Introducing the Xbox 360's Xenon CPU

The Xenon processor was designed from the ground up to be a 3-core CPU, so unlike Cell, there are no disabled cores on the Xenon chip itself in order to improve yield.  The reason for choosing 3 cores is because it provides a good balance between thread execution power and die size.  According to Microsoft's partners, the sweet spot for this generation of consoles will be between 4 and 6 execution threads, which is where the 3-core CPU came from. 

The chip is built on a 90nm process, much like Cell, and will run at 3.2GHz - also like Cell.  All of the cores are identical to one another, and they are very similar to the PPE used in the Cell microprocessor, with a few modifications. 

The focus of Microsoft's additions to the core has been in the expansion of the VMX instruction set.  In particular, Microsoft now includes a single cycle dot-product instruction as a part of the VMX-128 ISA that is implemented on each core.  Microsoft has stated that there is nothing stopping IBM from incorporating this support into other chips, but as of yet we have not seen anyone from the Cell camp claim support for single cycle dot-products on the PPE. 

The three cores share a meager 1MB L2 cache, which should be fine for single threaded games but as developers migrate more to multi-threaded engines, this small cache will definitely become a performance limiter.  With each core being able to execute two threads simultaneously, you effectively have a worst case scenario of 6 threads splitting a 1MB L2 cache.  As a comparison, the current dual core Pentium 4s have a 1MB L2 cache per core and that number is only expected to rise in the future. 

The most important selling point of the Xbox 360's Xenon core is the fact that all three cores are identical, and they are all general purpose microprocessors.  The developer does not have to worry about multi-threading beyond the point of getting their code to be thread safe; once it is multi-threaded, it can easily be run on any of the cores.  The other important thing to keep in mind here is that porting between multi-core PC platforms and the Xbox 360 will be fairly trivial.  Anywhere any inline assembly is used there will obviously have to be changes, but with relatively minor code changes and some time optimizing, code portability between the PC and the Xbox 360 shouldn't be very difficult at all.  For what it is worth, porting game code between the PC and the Xbox 360 will be a lot like Mac developers porting code between Mac OS X for Intel platforms and PowerPC platforms: there's an architecture switch, but the programming model doesn't change much. 

The same cannot however be said for Cell and the PlayStation 3.  The easiest way to port code from the Xbox 360 to the PS3 would be to run the code exclusively on the Cell's single PPE, which obviously wouldn't offer very good performance for heavily multi-threaded titles.  But with a some effort, the PlayStation 3 does have a lot of potential.

The Consoles and their CPUs Xenon vs. Cell
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  • tipoo - Wednesday, August 6, 2014 - link

    That was such bullshit. RSX was worth under 200Gflops, Cell about the same and much harder to extract that much from, Xenos was over 200, Xenon was around 100. Nothing was near the terraflop range except in marketing bullspeak. Reply
  • LanceVance - Saturday, June 25, 2005 - link

    #59

    "Does every xbox game needs to be playable? No."
    "How many of you still play your old games? Market research shows not a lot."

    Backwards compatibility is a feature. It's just like any other feature on a mass market consumer product. Some people value it and others don't.

    You clearly don't value that and won't consider it when making consumer decisions.

    Other people clearly DO value that and you are trying to persuade them not to. That's none of your business. If people value a specific feature they have every right to consider it in their buying behaviors.

    Sorry, I know this post has now gone way off topic from the original article of technical analysis. Beautiful article; but any open forum on such a political topic is doomed to degenerate into this.
    Reply
  • knitecrow - Saturday, June 25, 2005 - link

    software emulation is difficult and takes a lot of man power to get right.

    The main advantage for microsoft was that they didn't have to stuck with poor business and engineering design choices of the past.

    Does every xbox game needs to be playable? No. I don't care for games like "big rigs"
    I abviously want halo to work, but also the lesser known good titles on xbox ... like Panzer Dargoon Orta, Kingdom Under Fire, Otogi 1 & 2 and so on.

    How many of you still play your old games? Market research shows not a lot.I have an 80+ games library for the xbox. I don't mind.
    Reply
  • BenSkywalker - Saturday, June 25, 2005 - link

    milomnderbnder21-

    They are flipping consumers off. Sony and Nintendo at this point are both adding costs to their consoles to incorporate hardware to make certain that their systems have full compatability with the prior generation. MS has decided that you and I are not good enough to offer that same assurance. They have decided to save themselves a couple of dollars and render useless all of their games they can't get running on their new platform so they can save a few dollars. They will take a shot at software hacks- if they don't work we are out of luck. They are also stopping production of current gen XBox hardware. I have about thirty games for my XB currently, what do I do with them if my XB dies and they aren't supported by XB360(which there is no assurance they will)? I keep my legacy hardware around, back to my 2600, and my games.
    Reply
  • Olaf van der Spek - Saturday, June 25, 2005 - link

    > Compared to the built in 5X CAV DVD drive in the Xbox, the hard drive offered much faster performance. With the Xbox 360, the performance demands on the hard drive are lessened, the console now ships with a 12X CAV DVD-DL drive.

    Aren't all read-only DVD drives dual-layer?
    Reply
  • Starglider - Saturday, June 25, 2005 - link

    I'm a game programmer and I take issue with the statement on page 4 that BSP collision detection benefits from branch prediction. It doesn't; it's one of the rare types of code where the branches are effectively impossible to predict. The algorithm /does/ benefit heavily from speculative execution, but as I understand it neither the XBox360 or the PS3 are capable of this. As such this is one area where PC style processors have an advantage; neither console is going to beat a modern PC at SuperPi. Reply
  • devilzblood - Friday, June 24, 2005 - link

    altho this is my first post at anandtech, i have been reading ur articles from the geforce 3 launch.
    newayz.....im posting here coz im wondering if neone knos this.....what degrees do Anand and Derek hold?? they seem to be such a bank of information, i never thought it was humanly possible to know so much about computers..needless to say im impressed by u people..and all i would like to say about the article is that it was an informative and enjoying read
    Reply
  • milomnderbnder21 - Friday, June 24, 2005 - link

    #46

    MS is by no means "flipping off it's supporters" with regards to backwards compatibility. They have flat out stated that it is there goal that EVERY Xbox game be compatible on the 360, but they simply cannot guarentee it. In any case, look for a majority of them to be so. And if they can't get everything working, I'm not going to miss outlaw golf on my 360...
    Reply
  • MDme - Friday, June 24, 2005 - link

    #50

    Sony WILL support 1080p. They are supporting it so that they can BRAG about it. It's all about the hype, even if they only have 1 game supporting it, they will BRAG about it. heck, if you really think about it, if sony played a video (H.264) at 1080p then ran the game at 720p they will still claim, WE HAVE SUPPORT FOR 1080p. It's all marketing. Even X360 can claim this.
    Reply
  • finbarqs - Friday, June 24, 2005 - link

    bla bla bla, which one is better? Reply

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