Author: Robert Hyatt
Date: 21:24:45 10/26/01
Go up one level in this thread
On October 26, 2001 at 23:04:56, Tom Kerrigan wrote: >On October 26, 2001 at 22:33:47, Robert Hyatt wrote: > >>On October 26, 2001 at 21:43:35, Tom Kerrigan wrote: >> >>>On October 26, 2001 at 21:19:11, Robert Hyatt wrote: >>> >>>>>"The floating point unit has 32 32-bit non windowed registers, which must be >>>>>saved on a per-context basis" >>>> >>>>Memory fails as age increases, apparently. :) >>> >>>Maybe FPUs are studied in a semester of comp org that you didn't teach. >> >>Actually FPUS really aren't touched on in a one-semester architecture >>course. With pipelines, cache, memory management, plus a few specific >>architectures, time runs out pretty quickly. >> >> >>> >>>>There is only _one_ data path _into_ the CPU. I was originally talking about >>>>the 64 bit chunks that can flow into the cpu from outside. And that is a >>>>real bottleneck on Intel boxes, still. IE you can't possible load >>>>instructions, int data, and fp data, fast enough if you have to use memory. >>>>And the classic SPEC benchmarks tend to stream data like crazy... >>> >>>This is going off on a tangent; Intel's decision to use a 64-bit FSB is almost >>>certainly based on price/performance goals and not the bitiness of any processor >>>internals. The FSB is 64-bit, the L2 bus is 256-bit, the SSE datapaths are >>>128-bit, the x87 FPU is 64-bit (I believe), the core is 32-bit... all design >>>decisions determined by any number of factors. It would have been a small amount >>>of work to make the P4 a 64-bit chip instead of a 32-bit chip; this wasn't done >>>almost certainly because the need for 64-bit is too small to justify a new >>>instruction set. Or they didn't want the P4 to compete directly with the Itanic >>>(and kick it in the nuts). AMD seems pretty happy to go the 64-bit route with >>>x86-64 and minimal changes to the Athlon design. >>> >>>-Tom >> >>In any case, I still believe the _driving_ force for 64 bit machines is not >>memory, since I still don't see any > 4gig machines lying around. But I do >>see a lot of people comparing FP performance to choose their next >>high-performance workstation. The best example here is still the Cray. With >>a 32 bit address bus, but a huge data path. Ditto for comparing the processors >>made by everybody, to the intel X86. Everybody has done 64 bit processors, >>but hardly any go beyond 2^32 address lines. Seems to me that it is for >>reasons other than address space, based on that... > >Well, I know that a lot of noise was made even a few years ago about certain OSs >not supporting memory over 2GB. I also know that many of your nicer [non-Intel] >MP systems ship with many, many GB of RAM. Sure. And today, when you ask about large-memory systems, the topic generally drops around to the Cray machines, particularly the Cray-2, and now the C90/T90 with 32 gigs (4 gigawords). Most of the "MP systems" are not shared memory, so with 128 processors, and 128 gigs of ram, you still only need 30 bits of address space. (IE IBM SP for one, CM5 for another, big alphas for another, etc.) > So somebody out there needs it. It's >possible that the demand has been low due to memory prices, but with prices in >the basement right now, I expect many more people will want > 4 GB RAM. I only >have 512MB myself, but I know many people who are up past 2 GB already. > >-Tom I know a _few_ that are at 2 gigs. And I know a couple that are using 4 gigs. So the demand is there in very low levels. And no doubt it will grow as systems and apps grow. But note that 64 bit architectures were around in the middle 60's... (60 and 64 bits). They were obviously done for something _other_ than address space...
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