PCI bus freq with CPU freq at (X * 40)Mhz?

PCI bus freq with CPU freq at (X * 40)Mhz?

Post by Iain Bai » Wed, 06 Mar 1996 04:00:00




: 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
cards.

(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.

iain

 
 
 

PCI bus freq with CPU freq at (X * 40)Mhz?

Post by Chris Mauri » Wed, 06 Mar 1996 04:00:00



: : 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
: cards.

Reduce the life expectancy?  How?  My experience has been that
overclocked cpu's/cards either work or don't work from square
one...they don't work and then fail.  If you have different
experience please share.  I'd hate to think I'm risking expensive
cpu's and #9 graphics cards...
:
: 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.

I do the same.  No problems.

: : 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.

Yup.  My play machine is currently a 486/120 with #9 GXE-64-Pro
VLB card + 32mb RAM + ancient ISA SCSI card + quad speed CD-ROM
and it is a screamer.  Back of the napkin benchmarks (for me) put
it at somewhere between a P75 and P90 in terms of performance and
I was able to leverage "spare parts" to do it.

Unless you're doing heavy number crunching, I've found that
once you've passed Dx2/66 performance the cpu becomes almost
irrelevant in terms of how "useful" a system is.  Modern
video and IO cards (major cycle stealers of old) usually
now have enough "brains" to take their orders and run with the
ball, leaving the cpu to worry about other more important tasks.

When it gets to the point that P166 mboards cost as much as
toasters, I'll go for more cpu brawn.  :-)

: : 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...

Really....According to AMD's web page, it's almost identical in
performance to a 486/120...

: :      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.

I might try one for grins, but AMD's own benchmarks put it almost
exactly where the 486/120 is...I'm not sure I understand their
marketing...

Chris
--
Christopher Mauritz         | For info on internet access:

Mordor International        | http://www.mordor.com/
201/212/718 internet access | Modem: (201)433-7343,(212)843-3451

 
 
 

PCI bus freq with CPU freq at (X * 40)Mhz?

Post by John Shalamsk » Sat, 09 Mar 1996 04:00:00




>> : Reduce the life expectancy?
>The biggest risk is that you don't need to notice the problem
>immediately.  The most fatal source of failures on an overheated chip
>are whiskers in an Al conductor layer that finally break an insulation
>layer, hence causing a shortcut, or so-called ``electro migration'',
>thinning out a conductor layer, which causes it to overheat the more
>until it's broken.

>Both effects mostly happen with local hot spots on the chip.



The general rule of thumb for semiconductors is that lifetime is
halved for every 10 degrees Celsius that the operating temperature
rises.  Does anyone know what happens to the temperatures inside CPU
chips when the clock doubles?  All it takes is one hot spot to
develop, and poof!  No more CPU.

John


dba MLH Consulting                               phone (808) 521-3141
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PCI bus freq with CPU freq at (X * 40)Mhz?

Post by Willy TARRE » Thu, 14 Mar 1996 04:00:00






: |: : 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.)?
: [deleted]
:  
: |: 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
: |: cards.
: |
: |Reduce the life expectancy?  How?  My experience has been that
: |overclocked cpu's/cards either work or don't work from square
: |one...they don't work and then fail.  If you have different
: |experience please share.  I'd hate to think I'm risking expensive
: |cpu's and #9 graphics cards...

: Sure can, power-dissipation is proportional to speed, and power-
: dissipation spells *heat*; that's why high-speed CPUs require heat-
: sinks and fans.  Look at some semiconductor data-sheets: you'll see
: "ABSOLUTE MAXIMUM RATINGS", temperature, Power Dissipation, and
: voltage.  "Stresses above those listed under ABSOLUTE MAXIMUM
: RATINGS my cause permanent device failure.  Functionality at or
: above these limits is not implied.  Exposure to absolute maximum
: ratings for extended periods may affect device reliability."

: The above is an excerpt from an IC data-sheet, and is typical.

I had an experience with an AMD DX2/80 3.3 Volt which ran about one year
120 Mhz under 5V ! I was surprised to discover that when I set it back to
80 Mhz under 3.3V, it sometimes hanged, so I had to set it back to 5V !
My conclusion was:  The CPU finally accomodated itself to the higher voltage !

But I've never had problems with overclocked CPUs at the same voltage.

Willy

 
 
 

1. Lilo "L 40 40 40 40 40 40..."

Greetings,

I have set up a Linux From Scratch system on a 1.2Gb Maxtor hard drive. It
works fine on my two Athlon systems, but when I put the drive in my Pentium
100, the system results in "L 40 40 40 40 40 40..." before showing the Lilo
menu.

My Lilo.conf contains the "lba32" entry which should mean this would work,
but it doesn't.

Any ideas?

Regards,
Jon

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