Memory - Upgrading and Repairing PCs - page 320

As you can see from Table 6.2 , as clock speed increases, cycle time decreases proportionately, and
vice versa.
Over the evolutionary life of the PC, main memory (what we call RAM) has had a difficult time
keeping up with the processor, requiring several levels of high-speed cache memory to intercept
processor requests for the slower main memory. More recently, however, systems using DDR,
DDR2, and DDR3 SDRAM have memory bus transfer rates (bandwidth) capable of equaling that of
the external processor bus. When the speed of the memory bus equals the speed of the processor bus
(or some even multiple thereof), main memory performance is closest to optimum for that system.
For example, using the information in Table 6.2 , you can see that the 60ns DRAM memory used in the
original Pentium and Pentium II PCs up until 1998 works out to be an 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