CASLatency

CAS latency, often abbreviated CL, is a memory timing parameter used to describe the delay between sending a READ command to a DRAM module and the moment data begins to appear on the module’s data bus. It is expressed as a number of memory clock cycles and is one component of the broader set of memory timings, which also include tRCD, tRP, and tRAS. A memory module’s timings are usually written in a sequence such as CLx-y-z-w, where x is the CAS latency.

Because CL is measured in clock cycles, the actual delay in nanoseconds depends on the memory’s operating clock. For example, a DDR4 module with a base clock of 1600 MHz (often described as DDR4-3200 due to its data rate) has a period of about 0.625 nanoseconds per cycle. Therefore CL16 corresponds to roughly 10 nanoseconds of latency, while CL18 would be about 11.25 nanoseconds, all else being equal. In practice, the impact of CL on real-world performance also depends on the memory frequency, other timing parameters, and the memory controller.

Lower CAS latency generally reduces latency, but higher memory frequencies can offset higher CL values, so overall performance is a balance between CL and frequency, as well as other factors like bandwidth and system workload. Users can often adjust timings in BIOS/UEFI or enable profile presets (such as XMP/DOCP) to achieve a desired CL, subject to stability. CAS latency is a useful, but not sole, predictor of memory responsiveness.