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RE: Chris (IP: ---.student.iastate.edu) - Posted on 2004-01-22 22:34:05
I'm sure this is mostly concerning the recent Athlon64, as it is the first 64 bit x86 chip that I have heard of.
The benefit with it isn't 64 bit integers, as stated above it's that it has 16 registers per unit where Athlon has 8. Pentium 4 has 8, and I believe I read somewhere it has 112 other invisible registers. Doubling the registers is definitely a good thing as one can lose clock cycles even when requesting information from cache; and with an 18/20 stage pipeline losing a cycle is a big deal.
True, however, RISC chips have historically aways had a huge number of registers, also, regarding the "112 other invisible registers", I assume you're referring to the renaming registers which are nice hack to providing more registers without actually providing more.
The thing with RISC is that, in theory, if an application is tweaked to the max, that is, using ever possible feature of the chip; the huge number of registers, ISA enhancements like VIS and use a top notch compiler, the RISC chip should perform better, however, in reality one cannot spend the amount of time required to actually get software performing that good.
Also, we have to move eventually as 32bits can only hold the epoch till 2038 if I remember correctly. There are of course other reasons why a 64bit machine is cool. And a less than 20% performance loss in those tests is a small price to pay, as die size decreases we will make up for those speed issues in increased clock speeds within a year or two.
One also has to take into account that we've reached a point of deminishing returns. Instead of the Intels of the world working on making their processor more efficient, that is, doing more work per-clock cycle, they push the pipe line out to the moon and back just so they have the ability to boast in the clock speed hype-a-thons that occur in the local computer rags like zdnet.
The benchmarks seem pretty predictable though, and it's true that for most people's needs there is no point in having a 64bit integer unit. But in 5 years there most likely will be, so we might as well start upgrading now.
If possible, I'd love to see a correct benchmark of a Athlon64, since all I have seen so far have been 32bit comparisons. I want to see it's 64bit mode compared to it's 32bit compatibility mode.
IIRC, x86-64 has compatibility and long mode. Longmode is a native x86-64 application. Long mode covers both 32bit and 64bit, meaning, one can have a long mode 32bit application featuring all the benefits of x86-64 ISA without the need to re-tune the application.
RE: JCS (IP: ---.schaferhsv.com) - Posted on 2004-01-22 22:39:30
Apart from tricks like PAE and whatever is in Panther that does the same thing, your memory addressing comment is quite correct. An Opteron running Linux x86-64 is a 64-bit system. A Xeon with 8 GB of RAM - and using it - is not.
What is not spoken about is the fact there are limitations and issues with PAE. Firstly applications must be able to understand PAE, without that, applications will still only see the max, also, there is a performance penalty for it too. btw, A Xeon with 8GB of memory uses PAE with 36bit addressing thus giving a max of 32GB of addressable memory. x86-64 on the other IIRC< addresses around 42-44bits.