[Ham-Computers] RE: WTB..Ram

Hsu, Aaron (NBC Universal) aaron.hsu at nbcuni.com
Mon Mar 10 17:57:40 EST 2008


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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