Memory - Upgrading and Repairing PCs - page 321

of DDR2, first at 400MHz and then at 533MHz. DDR2 memory continued to match processor bus
speed increases in PCs from 2005 to 2006, rising to 667MHz and 800MHz during that time. By 2007,
DDR2 memory was available at speeds of up to 1066MHz. By late 2007, DDR3 came on the market
at speeds of 1066MHz, with 1333MHz and 1600MHz appearing in 2008. In 2009, DDR3 memory
became the most popular memory type in new systems, and faster speed grades of 1866MHz and
2133MHz were added. When DDR4 is released in 2013, it will start at a speed of 1600MHz, with
future speeds of up to twice as much. DDR4 should become popular in new systems in 2015. Table
6.3 lists the primary types and performance levels of PC memory.
Table 6.3. PC Memory Types and Performance Levels
Another specification to consider that is related to speed is the CAS (column address strobe) latency ,
which is often abbreviated as CL . This is also sometimes called read latency , and it's the number of
clock cycles occurring between the registration of the CAS signal and the resultant output data, with
lower numbers of cycles indicating faster (better) performance. If possible, choose modules with a
lower CL figure because the motherboard chipset reads that specification out of the SPD (serial
presence detect) ROM on the module and takes advantage of the lower latency through improved
memory controller timings. Figure 6.3 shows the memory timing and SPD information as reported by
CPU-Z ( www.cpuid.com ) for a system with DDR3-1600 SDRAM.