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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  • calimero - Wednesday, July 6, 2005 - link

    btw Anand article was "full of shit" (sorry but that is the right phrase) and it's not odd that Anand pull it. It's quite embarassing for Anand; someone already told: one thing is to write test of CPU speed and speed of graphics card in games... and another to analyse CPU architecture.
  • jwix - Tuesday, July 5, 2005 - link

    Creathir - the article was reposted on other forums around the net. Here is the story in summary - Sony & Microsoft have both overhyped the processing power of their cpu's by using clever marketing speak. It turns out the processor designs are uneccessarily complicated, inefficient at crunching today's game code, and unlikely to be useful when game code finally becomes fully multi-threaded in the coming years. Why microsoft and sony didn't go with an Intel or AMD design, I don't know. The article speculates that both companies wanted IP rights to the cpu, maybe that's the reason.
    The GPU's on the other hand look plenty powerful. They should both be relatively equivalent in performance to the R520 and the current 7800 GTX.
    Bottom line - the new consoles will be quite powerful compared to the previous generation. However, PC's will still be more powerful, and wil remain the platform of choice for high end gaming. Something I was glad to read as I just built a new pc.

  • steveyoung123456789 - Friday, December 9, 2011 - link

    wow your so smart! faggit Reply
  • creathir - Saturday, July 2, 2005 - link

    I had read a good portion of the article, but had been pulled away (thought to myself I'll just reread it later) and was upset to find it was gone. I have never seen this here at Anandtech, and Anand has not made a single comment on his blog about it. I suppose some fact was incorrect? Maybe Sony/Microsoft decided they would SUE him over the article? I bet the most logical answer is this, Tim Sweeney saw the article, and even though Anand referenced the "anonymous developer", he had earlier mentioned in his blog he had been waiting for some answers from Tim. I would bet this "outed" his source, much like the LA Times outed their source recently for a Grand Jury. This outing probably was followed by a request by Tim to pull the article. I would have to bet we will see it soon enough, reworked, reworded. Whatever the case, Anand, it was a good article, you should be sure to repost it.
    - Creathir
  • steveyoung123456789 - Friday, December 9, 2011 - link

    o someone can read!! yay! Reply
  • linkgoron - Thursday, June 30, 2005 - link

    blckgrffn, THIS IS NOT i repeat NOT the article you think it is. Reply
  • blckgrffn - Thursday, June 30, 2005 - link

    Yes it is back up! :D

  • jwix - Thursday, June 30, 2005 - link

    Last night, around 10:00pm EST, I surfed over to the Anandtech home page to see what was happening. I was greeted by Part II of the article (Xbox 360, Sony PS3 - a hardware discussion). Did anyone else read this article last night. I was only able to read the first 2 pages before the article was pulled off the website. Why would they post it and then pull it so quickly? And why has not been reposted since?
    The story it told was unbelievable - basically, the floating point processing power of both the Sony and Xbox processor was less than half of your average Pentium 4. Anand went into detail on how and why this was the case. His sources apparently were confidential, but definitely industry insiders ( developers). I wish I could have finished reading the article before it was pulled. Did anyone read the whole article?
  • ecoumans - Thursday, June 30, 2005 - link

    Physics Middleware will be Multithreaded and heavily optimized for Cell's 7 SPE's. This makes life easier for gamedevelopers, and it changes the story about CPU usage... Same story for sound etc. Reply
  • Houdani - Tuesday, June 28, 2005 - link

    29: In order to turn off the "sponsored links" go to ABOUT in the top left menu and turn off INTELITEXT.

    I think this setting is stored in a cookie, so you will need to do this everytime you clear your cookies.

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