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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  • BenSkywalker - Sunday, June 26, 2005 - link

    ""One thing is for sure, support for two 1080p outputs in spanning mode (3840 x 1080) on the PS3 is highly unrealistic. At that resolution, the RSX would be required to render over 4 megapixels per frame, without a seriously computation bound game it’s just not going to happen at 60 fps." -- Quote from page 10"

    First off 1080p doesn't support 60FPS as of this moment anyway, and there are an awful lot of games on consoles that aren't remotely close to being GPU bound anyway. Remember that the XBox has titles now that are pushing out 1080i and the RSX is easily far more then four times the speed of the GPU in the XBox.
  • tipoo - Wednesday, August 6, 2014 - link

    "RSX is easily far more then four times the speed of the GPU in the XBox."

    It's funny reading these comments years later, and seeing how crazy the PS3 hype machine was. I assume this insane comment reffered to the 1 terraflop RSX thing, which was a massive joke. RSX was worse than Xenon not only in raw gflops (180 vs over 200 I think), but since it didn't have unified shaders it could be bottlenecked by a scene having too much vertex or pixel effects and leaving shaders underused.
  • calimero - Sunday, June 26, 2005 - link

    Here is one tip about Cell:
    to play MP3 files (stereo) on PC you need 100MHz 486 CPU. Atari Falcon030 with MC68030 (16MHz) and DSP (32MHz) can do same thing!
    Everyone who know to program will find Cell outstanding and thrilling everyone else who pretend to be a programer please continue to waste CPU cycles with your shity code!
  • coolme - Sunday, June 26, 2005 - link

    "Supporting 1080p x2 may seem like overkill,"

    It's not gonna support 1080p x2

    "One thing is for sure, support for two 1080p outputs in spanning mode (3840 x 1080) on the PS3 is highly unrealistic. At that resolution, the RSX would be required to render over 4 megapixels per frame, without a seriously computation bound game it’s just not going to happen at 60 fps." -- Quote from page 10
  • nevermind4711 - Sunday, June 26, 2005 - link

    People have different ways of expressing the frequency of DDRAM. The correct memory frequency of 7800GTX is 256MB/256-bit GDDR3 at 600MHz, but as it is double rate some people say 1200 MHz.

    In the same way you can say the RSX memory is operating at 1400 MHz. How else could 128 bit result in a memory bandwidth of 22 GB/s for the RTX?

    #64 knitecrow, who is your source that the RSX does not contain e-dram, or is it just speculation?

    Besides, your conclusion from extrapolating the transistor count may be correct, but assuming the transistor count is proportional to the number of pixel pipelines is a rather big simplification, there is quite a lot of other stuff inside a GPU as well, stuff that does not scale proportionally to the pixel pipelines.
  • Furen - Sunday, June 26, 2005 - link

    The RSX is supposed to be clocked higher but will only have a 700MHz, 128bit memory bus (as opposed to the 1200MHz, 256bit memory bus on the 7800gtx). Reply
  • knitecrow - Saturday, June 25, 2005 - link

    too bad you don't speak marketing.
    When they say near.. it means very close. Could be slightly under or over. If it was something like 320M... they will be hyp3ing 320M.

    #62 too bad you are wrong

    with 300M transistors, the RSX is a native 24 pixel pipeline card

    You can extrapolate the number by looking at:
    6800ultra - 16 - 222M
    6600GT - 8 - 144M

    it has no eDRAM.

    The features remain to be seen, but its going to be a G70 derivate -- just like XGPU for the xbox was a geforce3 derivative.

    There is absolutely no evidence to suggest that the RSX is going to be more powerful than 7800GTX.

    Just because a product comes out later doesn't make it better

    Exhibit A:
    Radeon 9700pro vs. 5800ultra

  • Darkon - Saturday, June 25, 2005 - link Reply
  • Dukemaster - Saturday, June 25, 2005 - link

    I think it is very clear why the RSX gpu has the same number of transistors but still is more powerfull then the 7800GTX: the 7800GTX is a chip with 32 pipelines with 8 of them turned off. Reply
  • nevermind4711 - Saturday, June 25, 2005 - link

    Interesting article. However, I find it strange that Anand and Derek do not comment on the difference in floating point capacity between the combatants. 1 TFlops for X360 vs. 2 TFlops for PS3. For X360 we know that the majority of flops come from the GPU, where probably the big part consists of massively paralell compare ops and such coming from the AA- and filtering circuitry integrated with the e-DRAM.
    It would be very interesting to know how the RSX provides 1.8 TFlops. I do not think the G70 has a capacity anything near that. Could it be possible that Sony will bring some e-DRAM to the party together with AA and filtering circuitry similar to X360. After all Sony has quite some experience of e-DRAM from PS2 and PSP.
    Anand and Derek wrote "Both the G70 and the RSX share the same estimated transistor count, of approximately 300.4 million transistors." Where do this information come from? Sony only said in its presentation the RSX will have 300+ mil t:s. G70 we now know contains 302 mil t:s.
    #48: Sony may very well have replaced some video en/de-coding circuitry of the G70 with some e-dram circuitry.

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