Slashdot Mirror
Under the Hood of the Xbox 360
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."
PowerPC's mainly an IBM-designed and promoted architecture, borne from the Apple-IBM-Motorola alliance.
Apple are simply one of IBM and Motorola's (now Freescale) customers.
What's the frequency, Kenneth?
This is probably a JTAG block. Depending on what it's built for, you get:
- 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.
It's probably the JTAG debug logic. Most modern CPUs use a JTAG port (full-duplex, multi-device serial port) to provide access to internal CPU state, as well as providing hooks for starting and stopping the processor.
/ 3675.html
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
"PS3 will have better graphical performance, up to 2x. High definition. Blu-ray. Up to 7 wireless controllers - those actually mean something. Xbox 360, will just have improved graphics over the original Xbox."
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 average Joe with a TV is going to be able to support seven players? Maybe the rare Super Smash Bros. style game, but nothing big.
The Xbox 360 has several things the PS3 does NOT have:
1) Xbox Live. The Playstation 3 has no unified online service at all and has no plans to. Xbox Live is an awesome way to play your video games online. One fee. No ten bucks a month here, five bucks a month there... $50 a year.
2) Time. Xbox 360 is here now whereas the PS3 is going to offer comparable hardware and games in a year.
3) Developer backing. Bungie and Rare are both developing for the Xbox 360, and that's only naming two big name developers. Also Final Fantasy will be coming to Xbox 360 too. EA is also signed on.
The question is why NOT buy an Xbox 360? Would you rather wait and get less for the same, or maybe more, amount of money?
-Eric Smith
Games often have far smaller cache requirements than many other applications, and as a result, it is preferable to go with a higher speed cache and higher CPU speed than a slower but larger cache/CPU.
:)
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.)
retrorocket.o not found, launch anyway?
Not a chance :) The COP interfaces contain bit vectors that are thousands of bits long, and only a handful of those bits are actually safe/useful to interact with for debugging the chip. Thats where the NDAs come in.
That is until you realize that the features that IBM and MS removed from the 360's CPU because they weren't needed for gaming cripple the CPU for most other applications.
retrorocket.o not found, launch anyway?
Hate to burst alot of bubbles, BUT:
0 -2.ars/2
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.
This is a fully PowerPC compliant chip. Make no mistake. It WILL run standard PowerPC code, as will Cell BE. From where did you get that the singe core G5 costs 5-600 dollars? An iMac G5 (singe core PowerPC 970FX costs $1300 and that's a complete computer with a GPU, harddrive, DVD-burner, webcam, 512 MB RAM, an 17" TFT, package, shipping, advetising and about 30% margin.
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!
- Henrik
- when the Shadows descend -