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Under the Hood of the Xbox 360
Posted by
ScuttleMonkey
on Mon Dec 12, 2005 03:09 PM
from the love-the-smell-of-solder-in-the-morning dept.
from the love-the-smell-of-solder-in-the-morning dept.
An anonymous reader writes "IBM DeveloperWorks is running a behind the design story for the making of the Xbox 360. The 360 has but a single chip with 165 million transistors for it's CPU " From the article: "This chip is in fact a three-way symmetric multiprocessor design. The three PowerPC cores are identical, except that they are physically reflected through the X and Y axis. Each of the CPU cores is a specialized PowerPC chip with a VMX128 extension related to (and partially compatible with) the VMX instructions in the G4 and G5 CPUs. The three CPU cores share a 1MB Level2 cache. Each processor has 32KB each of data and instruction Level1 cache. The chip's front-side bus/physical interface has a 21.6GB/second bandwidth, and runs at 5.4GHz."
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Anybody currently working on CPUs etc.? (Score:4, Interesting)
Re:Anybody currently working on CPUs etc.? (Score:5, Informative)
- instruction single-step
- register and memory peek/poke
- control suitable for burning on- and off-chip flash/eeprom
- trace buffer that contains the most recently-executed opcodes
- breakpoints
- access to profiling and instrumentation registers
JTAG is a serial protocol that runs much, much slower than the core -- but it's an extremely nice way of getting into a running chip and poking around.
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Re:Anybody currently working on CPUs etc.? (Score:5, Informative)
JTAG is a hardware test standard, but chip vendors define their own extensions to it to provide software debug hooks. Most PowerPC chips use what's called the Common On-Chip Processor (COP), which is controlled through the JTAG port. The specific details of COP and its implementation on each chip is proprietary, and usually only available to IBM, Freescale, and a few select tool vendors with NDAs. Here's a link to some more information on PowerPC COP:
http://www.elecdesign.com/Articles/ArticleID/3675
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Re:Anybody currently working on CPUs etc.? (Score:3, Informative)
Tiny cache... (Score:4, Interesting)
Re:Tiny cache... (Score:5, Insightful)
A big cache is good when you are dealing with programs that could access ANY data at ANY time (or you're running multiple programs). With games, that doesn't really happen. Game code is sort of "linear", and you're only running one at a time.. So the cache can be filled with what needs to be there, and nothing else.
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Re:Tiny cache... (Score:5, Insightful)
This is a game console, not a multi-tasking general purpose PC. This is reflected in both the power a programer has over the hardware, and it is reflected in the fact it depends on the programmer to make up for cost saving measures implemented in the hardware.
You're thinking about thread scheduling the way a general purpose PC would schedule threads. It doesn't work that way in a game system. When talking about threads on a game console, we're talking about hardware threads. The programmer decides what core runs what logic, when it runs that logic; there aren't any context switches, and threads don't "change" cores when running.
The cache design on this chip is actually quite cool. If you're only going to use a piece of data once (or infrequently), you can instruct the processor to not store the data in the L2 cache (leaving data that you're using more freqently in the cache). Additionally, the L1 and L2 cache can hold separate data, and a load into the L1 cache doesn't require a storing that data into the L2 cache. When writing data out to main memory, a program can instruct the processor not to store it in the cache. Software can also reserve chunks of cache for whatever purpose it wants. Finally, a direct link between the CPU and the GPU can be established where the GPU reads data directly off of the L2 cache as the CPU generates it.
These abilitys make it possible to utilize L2 cache better than you would with any general purpose PC. However, it requires more thinking and more effort to do so. That's the price/performance tradeoff you get.
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Re:Tiny cache... (Score:4, Funny)
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PS2 EE Has The Same Philosophy (Score:5, Interesting)
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Re:PS2 EE Has The Same Philosophy (Score:3, Interesting)
Not for games (Score:5, Informative)
The Celeron in the 300A era are one of the best examples of this. They had half the cache of their Pentium III counterparts, BUT the P3 cache ran at half the CPU speed while the Celeron cache ran at full speed. The Celeron's performance was crap despite the faster cache for many applications (including server machines and most office applications) due to its smaller cache, but gamers discovered that for games, the situation was exactly the opposite - clock for clock the Celeron was significantly faster than the P3 due to the fact that most games in that era could fit almost all of their rendering pipeline within even the Celeron's small cache. Rare cache misses and twice the cache speed = much better performance. It also happened that that on-die cache allowed the Celerons to be overclocked like crazy, a significant added bonus.
