Inside Microsoft's Xbox 360
by Anand Lal Shimpi, Kristopher Kubicki & Tuan Nguyen on November 16, 2005 5:09 AM EST- Posted in
- Systems
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.
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):
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:
We have previously discussed the Xbox 360's CPU in much greater detail, which you can read about here.
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DrZoidberg - Wednesday, November 16, 2005 - link
Aww.. u guys shouldnt say that. Now it doesnt sound that amazing anymore.
24 pipe x800 doesnt sound as good as 48 pipe x1800xt.
Still great work disassembling console. It looks like it takes a while to disassemble it. Would this mean it would be hard to mod xbox360?
jkostans - Wednesday, November 16, 2005 - link
Well if a 700MHz P3 with 64mb ram and a geforce 4 can run doom 3 then imagine what you can do with mid-range hardware today. There is a lot of processing power in that box, especially for the price. I mean yeah a high-end PC is more powerful, but that's the way it's always been. I'm not a console person, mostly because I like shooters and a mouse/keyboard is completely necessary. Did microsoft change their policy towards keyboards and mice? I would love to lay the smack down on my friends in halo, but gamepads suck! Also whatever happened to the console being able to play with a pc over the internet? Didn't dreamcast have something like that?coldpower27 - Wednesday, November 16, 2005 - link
Heh, they should say that if the computational power of each pipeline is inferior to the dedicated Pixel/Vertex Pipes used in the R520. They are more versatile in the fact that they can execute both pixel/vertex shaders, but in exchange for that versatility each pipe is made more simple, and there are more of them over conventional GPU's.CZroe - Wednesday, November 16, 2005 - link
Why didn't they take apart the HDD unit's enclosure? Wouldn't be any less interesting. I'd sure like to know what brand, size and interface it uses. True capacity?DRavisher - Wednesday, November 16, 2005 - link
Have I understood correctly that the GPU and CPU share a 22.4GB/s 512MiB memory? Isn't that kind of low compared to PC graphics?Pythias - Wednesday, November 16, 2005 - link
Nope. Its on par with low/midrange. But they be making games with it for years to come. Unlike the current flavor of the month gpu.
Slaimus - Wednesday, November 16, 2005 - link
The article says the chip has the processing power of a 24-pipeline R420, but only about 1/3 of its memory bandwith. Although you do have to keep in mind the bandwith that does not get wasted on AA thanks to the daughter chip.lexmark - Wednesday, November 16, 2005 - link
with the tedious security features, i wonder how many people will bother modding with the new $399 version...BigandSlimey - Wednesday, November 16, 2005 - link
Looking at the internal shots it highlights the huge area taken up by the DVD drive, I wonder if they used laptop DVD drive tech, they could've made the console much smaller.tuteja1986 - Wednesday, November 16, 2005 - link
yeah... but then we would get a crappy laser that would die !