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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  • KoolAidMan1 - Friday, May 27, 2011 - link

    "The example given by headbox is a prime example."

    I can't think of any Mac user who would really try this. I mean, I plug my gaming PC into my 27" iMac as a primary display through the mini-DP port, and I figure I'm in an extreme minority of users. People who plug consoles and BR players and who would need a converter box is be an even tinier number of users.

    Don't get me wrong, I'm annoyed that the 2011 iMac has new Target Display Mode requirements, but the limitations of prior models in terms of using set-top boxes (consoles, Blu Ray players) isn't statistically a big enough number to get bent out of shape over, IMHO.
  • Tros - Friday, May 27, 2011 - link

    "How are you going to upgrade the motherboard on a proprietary and overpriced all in one?"

    This is as much of a criticism on laptops, tablets, smartphones, as all-in-one units. And guess what: That proprietary junk has been of great value to a lot of people, especially if it has an aesthetic appeal.
  • Penti - Friday, May 27, 2011 - link

    While certainly great for what they for probably, it's definitively not the same thing using 23" 1920x1080 120Hz TN-screens with Nvidia 3D Vision as with a good IPS or PVA screen with proper viewing angles and for mostly other uses then gaming. Of course you need your PC to your triple display gaming machine and a strong GPU too.
  • rubaiyat - Wednesday, September 7, 2011 - link

    Do you actually use your computer for much besides gaming?

    I'd rather have an excellent monitor than some dodgy and essentially useless 3D 'feature'.

    But that separates consumers in all areas. Those who will see a movie because it has (very loud) surround sound and pseudo 3D with lots of explosions, and those who will see a movie because it actually IS a good movie.

    My iMac27 has a brilliantly sharp and accurate 27" 2540x1920 display, that thankfully is not 3D nor runs generally awful Windows grade video.
  • nafhan - Friday, May 27, 2011 - link

    I'm fine spending more on a monitor than a desktop... However, I'll usually go through two or three desktop hardware upgrades before I replace my monitor.
  • fitten - Friday, May 27, 2011 - link

    And when the monitor is the majority of the cost of the system, it makes sense to reuse it for future upgrades. Monitor technology seems to evolve slower than the rest of the system so barring some major changes, keeping it for several upgrades won't 'set you behind' any. So, reusing a monitor is an extremely cost efficient technique to keep your computer 'modern'.
  • Spivonious - Friday, May 27, 2011 - link

    Exactly. I've had the same NEC (Mitsubishi tube) CRT on my desk for 15 years. The picture is still fantastic and blows any TN-panel LCD out of the water.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Good thing they use the best IPS panels in these things. :)
  • Guspaz - Friday, May 27, 2011 - link

    Colour-reproduction wise, perhaps, although that depends on the backlight. In terms of detail and a crisp image, even a cheap TN panel will destroy an old CRT, and IPS panels will match or surpass such a CRT.

    I'm reminded of a friend of mine, who for years (until perhaps 2-3 years ago) insisted that his old CRT monitor was fine, despite the fact that it was so out of focus that 14 point text was unreadable. He finally relented and upgraded when we proved to him that he was in denial when we realize that the reason he didn't have trouble reading on the monitor was because he increased the text size by 200-300% when he used the monitor. Now, I'm not suggesting that your monitor is out of focus, a good CRT monitor can have excellent sharp detail. But even the best of them comes nowhere close to a half decent LCD.
  • KoolAidMan1 - Friday, May 27, 2011 - link

    Not at all. Sell the iMac and get a better monitor that comes in the next update, it happens every 2-3 years. Resale value is also high.

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