The Xbox 360's CPU takes the whole idea much farther. While most desktop CPUs are designed to perform well over the widest range of situations (with some tradeoffs always being evident - note that Athlons eat P4s for lunch in many cases such as games, while Athlons do actually lose most of their advantages in performance per clock cycle when performing video compression and decompression because most video codecs don't have significant amounts of branching resulting in pipeline stalls from branch mispredictions.) The Xbox 360 CPU goes a step further by optimizing for one thing and one thing only - gaming. Instruction reordering which is critical in most desktop CPUs turns out to be not as necessary for gaming (specifically graphics rendering), and as a result the 360 drops instruction reordering capability completely in favor of having multiple cores at a low cost. (Instruction scheduling takes a LOT of die space in modern CPUs compared to the size of the rest of the CPU core.)
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What a Difference a Year Makes (Score:3, Funny)
Imagine... (Score:5, Funny)
With a bit of a mind flip... (Score:4, Funny)
Must be this "reversible computing" I keep hearing about...
The most important question. (Score:4, Funny)
Re:The most important question. (Score:5, Funny)
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Dear Apple (Score:5, Interesting)
Seriously, though, these are fascinating little beasts. It looks as if the concept has its roots in the Transputer, which also relied on fast and narrow point to point external links. When I first read the blurb I guessed from the description that there were 4 cores per chip and the bad ones were disabled to get the yield up, but clearly the yield is much much better than that. However, anybody silly enough to think about overclocking will need to note that the working CPU voltage is hard coded; it looks like, to get the yield at the clock speed, each device has to be individually tuned. Which suggests that the tolerances for reliable functioning are tight. Perhaps the overall error rate is not good enough for a truly general purpose computer which needs to be able to tolerate a range of operating conditions without significant error. Which doesn't suggest a range of motherboards and retail boxed processors any time soon. Just like Apple, in fact. This reminds me of good old ECL based computers (whose CPU voltage had to be adjusted on the fly for reliable operation rather than set up once for all, but I'm sure you take the point).
It's perhaps a pity that the design teams for the Mac Mini and the XBox couldn't be locked up in a development lab with a progressively increasing caffeine level in the coffee until they create the hybrid that would really be the future of home computing. Apple's thermal management and sound level control, IBMs obvious chip development capability, and Microsoft's willingness to spend some of its cash pile would be a formidable combination. The trouble is, you'd probably end up with Apple's's ability to design chips, IBMs willingness to lose money, and Microsoft's thermal management and general aesthetics.
Can You Hack It? (Score:3)
Shattered Beowulf Dreams (Score:5, Informative)
The Xenon CPU IS NOT the same as 3 G5's all on one chip! Read the arstechnica article here:
http://arstechnica.com/articles/paedia/cpu/xbox36
Basically it says: "The basic idea behind both Cell and Xenon is to make the execution core less complex by stripping out hardware that's intended to optimize instruction scheduling at runtime. Neither the Xenon nor the Cell have an instruction window, which means that these two processor designs largely forget about instruction-level parallelism. Instead, instructions pass through the processor in the order in which they're fetched, with the twist that two adjacent, non-dependent instructions are executed in parallel where possible."
This means that standard PPC code (OS X, etc) WILL NOT RUN on this. This is also the reason that IBM is selling these things at only $106 a pop to M$. Have you checked the prices for SINGLE CORE G5s for Apple? Their like $600-700 a piece! So, I am guessing that stripping these down makes them much easier and therefore faster and cheaper to mass produce, and therefore the price difference.
Anyway, there are reports that only one core is availble to intitial game developers, and one of the cores is strictly for M$ bullshit content protection TC such as the hypervisor, etc.
Not to mention from the article:
Microsoft and IBM engineers worked together during the definition phase of the project to specify a design to satisfy the constraints of a mass-produced consumer device
Sounds like a shitload of TC shit build right into the chip, so I am NOT holding my breath for linux to be ported to this (not that I wouldn't be thrilled to see this). Cetainly not when the port to STI Cell architecture has been under dev for what, over a year? Damn, can't wait for PS3 release.
