CompTIA A+ Certification All-in-One Exam Guide

Figure 4-17 Double-sided DDR SDRAM
Latency
Different RAM responds to electrical signals at varying rates. When the
memory controller starts to grab a line of memory, for example, a slight delay
occurs; think of it as the RAM getting off the couch. After the RAM sends
out the requested line of memory, there’s another slight delay before the
memory controller can ask for another line—the RAM sat back down. The
delay in RAM’s response time is called its latency, but shorthand like CL17
or CL19 uses initials for the technical name: column array strobe (CAS)
latency.
If both have the same speed rating, RAM with a lower latency—such as
CL17—is slightly faster than RAM with a higher latency—such as CL19—
because it responds more quickly. The CL refers to clock cycle delays. The
17 means that the memory delays 17 clock cycles before delivering the
requested data; the 19 means a 19-cycle delay. Because it’s measured in
clock cycles, CL is relative to the clock speed of the RAM—a 19-cycle delay