously no support for overclocking.

The other major change from the MacBook Pro is that the Thunderbolt controller no longer branches off of Sandy Bridge's on-die PCIe controller. Let's look at Intel's Z68 block diagram:

With Westmere (Clarkdale/Arrandale) Intel integrated a 16-lane PCIe 2.0 controller onto the processor die. The same controller made its way into Sandy Bridge. Traditionally these lanes are used for discrete graphics, the other major bandwidth hog within your system however the lanes are open to be used by PCIe device. In the 2011 MacBook Pro Apple used four of these lanes for Intel's Thunderbolt controller, leaving the discrete AMD GPU with a x8 interface. Even high end desktop GPUs aren't limited by a x8 PCIe 2.0 interface so there was no real performance penalty.

On the 2011 iMac however, Apple gives the discrete AMD GPU all sixteen lanes from the CPU. The Thunderbolt IC (which carries a different model number) branches off the Z68 chipset, which has 8 PCIe 2.0 lanes by itself.

Only four lanes are used by Intel's Thunderbolt controller, the remaining lanes are used for things like Bluetooth and WiFi. Do the math and you'll realize that four PCIe 2.0 lanes are only good for 20Gbps of bandwidth, plus DMI between the Z68 chipset and Sandy Bridge is limited to 20Gbps itself. A single Thunderbolt port is capable of 20Gbps of bandwidth (10Gbps in each direction), so that works out well (if you don't use any of the other PCIe devices in the system at the same time). While the 21.5-inch iMac has a single Thunderbolt port, the 27-inch model has two. That's a total of up to 40Gbps of bandwidth to Thunderbolt devices, but only 20Gbps to the controller itself. Don't be fooled by the presence of two Thunderbolt ports on the 27-inch iMac, you don't get any more bandwidth than you would on the 21.5-inch model - you can just hook up more displays.

Each Thunderbolt port on the iMac is good for at least one display output depending on what GPU you have driving it. The high end 27-inch iMac has two Thunderbolt ports, each of which is capable of driving two displays. Yes, you could theoretically have a 5-display setup driven off of a single iMac (given that even the upgraded iMac only has a 1GB frame buffer I wouldn't recommend doing such a thing).

The Thunderbolt ports aren't exclusively for video output, you can use one of the ports for video input. While you can use DisplayPort cables for video output, to make video input work you need to output video from a Thunderbolt port using a Thunderbolt cable. That means, at least today, the iMac can only accept video from a 2011 MacBook Pro (or technically another iMac) using a cable that doesn't yet exist on the market. You can expect to see Thunderbolt cables appear in stores very soon though.

Video aside, you will also be able to use the Thunderbolt ports for upcoming Thunderbolt devices as well. Each port supports a daisy chain for up to six Thunderbolt devices, meaning you can hook up a total of 12 devices to a 27-inch iMac.

The CPU Selection The GPU
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  • Exodite - Friday, May 27, 2011 - link

    Of course they are, I don't care the slightest about color fidelity or 178 degree viewing angles.

    The way I prefer to set up my displays will undoubtedly destroy and form of color calibration anyway.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    I hate TN panels, they're horrible. You're fortunate in that you don't care how things look, it is definitely cheaper.
  • Exodite - Friday, May 27, 2011 - link

    Strictly speaking I do care about how things look, I just don't consider IPS technology necessarily better for me personally.

    Being more prone to ghosting, and far more expensive, is far more a drawback than the better color fidelity, gamut and viewing angles are advantages. Again, for me personally.

    Unfortunately there are no post-1080P displays targeted at anyone but the graphics professional. It's not just the panel technologies that are more expensive either, 27" - 30" displays tend to incorporate USB hubs, chargers, elaborate stands and exotic connectivity options that are all equally useless for me.

    I'd love to see a minimalistic, matte, 2560x1440/1600 TN display with reasonable pricing.

    Well, I wouldn't mind a IPS, PVA or other type of panel either if those end up more reasonably priced. I just don't particularly care about the advantages.

    Then again I'm the guy that can't easily spot pentile matrix patterns or see any point in high-fidelity audio.

    So I suppose you're right, I'm probably blessed - or at least my wallet thinks so. :)
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Vertical viewing angles are narrow enough and corner-to-corner uniformity is uneven enough to begin with on 23" monitors and the bulk of 13" laptop displays.

    A TN panel on a 27" would make these limitations even more apparent. I think you'd be happier with a higher number of smaller TN panels, as you're doing now. Even if the deficiencies don't bother you, there is a point where increasing size makes them even more obvious. When only a 30% band of the display looks anything close to uniform at any one time, I reckon that would bug nearly anybody. :)

  • Exodite - Saturday, May 28, 2011 - link

    You'd be surprised I suspect, as I can turn the second display quite a bit away before it gets to the point where it matters.

    There are quite a few 27" TN panels on the market already, though only 1080P ones so far, and they do sell so it can't be that bad.

    I'm very happy with my two 19" 1280x1024 displays though, I just wish we'd see more progress in that area.

    I got the displays in 2006, IIRC, and the push for 1920x1200 were going well... and then 1080P came into the picture and the whole market regressed to that and haven't budged since.
  • xSauronx - Friday, May 27, 2011 - link

    Did you even read the article?

    " is the experience with having 2560x1440 resolution over 1920x1080 better then the experience that you get with $1600 worth of hardware over $700? I think I would say No."

    You might, but Anand actually mentioned that he prefers one large monitor over two smaller ones with lower resolutions.

    Id be tempted to agree, but 2 1080p monitors fit into my budget easier than did a 27 or 30" with high resolution.
    /moneys, oh moneys. \
    //mostly happy with these two guys, so oh well
  • kuk - Friday, May 27, 2011 - link

    It's possible to use only the 27" display hooked to another computer (look for target display mode), though there still the space and power penalties, as the whole iMac system has to be running.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    I addressed Target Display Mode in my post below. The Thunderbolt port has changed the requirements for TDM, now you need a Thunderbolt equipped computer to use it as a video source. For the time being the only computers you can use as a secondary source for the iMac display are other 2011 Macs equipped with Thunderbolt.

    I wasn't planning on replacing my current (non-Thunderbolt) 27" iMac, but this more than sealed the deal. Aside from being a Mac, my iMac also functions as the primary display for my gaming PC. The new iMacs are a step backward in this regard, at least until Thunderbolt ports show up in more computers next year or someone releases an adapter.
  • tipoo - Friday, May 27, 2011 - link

    Ah, interesting. That's quite a step backwards IMO. You could always get switcher boards I guess, but that's added cost and not very elegant. And there's no guarantee that even Mac's in 2-4 years will use the same thunderbolt cable.

    Is it possible to go HDMI-Thunderbolt, say for a console or future laptop?
  • tipoo - Friday, May 27, 2011 - link

    Also I've looked this up and someone on the Apple support forums said only the 15 and 17 inch MBP's could drive the iMac's display, is that true? The thunderbolt equipped 13 inch and MBA can't?

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