Daddy Bob

DADDY BOB'S COMPUTER Q & A

 

February 10, 2008

Q. On a memory module there is a designation called CL-2.5. I pretty much understand what the other numbers mean, but I am lost when it comes to the CL number. Can you explain what it is?

A. It will be difficult to explain this without getting technical, but I'll give it a try.  CL stands for CAS Latency, where the CAS is for Column Address Strobe, or sometimes better explained as Column Address Select. Picture the DRAM, Dynamic Random Access Memory, module as being composed of an array of capacitors, we'll call cells, arranged in an orderly grid of columns and rows much like an Excel spreadsheet. CAS Latency, or the CL number is the time in CPU clock cycles (explained at end of this article) that elapses between the time the memory controller tells the memory module to access a particular column in the current row, until the first bit of that data can be read from the module's output pins.

To try to explain further, data is stored in memory cells that are identified by a memory bank, a row, and a column. To access data stored in memory, the controller first selects a memory bank, then a row then a column. Once the particular cell is located, the controller requests the data that is stored in that physical cell. The CL number is the number of clock cycles that elapse from the time the request for data is sent to the specific memory cell until the first bit of that data is transmitted from the module and becomes available for the CPU to use.

So, what does all this mean? When selecting a RAM module, the lower the CL number, all other things being equal, the less time it will take to fetch data from it. The less time it takes, the faster it will operate. But, there is something else to consider. The CL number only specifies the delay between the request and the availability of the first bit of data. The clock speed specifies the latency between bits of data. Thus, when reading bursts of data, which is the norm, a higher clock speed can be faster in practice, even with a worse (higher) CL number.

Lower latency modules are more difficult to manufacturer, and thus, will cost more. This will explain why it may be less expensive to buy a 1 GB module with a CL-5 than a 512 MB module with a CL-2 when all other specs are equal. In most cases, assuming the computer can use either, the best overall performance gain will be experienced with the larger capacity module. In the rare case when the computer has all the memory it can use, the lower CL number may produce a higher performance gain and be worth the extra cost.

Most RAM supports multiple clock speeds and CL numbers, with varying performance. RAM with a lower rating than the computer specifies should not be used and can cause crashes, but higher rated memory can be installed into a system with a lower rating. 200 MHz rated RAM can be installed in a system with a 133 MHz memory bus, BUT it will only run at 133 MHz. RAM speed ratings are best understood as speed limits, rather than operating speeds. Therefore, installing faster RAM into a computer than the computer is designed to use for will not cause it to operate any faster.

So to conclude, the CL rating of memory is an important consideration if all other things remain equal, and the lower the number the better. However, it is a combination of the DRAM speed and CL number that should be considered when purchasing memory. Faster speeds will offset higher CL numbers so in general, assuming the computer can use either, a module rated at 667 MHz and CL-5 would be preferred over one rated at 133 MHz at CL-2. All this in mind, the best advice is to use what the computer manufacturer recommends and has pre-installed if it is available.

CPU Clock Cycle:

A clock cycle is the time it takes to progress from any point on the sine wave to a like point on the next sine wave, usually from peak to peak or valley to valley. The number of waves or cycles per second, called Hertz, is the frequency and it is controlled on most computers by a crystal oscillator. On a CPU rated at 1 GHz, there are 1,000,000 cycles per second, and its clock cycle is therefore 1 millionth of a second. So, on this particular system, the time represented by CL-2 would be 2 clock cycles or 2 millionth of a second.

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