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About the AGP bus



 
 
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  #81  
Old January 1st 05, 09:11 AM
Robert Hancock
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J. Clarke wrote:

PCI-X is too slow for current graphics cards. It goes up to 133 MHz on
64 bit, but this is only the same speed as AGP 2X. And it would likely
have major problems scaling faster in speed because it has such a wide
parallel bus. A serial bus like PCI Express is much easier to scale up
in terms of speed (look at SATA).



(a) PCI-X 2.0 goes to 533 MHz
(b) What about SATA? The second revision is not quite as fast as parallel
SCSI has been for years, and a lot less flexible besides.


As far as PCI-X 2.0, I would guess it's not going to get much or any
faster than that, and the cost is huge. From the PCI Express white
paper: "Close investigation of the 1990’s PCI signaling technology
reveals a multi-drop, parallel bus implementation that is close to its
practical limits of performance: it cannot be easily scaled up in
frequency or down in voltage; its synchronously clocked data transfer is
signal skew limited and the signal routing rules are at the limit for
cost-effective FR4 technology. All approaches to pushing these limits to
create a higher bandwidth, general-purpose I/O bus result in large cost
increases for little performance gain."

As far as SATA, the bus doesn't need to be as fast as SCSI since it is a
point-to-point bus which is not shared (similar to PCI Express). Also,
the performance of current SCSI setups comes at a high cost - SCSI
controller hardware seems about an order of magnitude more expensive in
many cases..

--
Robert Hancock Saskatoon, SK, Canada
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  #82  
Old January 1st 05, 09:12 AM
Robert Hancock
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J. Clarke wrote:
Robert Hancock wrote:


J. Clarke wrote:

I see PCI Express as more of a "stuck with" than a "must have". I think
we would all have been better served if they had instead of PCI Express
put
PCI-X in their chipsets as a standard feature. But that wouldn't have
forced one to upgrade any other components in order to replace a
motherboard.


As I mentioned in another post, PCI-X is not sufficient for use on a
video card these days. The fastest version is only the same speed as AGP
2X, we already have AGP interfaces 4 times faster than that..



The fastest version is the same speed as AGP 8x, which, given that no video
board currently on the market is bottlenecked at _4_x, would appear to be
adequate for video.

Your information, from whatever source, is not current.


Yes, PCI-X does go up to 533 MHz now, however it's a) extremely
expensive and b) not likely to get much faster, whereas PCI Express has
a clear upgrade path for the future..

--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from
Home Page:
http://www.roberthancock.com/
  #83  
Old January 1st 05, 03:54 PM
J. Clarke
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Robert Hancock wrote:

J. Clarke wrote:

PCI-X is too slow for current graphics cards. It goes up to 133 MHz on
64 bit, but this is only the same speed as AGP 2X. And it would likely
have major problems scaling faster in speed because it has such a wide
parallel bus. A serial bus like PCI Express is much easier to scale up
in terms of speed (look at SATA).



(a) PCI-X 2.0 goes to 533 MHz
(b) What about SATA? The second revision is not quite as fast as
parallel SCSI has been for years, and a lot less flexible besides.


As far as PCI-X 2.0, I would guess it's not going to get much or any
faster than that, and the cost is huge. From the PCI Express white
paper: "Close investigation of the 1990?s PCI signaling technology
reveals a multi-drop, parallel bus implementation that is close to its
practical limits of performance: it cannot be easily scaled up in
frequency or down in voltage; its synchronously clocked data transfer is
signal skew limited and the signal routing rules are at the limit for
cost-effective FR4 technology. All approaches to pushing these limits to
create a higher bandwidth, general-purpose I/O bus result in large cost
increases for little performance gain."


Remember when 100 Mb/sec LANS cost $10K/NIC? Time passes, Moore's Law
applies, what was impossible in the '90s is cheap in the 21st century.

As far as SATA, the bus doesn't need to be as fast as SCSI since it is a
point-to-point bus which is not shared (similar to PCI Express). Also,
the performance of current SCSI setups comes at a high cost - SCSI
controller hardware seems about an order of magnitude more expensive in
many cases..


You miss the point entirely. You're claiming that the serial bus has some
inherent performance benefit. If that was the case then SATA would be able
to outperform SCSI in all regards at lower cost. Instead it only allows
one device per cable, runs at a lower speed, and has about a tenth the
allowable cable length, which means that it is not a high performance
connection in any regard. Further, if there is no need for high
performance in SATA then why is there a 3 Gb/sec version of it?

As for SCSI controller hardware being "an order of magnitude more
expensive", where can one get an SATA2 host adapter at any price? The only
ones I can find are built into motherboard chipsets. Regardless, SCSI is
targetted at a different market--the price is set by marketing
considerations, not by manufacturing cost. Unless you believe that it
costs more to make a multiple-die-shrunk 5 year old chip design than it
does to make a brand new one.

--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
  #84  
Old January 1st 05, 03:58 PM
J. Clarke
external usenet poster
 
Posts: n/a
Default

Robert Hancock wrote:

J. Clarke wrote:
Robert Hancock wrote:


J. Clarke wrote:

I see PCI Express as more of a "stuck with" than a "must have". I think
we would all have been better served if they had instead of PCI Express
put
PCI-X in their chipsets as a standard feature. But that wouldn't have
forced one to upgrade any other components in order to replace a
motherboard.

As I mentioned in another post, PCI-X is not sufficient for use on a
video card these days. The fastest version is only the same speed as AGP
2X, we already have AGP interfaces 4 times faster than that..



The fastest version is the same speed as AGP 8x, which, given that no
video board currently on the market is bottlenecked at _4_x, would appear
to be adequate for video.

Your information, from whatever source, is not current.


Yes, PCI-X does go up to 533 MHz now, however it's a) extremely
expensive


So is PCI Express if you're running any kind of hardware that needs more
performance than regular PCI delivers.

and b) not likely to get much faster,


In your opinion.

whereas PCI Express has
a clear upgrade path for the future..


Clear upgrade for what purpose? What's coming along that needs more than 10
GB/sec?

PCI Express exists for one purpose, to put money in Intel's pocket. If you
want to buy the hype, go right ahead. But Intel implements all-up PCI-X in
_every_ PCI Express chipset they make (read the specs if you don't believe
me), so by your reasoning those chipsets, with both PCI-X and PCI Express,
should cost more than chipsets that implement PCI-X alone.

So much for your argument about cost.


--
--John
Reply to jclarke at ae tee tee global dot net
(was jclarke at eye bee em dot net)
 




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