PC Hardware A Beginner’s Guide

Chapter 7: Computer Memory^135

CPU), RAM only requires perhaps 50 to 60ns to do its part. The significance here is that
the closer the RAM’s speed is matched to that of the data bus and CPU clock, more data
willbetransferredfromRAMtotheCPUandothercomponentsofthePConeachcycle.
Another speed in the PC that must be considered is the speed of the data and address
buses. Like the CPU, the bus transfer speed is in megahertz, which represents the speed
used to move data and instructions between structures, such as the CPU and memory.
Most RAM manufacturers include online guides on their Web sites to hel pmatch RAM
and RAM speeds to bus and CPU speeds. Table 7-2 contains a sampling of RAM speeds
and matching bus speeds.

Figure 7-3. The CPU interacts with RAM through wait states

Data Bus RAM

20MHz 50ns

25MHz 40ns

33MHz 30ns

50MHz 20ns

66MHz 15ns

100MHz 10ns

133MHz 6ns

Table 7-2. RAM/Bus Speeds