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Design Philosophy of the IBM PowerPC 970
D.J. Hodge writes "Ars Technica has a very detailed article on the PowerPC 970 up that places the CPU in relation to other desktop CPU offerings, including the G4 and the P4. I think this gets at what IBM is doing: 'If the P4 takes a narrow and deep approach to performance and the G4e takes a wide and shallow approach, the 970's approach could be characterized as wide and deep. In other words, the 970 wants to have it both ways: an extremely wide execution core and a 16-stage (integer) pipeline that, while not as deep as the P4's, is nonetheless built for speed.'"
What's the difference between the Power4 and the PowerPC 970? As a Mac guy, I've been following all of the rumors and announcements with interest but I keep seeing the PPC970 referred to as a scaled-back version of the Power4.
Why wouldn't Apple go with the Power4 over the PPC970? And I already know that nothing official has been announced by Apple and that this is all probably going to be a lot of sturm und drang signifying nothing, but that's what keeps us Mac guys going I guess.
I recall IBM's PPC boards going for over a grand, which is (to me) far too much. Especially when it was a 'G3' chip.
Even if the new chip is faster, will I be able to buy 2 pentium 4's (5?) for the price of it?
I think that since this is a 64 bit chip, why not compare it with other 64 bit consumer desktop chips (ie, AMD Clawhammer)? A lot of Intel's questionable moves (12K micro-ops instruction cache?) for the P4 were obviously not copied by AMD, and x86-64 seems to be the 64 bit desktop chip of the future.
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The PowerPC 970 has other potential customers as well, though, not the least of which is IBM itself who, with its large investments in Linux, would love to see a high-performance, 970-based 4-way or 8-way SMP Linux desktop workstation halt the steady flow of former 64-bit *NIX workstation users who began switching to Wintel hardware in the late 90's.
Before all my fellow Mac users start A) thinking about going to Linux B) drooling C) wondering about Darwin or D) some combination of the above, let me remind you that Darwin scales very well. You can now return to your previous state of awe.
PS - How much you want to bet good ol Steve is already having wet dreams about doing the traditional Photoshop test at a Macworld with 4-way SMP?
*Sigh* The size of a CPU's address/data bus does not reflect a processor's "bitness". 64-bit means that it has a 64-bit word size (as opposed to the 32-bit word size on x86 processors), 64-bit registers, etc. Most 64-bit CPUs don't actually have a 64-bit address bus. Like you said, the Alpha's is 48-bit. This is usually done to keep the pin count down to a sane level (if you need all the physical RAM that a 64-bit address bus would provide, you need something bigger than a desktop CPU). You can expect that as 64-bit chips become more common on the desktop to see somebody introduce a 64-bit CPU that has a 64-bit address bus just so that they can say, "Hey, look, we have a 64-bit address bus, the other guys only have a 48-bit one!" and (like you are trying to do) will insinuate that this means the competition's CPUs aren't "true" 64-bit (even though they are).
I dunno 'bout Macs (I don't know the M68k's "bitness"), but Intel introduced the 386 (their first 32-bit CPU) in 1986. And I certainly don't think the M68k was a RISC processor.
at current prices and projected prices, 512 gigabytes or RAM will barely cost more than a couple of the fastest processors of this type.
Really? I would LOVE to be able to buy 512 gigabytes of RAM for the cost of a couple of fast desktop processors. Don't forget that the PowerPC 970 is meant to be a desktop processor.
Because that is where most of the desktop CPU money is going, some of the high end, and frighteningly enough a fair bit of embedded CPU money too.
In short if you can navigate the patent mine field, the brutal competition mine field, and deal with the instruction set making things a royal bitch doing an x86 CPU is a total no-brainer.
Other then needing a whole new decoding front end, and being forced to use a trace cache because decoding multiple instructions in x86 land is very hard... the instruction thing isn't a big deal. Handling the odd-ball 80 bit FP format is. So is emulating all of the trap stuff and the other little odd bits close to the instructions set (like the MMU).
A big pain. But with much of the effort not being where folks think it is!
What I really want to know is how much this chip is going to cost. If its cheap for Apple to put 2 or 4 of these in a machine, then how much will it matter that an expensive P4 (P5) out performs it? Hmmm.... The current Wind-Tunnel G4s raised a few eyebrows when it first came out do to the new case design. These things were designed to disapate heat! A HUGE (7 lbs) heat sink w/ matching fan, a small case fan, 2 fans on the power supply, and a ton of ventalation in the back. WAY more cooling that those 2 little G4s require. I think Apple is trying to avoid the fiasco it had with the Sawtooth (1st gen) G4s where they just slapped a G4 onto a G3 mobo. This time around, I believe they're releasing a new mobo first and then put a new proc in it down the road. I've also read stuff in forums suggesting that the power supply for the Wind-Tunnel had way more juice than the system currently demands. Can anyone out there do the math on this? We know how much power the PPC 970 eats. Can we figure out how much heat the Wind-Tunnel case is designed to disapate? What about how much power the power supply is putting? With these numbers, can we figure out how many PPC 970 the Wind-Tunnel case could power and cool? I've been suffering with a 266MHz G3 iMac, and I refuse to upgrade until Apple comes out with a system that really is worth that premium they charge, and a G4 is not it.
IT IS ONLY 40 BITS not 64
Your desire to use address pins (or is it max pinned space per process?) to measure size puts you in a distinct minority. That doesn't make you wrong. But neither does it help make you right in this particular jungle.
Systems whose physical addressing match their claimed "bitness" are probably in the minority.
Some systems provide more physical addressing than register width (later PDP-11s, 8086, S/390), some less (68000, classic CDCs, early POWER). The 970 falls into the less category. Nothing unusual there.
Apple, like EVERY OTHER OS KNOWN, will steal a bit or two
Some bits come from physical addresses, some from virtual addresses. These should be addressed [pun slipped in, sorry] separately. AIX, btw, steals less than one bit. Linux can also be configured to steal less than one bit. (Assertions I can get away with no loss of credibility, since AC's have none to start with.) Were you frightened by a VAX in your formative years?
Why do fanboys mod stuff like this down?
Because we can't figure out why someone who needs 512GB, or 1TB, or more (which is it?) cares that a Linux process is limited to 1GB and not 2GB or 4GB.