Column Address Strobe Latency

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The Column Address Strobe Latency ( CL or CAS Latency for short ), also called memory latency , is the time required to address a column in the main memory of a computer. The measure for this is the required number of clock cycles. The smaller this number the better. However, this has no practical significance when operating a home computer.

definition

The Column Address Strobe Latency (CL) measures the delay between addressing (addressing of a column) in a DRAM module and the provision of the data stored at this address. The required clock cycles are measured . The time required results from the duration of a clock cycle times the number of clock cycles required.

The CL indicates how many clock cycles the memory module needs to process the data delivered during the CAS before it can accept further commands or communicate the result. The higher the CL value, the more clock cycles are required for processing, which means that the delay that depends thereon also increases; the lower the value, the faster the memory bar.

The accelerating effect of lower CL timings, however, is usually overestimated. It is generally below 5% and is therefore not perceptible to the user. Nevertheless, many customers are willing to pay surcharges for such modules. The disappointing acceleration effect can mainly be explained by the ever more effective and larger caches on the processors , which already catch approx. 90-95% of all accesses. It should also be noted that there are other latencies that have an impact on performance.

In addition to CL, the information tRCD , tRP and sometimes also tRAS are also often given for memory bars , e.g. B. in the form CL17-17-17 .

example

With a real clock frequency of 400 MHz, a clock cycle takes 2.5 ns. A memory with DDR2-400 and CL-4 therefore needs 4 clock cycles times 2.5 ns = 10 ns to address a column. A CL-5 memory takes 12.5 ns and a CL-6 memory 15 ns. In addition, there are additional latencies, for example for line addressing, for refresh, and others.

supporting documents

  1. ↑ "cells race" c't 8/06, page 210 ff.

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