[Ham-Computers] RE: WTB..Ram

Mon Mar 10 22:22:03 EST 2008

Good tutorial Aaron,  thanks!

BTW, in some cases you don't even have to match "speeds", let alone latency.
I have a system here that was was a dog with WinXP and 256MB of PC2100.  I
upgraded it to 512MB and it went to "okay" performance.  (I know, not enough
ram for the XP memory hog, but it was a $300 "internet" computer off the
shelf at a big-box store.  It was enough for the kids surfing since they
aren't gamers).

Meanwhile, another desktop let the magic smoke out of the CPU and
motherboard (catastrophic heat sink/fan failure on the CPU) but it had 1GB
of PC2700.

The "internet" computer's motherboard only supported up to the PC2100 speed,
but the PC2700 "works", just by slowing down to accomodate the slower bus
speed.  1GB is way better than 512MB in that application - and there's no
"cost" since the PC2700 wasn't going to fit in the replacement computer
anyway.

And the internet computer's name on the in house LAN?  "Blondie" - it's
okay, just don't ask it to do too many things at once, because it's a little
slow and scatterbrained.

73,
Jay
AD5PE

-----Original Message-----
From: ham-computers-bounces at mailman.qth.net
[mailto:ham-computers-bounces at mailman.qth.net] On Behalf Of Hsu, Aaron (NBC
Universal)
Sent: Monday, March 10, 2008 17:58
To: I>Ham-Computers
Subject: [Ham-Computers] RE: WTB..Ram 

Brian sent me an e-mail asking for additional info about RAM specs.  I'm
posting my reply to the group as it's handy info for those who want to know.
Hope you don't mind, Brian.

RAM has more than one spec.  The MHz "speed" (e.g. PC100) or memory
bandwidth (e.g. PC2-6400) is the most commonly advertised or published spec,
but some systems require that other "timing" specs be met before the system
can use that RAM.

Four other commonly looked at RAM timing specs are CL (CAS Latency), tRCD
(Time, RAS to CAS Delay), tRP (Time, RAS Precharge), and tRAS (Time, Active
to Precharge Delay).  You'll often find these listed as CL-tRCD-tRP-tRAS -
for example, my system has PC2-6400 RAM spec'd at 4-5-4-15.

Each of these numbers specify the number of clock cycles that must pass
before data is available.  In the 4-5-4-15 example, there is a CAS Latency
of 4 clocks before you can access data in any particular address "column"
after selecting that column (the first "4" in 4-5-4-15).  BTW, PC2-6400 CL4
is considered "fast" RAM - "Normal" PC2-6400 RAM will have a CL of 5 or 6.
With the same RAM, there is a 15 clock cycle delay when addressing data in
any particular "row" after selecting that row (the "15" in 4-5-4-15).  You
get the general idea.

If your system is designed to work with CL2 RAM, then CL3 RAM will be too
"slow".  Essentially, the system will attempt to access the data after 2
clock cycles, but the RAM requires 3 clock cycles before it's ready.  The
system will still grab whatever data is there, but the data may be invalid.
"Grasshopper, grab the pebble from my hand...when you can take the pebble
from my hand, it will be time for you to leave."  Sorry...silly show
quote...

How does a computer know what will or won't work?  Most all RAM modules
since EDO RAM include SPD - Serial Presence Detect.  Over the years, the
implementation of SPD changed, but the idea is the same - the SPD contains
the info that describe the RAM module.  Since SDRAM days, the SPD is an
EEPROM chip on the RAM module and timings are programmed into it so the
system can determine if the module is compatible.

Some systems ignored the SPD and attempted to work with *any* RAM module.
However, system instability would be the result if "slow" RAM were used.
"Better" systems used the SPD and would balk if slow or incompatible RAM
were installed - Toshiba, IBM, and Compaq systems do this.  So if the system
is designed for CL2 RAM and you installed CL3 RAM, then the system will fail
POST and usually would show an "incompatible memory" message.  Realize that
it's not just the CAS Latency that could fail.  But, the other timings are
often in "sync" with the CAS Latency - lower CL will also usually mean lower
tRCD, tRP, and tRAS timings.

With systems made 5 to 10 years ago, the CL wasn't published by the system
manufacturer as a way to "force" you to buy their RAM - you paid more, but
it was *guaranteed* to work with the system.  Often, 3rd party vendors also
didn't publish the timings, but *guaranteed* that a particular RAM module
would work with that system - this prevented you from using their "memory
finder" and then using the specs to find cheaper memory from someone else.
Kingston still does this - specific modules for specific systems even though
most of the modules are exactly the same.  You pay less for Kingston than
you would HP/Compaq, but you could also pay less for another "generic" if
you know the specs - thus, Kingston also doesn't publish specs.

Most all systems today that use DDR/DDR2/DDR3 RAM will work with any RAM
timing - the memory controllers are designed to accomodate whatever modules
you provide.  However, performance will take a hit as a result of using
slower memory.  That's why you'll find "performance" memory brands
(Crucial's "Balistix", OCZ "Gold" and "Platinum", Kingston "HyperX", etc)
that offer "fast" RAM for those who want the ultimate performance (usually
gamers or graphics users, e.g. Photoshop).  Does it make a difference?
Absolutely...3% to 10%, depending on the speeds and timings.

Anyways, I hope this answers some questions.  Here are links to two articles
explaining in more detail some timing specs:

http://www.amd.com/us-en/Processors/ComputingSolutions/0,,30_288_13265_13295
%5E13335,00.html

http://www.hardwaresecrets.com/article/26/2

Now, if anyone has any PC2-6400 3-3-3-10 or PC2-8500 5-5-5-15 DDR2 modules
they'd like to donate, I would be more than happy to take them off your
hands!  =)

73,

  - Aaron, NN6O

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