Hardware Reference
In-Depth Information
capable ofequaling that ofthe external processor bus.When the speed ofthe memory bus
equals the speed of the processor bus (or some even multiple thereof), main memory per-
formance is closest to optimum for that system.
memoryusedintheoriginalPentiumandPentiumIIPCsupuntil1998worksouttobean
extremely slow 16.7MHz! This slow 16.7MHz memory was installed in systems running
processors up to 300MHz or faster with external processor bus speeds of up to 66MHz,
resulting in a large mismatch between processor bus and main memory performance. To
alleviate this performance gap, starting in 1998 the industry shifted to faster SDRAM
memory, which could match the 66MHz and 100MHz processor bus speeds in use at
that time. From that point forward, memory and especially memory bus performance has
largely evolved in step with the processor bus, coming out with newer and faster types to
match any increases in processor bus speeds.
By the year 2000, the dominant processor bus and memory speeds had increased to
100MHz and even 133MHz, called PC100 and PC133 SDRAM, respectively. Starting in
early 2001, double data rate (DDR) SDRAM memory of 200MHz and 266MHz became
popular. In 2002, DDR memory increased to 333MHz, and in 2003, the speeds increased
further to 400MHz. In 2004, we saw the introduction 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.
Table 6.3
lists the primary types and per-
formance levels of PC memory.
Table 6.3 PC Memory Types and Performance Levels