Wrong! (Score:4, Informative)
The cost of a processor is directly related to the die size and since the size of the Xenon is larger than the dual core G5 (about 130 million transisotrs compared to 165 million in the Xenon) there's a good chance that Xenon is actually more expensive than the PowerPC 970MP to manufacture.
Linux will run just fine on an Xbox 360 if one would fins a way around the DRM stuff. OSX too if Apple would want to. Same goes with the PS3. Hell.. Sony's boss even said so!
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Re:Wrong! (Score:4, Insightful)
You don't need these things with properly compiled code. But that's the trick; you can't take poorly compiled code and expect it to be performant.
Traditionally general purpose CPU's need the capability to execute out of order because they're running code that wasn't written for their current design -- they have to run code designed to run on x previous iterations of the processor. And traditionally, CPU manufacturers like to tout how their new processor runs x benchmark 50 times faster -- and they can't do that without out of order execution.
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Re:Shattered Beowulf Dreams (Score:3, Insightful)
"Reports", yeah, but this isn't true. Several of the lanuch games were multi-clore, including Kameo and Perfect Dark. Numerous developers have commented on this. And as far as I know, Hypervisor on IBM systems has nothing to do with content protection and it's difficult to see why it would be implemented here.
Sounds like a sh
Re:Just a quick question... (Score:5, Funny)
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Re:Its off the shelf parts... (Score:3, Informative)
Re:Its off the shelf parts... (Score:3, Insightful)
Re:Why is Microsoft using Apple parts? (Score:4, Informative)
Apple are simply one of IBM and Motorola's (now Freescale) customers.
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Re:Why is Microsoft using Apple parts? (Score:3, Informative)
The Power architecture is an IBM design intended for use in their large server machines. About the time that Motorola was struggling with producing better/faster 68000 chips, IBM designed a cut-down version of the Power chip called "PowerPC". Apple adopted PowerPC from IBM, thus leaving Motorola behind. However, Motorola realized that they were losing big business and licensed the PowerPC archit
Re:Why is Microsoft using Apple parts? (Score:4, Interesting)
Correct me if I'm wrong...
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Re:Why buy an Xbox 360? (Score:5, Funny)
The Xbox 360, on the other hand, is made by Microsoft.
The choice is yours.
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Re:Why buy an Xbox 360? (Score:5, Insightful)
- The Nintendo Revolution
I don't want to sound like too much of a fanboy, but what can these consoles really give me that my PC can't? I'd rather have a console thats sole focus isn't trying to outpace my PC in terms of graphics... but to push the limits with new controllers, unique games and not costing me my 1st born child or my left arm to acquire.
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HDTV (Score:3, Funny)
Because nothing quite compares to playing Nethack in High Definition.
Re:Why buy an Xbox 360? (Score:4, Funny)
Nintendo, I choose you!
</pokemon reference>
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Re:Why buy an Xbox 360? (Score:3, Informative)
Who says the PS3 will have two times the graphical performace? Speculation does, perhaps.
The Xbox 360 is high definition out of the box.
The Xbox 360 does NOT use proprietary disc formats that can, at the vendor or manufacturers will, brick your box.
WHO NEEDS SEVEN CONTROLLERS ON ONE BOX!? Jesus. What
Re:Why buy an Xbox 360? (Score:3, Insightful)
However, it does have Perfect Dark: Zero, Project Gotham 3, Kameo (don't knock it 'till you try it) and soon, DOA4 and Battlefield.
Battlefield + Xbox Live = Heaven.
Granted, I don't know a whole lot about what games are coming soon to the Xbox 360 for lack of time, but by the time the Playstation 3 comes out, Xbox 360 will have a lot more games by great developers and I'm going to vent
Re:Why buy an Xbox 360? (Score:5, Insightful)
Also, while the Revolution's controller does pose some unique possibilities for gameplay, it's only a controller. Any of the modern gaming consoles could implement such a controller. If it's that big a deal, expect MS and Sony to have their own versions by the end of 2006.
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Re:PowerPCs? (Score:4, Funny)
What's next, dogs and cats living together?
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Re:obligatory... (Score:3, Interesting)
Give it a few more weeks, it will.
Re:obligatory... (Score:4, Funny)
Long answer; yes with an if...
