1. PCI bus freq with CPU freq at (X * 40)Mhz?
: Sorry for ignorance, but what will be the frequency on a PC's PCI bus
: when the CPU runs at main frequency of 40MHz (DX2/80, DX4/120 i.e.)?
: I always thought that PCI will than run at 40MHz too.
: But was told today that it will be only 20MHz?! Oh God.
: How can I check this for sure, without running hated benchmarks,
: but only by exploration of MB jumpers?
PCI is specified to run at up to 33MHz. To accomodate faster CPU
clocks motherboards can provide the facility for PCI to run at
some fraction of the CPU. My motherboard (GA-486AMS) allows a
CPU:PCI ratio of 1:1 or 2:1, configured in the BIOS setup. In
principle other ratios could be used (e.g. 4:3), but I'm not aware
of any motherboards which support this.
Having said that, I have run PCI at 40MHz with a DX4-120, and it
worked (with AHA-2940 and S3-968). This made little or no difference
to kernel build times compared to 20MHz, although this is a crude
benchmark. Doing this could reduce the life expectancy of PCI
(A recent version of the PCI spec (2.1?) allows for faster clocks,
but I don't think there are many compliant cards available).
: I was also told that having 40MHz will probably cause
: an addition of extra wait states for cache and RAM access,
: thus making a box considerably _slower_ for real work
: than with 33MHz CPU main frequency, especially for UNIX-like OS
: (FreeBSD in my case, but who cares). Any comments on this one?
With 15ns cache and 70ns SIMMs I was able to run a DX4-120 with
0WS and 2-1-1-1 cache burst read, the fastest settings supported.
This is probably motherboard dependent.
: And, in case both statements are true, why bother looking
: at (X * 40)MHz 486 CPUs at all?
They are more of a win with VLB motherboards.
: P.S. I don't own a PCI box, but just planning to buy one, choosing
: the best bang-per-buck configuration now.
: Was considering AMD 160MHz (glorified 486, called 5x86, he-he :)
: chip, which has 40MHz main freq and 4x multiplier.
This is the 5x86-133, it's only 160MHz if you overclock it, and then you're
back where you started...
: Now I'm seriously considering AMD 133 part (this one is 33MHz, 4x).
: Guys have a very nice opinions on it.
For PCI, the AMD 5x86-133 makes more sense. I'm running one now,
and very happy with it.
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