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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  • MDme - Friday, June 24, 2005 - link

    now i know what to buy :) Reply
  • SuperStrokey - Friday, June 24, 2005 - link

    lol, thats funny Reply
  • bldckstark - Friday, June 24, 2005 - link

    Having a PS2 and an XBOX I was not even thinking about buying a PS3 since the XBOX kicks the PS2's ace. (IMHO). After reading this article I have much more respect for the PS3 and now I don't have any idea which onw I will buy. My wife may force me to buy the PS3 if the 360 isn't as backward compatible as most want it to be.

    Maybe I will just use my unusually large brain to create a PS360 that will play everything. Oooh, wait, I gotta get a big brain first. Then a big p3nis. Or maybe just a normal one.
    Reply
  • Furen - Friday, June 24, 2005 - link

    #37: supposedly yes. Since it will have to be through hardcore emulation there will be issues (but of course). It wont be fully transparent like the ps2 but rather you'll have profiles saved on your harddrive which will tell the system how to run the games. Reply
  • SuperStrokey - Friday, June 24, 2005 - link

    I havnt been following the 360 too much (im a self admitted nintendo fanboy), but will it be backward compatible too? I heard it was still up in the air but as PS3 is going to be and revolution is going to be (bigtime) i would assume that 360 will be too right? Reply
  • ZobarStyl - Friday, June 24, 2005 - link

    #32 is right: how many games get released for all 3 console with only minor, subtle differences between them? Most of the time, first party stuff is the only major difference between consoles. Very few 3rd party games are held back from the 'slower' consoles; most are just licensing deals (GTA:SA on PS2, for example). And if you look back, of the first party games lineup, XBox didn't have the most compelling of libraries, in my opinion. Reply
  • yacoub - Friday, June 24, 2005 - link

    imo, the revolution will be a loser in more than just hardware. i can't remember the last time i actually wanted to play any of the exclusive nintendo games. actually, i think for about one day i considered a gamecube for metroid but then i saw it in action at a friend's place and was underwhelmed by the gameplay. forget mario and link, give me splinter cell or gran tourismo or forza or... yeah you get the idea. Reply
  • nserra - Friday, June 24, 2005 - link

    #27

    If you read the article carefully, you will see that since they are "weaker" pipelines, the 48 will perform like 24 "complete" ones.

    I think with this Ati new design, there will be games where the performance will be much better, equal or worst.
    But that’s the price to pay for complete new designs.

    On paper Ati design is much more advance, in fact reminds the VOODOO2 design where there are more than one chip doing things. I think I prefer some very fancy graphics design over a double all easy solution.
    Reply
  • Taracta - Friday, June 24, 2005 - link

    With 25.5 Gbs of bandwith to memory, is OoO (Out Of Order processing) necessary? Isn't OoO and its ilk bandwith hiding solutions? I have an issue with regards to Anandtech outlook on the SPPs of the CELL processor (I could be wrong). I consider the SPPs to be full fledge Vector Processors and not just fancy implementation of MMX, SSE, Altivec etc, which seems to be Anandtech's outlook. As full fledge Vector Processors they are orders of magnitude more flexible than that and as Vector Processors comparing them to Scalar Processors is erroneous.

    Another thing, RISC won the war! Don't believe, what do you call a processor with a RISC core with a CISC hardware translator around it? CISC? I think not, it's a RISC processor. x86 did win the procesor war but not by beating them but joining them and by extension CISC loss. Just needed to clear that up. The x86 instruction set won but the old x86 CISC architecture loss. The x86 insrtuction set will always win, fortunately for AMD because the Itanium was to have been their death. No way could they have copied the Itanium in this day and age which come to think of it is very unfortunate.

    From you have the processor the runs x86 the best you will always win. Unless you can get a toehold in the market with something else such as LINUX and CELL!
    Reply
  • CuriousMike - Friday, June 24, 2005 - link

    If it's a 3rd party game, it won't matter (greatly) which platform you pick, because developers will develop to the least-common-denominator.

    In the current generation, about the best one could hope for is slightly higher-res textures and better framerate on XBOX over ps2/gc.

    IMO, pick your platform based on first-party games/series you're looking forward to. Simple as that.

    Reply

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