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Re:The amazing part isn't just the cpu technology (Score:5, Insightful)
NES looked better than nearly any computer of its day. Ditto Genesis and SNES. Playstation and N64 packed an incredible amount of power into a cheap bundle. Remember the hype about PS2's Emotion Engine? There were rumors that exporting it would be restricted because it was going to be classified as a super computer. People were saying Iraq was going to use it to guide missiles. Xbox literally was a cheap PC, but gave more bang for the buck than your average beige box.
Consoles do this by taking the right shortcuts. They have a very focused performance target for very specific tasks. No need to add anything more than the minimum. Plus, they sell more than nearly any OEM PC maker so they get good prices on the parts.
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Re:Flaimbait (Score:5, Insightful)
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Re:Flaimbait (Score:3, Informative)
It could be that the return rate is what Microsoft's official statement.
If there is a manufactured shortage of units, it could be that Joe sixpack is waiting for Wal*Mart to restock before returning their overheating Xbox for a newer one.
The news reports that they are out of the console but I haven't looked in the stores to verify.
Re:Flaimbait (Score:3, Informative)
The kind of "stress test" you propose would be unprecidented in the history of consumer electronics. It would also be the easiest way to make sure your production line is as inefficient as possible (think about the scale of what it is you are proposing). Finally, it still won't catch problems that are the result of damage occuring during transit.
In a production line you depend on your s
Re:Flaimbait (Score:3, Interesting)
It isn't reasonable to expect every unit to roll off the line and make it into your home flawless. Nothing would ever be make it out of the factory if that was the requirement.
It is reasonable to expect a company to stand by their product if you get one with a defect. By all accounts, that is happening; replacements arriving in 5 days, with everything handled via overnight shipping.
Justifying what I'm seeing here as "slashdot loves to bash
Re:Flaimbait (Score:3, Funny)
Re:Gameplay (Score:3, Interesting)
Yup because that worked so well before (Score:3, Insightful)
Re:Yup because that worked so well before (Score:3, Insightful)
One game can make you a big name if that game is big enough. Ask Id.
Bungie has never been prolific, but they have been at the forefront of science-fiction based first-person-shooter titles from the very beginning, with the Marathon series on the Mac (which was comparable to Doom graphically, with a better plot). The two Halo games have been widely acclaimed as console FPS milestones. There is reportedly a Halo movie coming out.
I think that qualifies
Re:What about the blades? (Score:4, Informative)
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Not exactly as I have understood it (Score:4, Interesting)
So the big difference is that the 360 is more like current multicore PC's while the PS3 seems to lean more towards a cluster setup like openmosix.
As to wich is better? Well look at recent PS2 games. They show such graphical improvement that it might be true that PS2 still has untapped capabilties. The X-box on the other hand is pretty much at its limit. This was clear by developers complaining the PS2 was hard to develop for and the x-box was easy. Same with the next generation.
Given that the cores have the same basic design (64bit power) and Sony claims the same or even higher clockspeeds it would be easy to assume that 7+1 core > then 3 cores. I also seen larger cache sizes being claimed and even faster bus speeds. Is it all true? And even if it is will game developers succeed in tapping those resources? And even if they do, will that result in fun games?
Remember that currently the fast majority of games do not take advantage of dual core PC's even hyperthreading is rarely supported worse having it on can sometimes degrade performance. Now imagine having to write your code in such a way that it can be split across 7 processors. OR is that central core in te Cell processor capable of splitting up non- threaded applications? (Just random quesswork). After all it is supposed to be become more then the current PS3 chip it is supposed to be included in the next generation of TV's and other entertainment products.
That would be a huge advancement. The holy grail of grid computing (the cell is supposed to be like that) were you no longer have to worry about the specifics of your enviroment but can just run your code and the system will take care of it.
What I find a far more intresting proposition that with the PS3 supposedly so powerfull yet also so similar to the 360 is that it might just be possible to run 360 games on the PS3.
As for using consoles for number work. Already being done with both systems. They are so cheap yet so powerfull that all you have to do is wait for someone to break them open. Same as PC GPU's are being used for number crunching work. However GPU's is no problem wereas circumventing the PS3 or 360's protections might be in more repressive goverments (such as found in the west).
All off the above is just random speculation based on hilarious press reports. Any resemblance to the facts is unlikely.
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Re:Oooh, shiny! (Score:3, Funny)