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Inside Microsoft's Xbox 360

 by Anand Lal Shimpi, Kristopher Kubicki & Tuan Nguyen on November 16, 2005 5:09 AM EST
91 Comments | Add A Comment
91 Comments

The Xbox 360 CPU

The original Xbox used a hybrid mobile Pentium III/Celeron processor, but for the 360 Microsoft went to IBM and got the rights to a PowerPC core.  The move to the PowerPC instruction set meant that there would be no direct binary compatibility with older Xbox titles, but the sacrifice was obviously deemed necessary by Microsoft. 

The CPU itself features three of these PowerPC cores and is currently manufactured on a 90nm process, however Microsoft will most likely be transitioning to 65nm as soon as possible in order to reduce the die size and thus manufacturing costs.  Remember that a die shrink from 90nm down to 65nm will cut the size of the CPU in half, and should be possible for Microsoft sometime before the end of next year. 


Click to Enlarge

All three cores are identical and feature a 2-issue pipeline and can only execute instructions in-order; we've already discussed the reasoning behind this decision here.  The impact of the in-order execution cores is generally a negative one on current game code, but by going with a much simpler core Microsoft was able to stick three of them on a die with hopes of making up for lost performance by enabling some pretty serious multithreading. 

Not only does the Xbox 360's CPU feature 3 cores, but each core is capable of executing two threads at the same time, making the CPU capable of simultaneously executing 6 threads.  Unfortunately, most titles appear to be only using one or two threads for the majority of their game code, with the remaining threads being used for things like audio encoding/decoding, real-time decompression of game data off of the DVD-ROM and video decoding. 

Microsoft has their own license to use and manufacture the CPU used in the Xbox 360, and thus we see their logo on the chip itself.  Microsoft cools the 3-core CPU using a fairly beefy heatsink outfitted with heatpipes (pictured below): 


Click to Enlarge

Airflow is supplied by the two rear fans in the Xbox 360; the air is channeled over the GPU and CPU heatsinks using a duct. The larger heatsink on the right is atop the CPU, the smaller heatsink is for the GPU:


Click to Enlarge

We have previously discussed the Xbox 360's CPU in much greater detail, which you can read about here.

Removing the Heatsinks from the Motherboard The Xbox 360 GPU
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  • Questar - Wednesday, November 16, 2005 - link

    " We roughly estimated the power of the Xbox 360 GPU to be similar to that of a 24-pipeline ATI R420 GPU."

    What do you base this on?

  • Snuffles - Wednesday, November 16, 2005 - link

    From the article:

    quote:
    Unfortunately, we have yet to hear from Microsoft if this means that all games must be internally rendered at 1280 x 720 or if they can be rendered at a lower resolution and upscaled later on. There have been discussions involving at least one Xbox 360 game (Project Gotham Racing 3), rendering internally at a lower resolution and having the Xbox 360's TV encoder upscale it to 720p.


    The reason for this can be found on Bizarre's Forums:

    quote:
    For those wondering why this information would be true, here's the technical reason for why the game would probably be running at 1024x600 interally. On the Xbox 360, there is a thing called DRAM which serves as the framebuffer for the image. It has 10MB of DRAM, which means it can store a 10MB image, and the advantage is that it can render that image very fast (this is what gives the Xbox 360 it's "free" AA capability). Now the thing is, that 10MB can only store a full HD image (1280x720 and above) WITHOUT Anti-aliasing. So, they have a system where games can split a frame into seperate sections called tiles so that each tile would fit into that 10MB of DRAM. However, tiling is something that may have to be planned for early in the game's development, and the final Xbox 360 development kits (which were the only ones to include the DRAM) were only made availible a few months ago. In turn, Bizarre may not have had time to build tiling into the game, and therefor wouldn't have been able to enable anti-aliasing when it's running at HD. So instead, what they've likely done is lowered the resolution to 1024x600 and enabled 2xAA (which coincidentally fits into the 10MB DRAM buffer perfectly), which allows them to use anti-aliasing without adding tiling into the game.


    http://www.bizarreonline.net/forum/viewtopic.php?t...">Link

    This essentially means that all later games which are built from ground up to include tiling, will feature free AA and a native 720p resolution. PGR3 had to go this route, because of lack of time with the final devkit and time restraints in order to make launch.

  • Questar - Wednesday, November 16, 2005 - link

    " We roughly estimated the power of the Xbox 360 GPU to be similar to that of a 24-pipeline ATI R420 GPU."

    What do you base this on?

  • Questar - Wednesday, November 16, 2005 - link

    " We roughly estimated the power of the Xbox 360 GPU to be similar to that of a 24-pipeline ATI R420 GPU."

    What do you base this on?

  • nourdmrolNMT1 - Wednesday, November 16, 2005 - link

    these screws are NOT torx screws. these are Hex Heads.

    you aint gunna be using a torx driver to remove them. there is a difference, and the T7, T8 are what comfirm this. Torx have 5 points, these have 6, and are deffinately completely different from torx as they are much harder to strip out.

    no, its Dice as proven above.

  • bldckstark - Wednesday, November 16, 2005 - link

    From wikipedia.com search for torx -->

    quote:
    Torx is the trademark for a type of screw head characterized by a 6-point star-shaped pattern (in the same way that flatheads, Phillips, Allen, and Robertson have flat, ×-shaped, hexagonal, and square tips, respectively). People unfamiliar with the trademark generally use the term star, as in "star screwdriver" or "star bits." The generic name is hexalobular internal driving feature and is standardised by the International Organization for Standardization as ISO 10664.


    A hex head is usually referred to as an Allen bolt or the infamous Allen wrench.

    And as I posted earlier, from dictionary.com, the definition of dice is as follows --> A small cube marked on each side with from one to six dots, usually used in pairs in gambling and in various other games.

    The item referred to in this in plural form is dies.

  • Ruark - Wednesday, November 16, 2005 - link

    "You can now lift up the motherboard out of the metal chassis."
    Less awkward, "You can now lift the motherboard out of the metal chassis."

    "Three star bit screw drivers in the following sizes: T6, T7 and T12"
    Is a three star ranked a little lower than a four star? ;) At first glance, I thought this was certainly a secure method of attachment if a screw driver with a three pointed star tip was needed. Torx, torx, torx!

    "After two corners have been lifted, the rest of the clamp springs loose and can easily be removed by hand."

    As these are not the small cubes used for gaming, the plural should be dies.

  • Ruark - Wednesday, November 16, 2005 - link

    "Using your start bit screw drivers, remove the silver screws. . ."

  • PhoneZ - Thursday, November 17, 2005 - link

    "..START bit screwdriver.."? Or is that supposed to be star? Since we feel the need to correct people.

  • Live - Wednesday, November 16, 2005 - link

    So how do we overclock it?
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