From: willmore@iastate.edu (David Willmore) Subject: Re: Intel, the Pentium and Linux Date: Mon, 26 Apr 1993 22:38:55 GMT
jerry@msi.com (Jerry Shekhel) writes:
>That I don't know, but it's interesting how things have changed. In the
>old days, my 286 would have to be pumped up to about 200MHz to keep up with
>my 12MHz R3000 (Personal IRIS). Not that I ever tried it, of course :-)
>But it used to take a CISC CPU so many cycles to execute an instruction
>that it would need a much faster clock than a RISC in order to offer similar
>performance. Now the tables have turned; it takes a 150MHz RISC Alpha to
>beat a 60Mhz CISC Pentium.
There are just too many factors at play to be able to say that. A well
designed CISC should *always* be able to execute more instructions than
a RISC processor *per cycle*. The differences lie in that the CISC chip
will have taken much more work to design and won't be able to be run at
the clock speed that the RISC processor can.
Most RISC processors are memory bound today, anyway. Halve the speed of an
Alpha and leave the memory interface alone and you won't change the speed
of the Alpha by 2X. I guess it really comes down to the fact that 'clock
speed' is a very misleading term. On the DX family, bus speed and CPU
speed agree. On the DX2 family, they don't. On the pentium, who knows.
The processor may have a faster clock internally that really runs the
works. The memory interface on the Alpha always runs at one half of the
CPU clock speed or *slower* anyway. Should a 150Mhz Alpha really be
called a 75Mhz Alpha? On the R4x00 family, it's the other way around.
The CPU runs at twice the rated bus speed. A 75Mhz R4400 really runs at
150Mhz internally.
From what I heard, the low end Alphas will be running at 80Mhz. That comes
out to a bus speed of 40Mhz. For the Pentium running at 60Mhz, it's bus
will run at 60Mhz, but will waste a cycle (Sorry, nothing runs *no waits*
at 60Mhz in a PC) so that it's effective rate will be 30Mhz. The speed of
these machines will depend *heavily* on how the memory interface is designed.
An Alpha (24064) will support up to 8Meg of cache without much support logic.
Heck, who needs main memory...
Too many variables to determine a meaningful solution. The only thing that
*is* for sure is that the next few years are going to be *really* interesting
for the PC market. The first PC shakeout left MSDOS and the Apple II family
alive. The next left the Mac and the PC alive. The last one killed off
many of the clone vendors. What will the coming Pentium/Alpha/PowerPC/et at.
shakeout do?
Later,
David
-- =========================================================================== willmore@iastate.edu | "Death before dishonor" | "Better dead than greek" | David Willmore | "Ever noticed how much they look like orchids? Lovely!" | ===========================================================================