PC Hardware A Beginner’s Guide

(^134) PC Hardware: A Beginner’s Guide
The CPU’s clock isn’t really a “clock” like the cuckoo clock on the wall. The system
clock sets the length and number of electronic cycles available in one second. These
cycles, which are the timing mechanism used to synchronize the movement of data and
execution of instructions, are measured in megahertz (MHz). Ahertzis one shift of the
clock’s electronic signal from high to low (or low to high). Amegahertzis one million hertz
in one second. A CPU with a clock speed of 600MHz operates on 600 million cycles per
second. To put this in terms of instructions, a single computer instruction, such as adding
two binary numbers that are already in the CPU’s registers, generally takes one CPU
cycle. So, theoretically, a 600MHz computer is capable of completing 600 million of these
instructions per second. Many processors are rated inMIPS (million of instructions per second).
Unfortunately, most processors cannot translate their megahertz ratings directly into
MIPS. Data must be moved in and out of the CPU’s registers to RAM, the hard disk, and
other destinations, and these actions also require clock cycles to complete.
CPU Wait States
It should also be noted that RAM, which operates in nanoseconds, is faster than most
CPUs. This suggests a problem, but the CPU works throughwait states, which are intervals
of a set number of cycles between CPU actions, such as data requests, reads, writes,
moves, etc., to allow the requests to be carried out. To read data from memory, the CPU
may use three wait states, as illustrated in Figure 7-3. The CPU issues the request for data
along with an address. Receiving the address and transferring it to the memory controller
uses about one wait state. Finding the data in memory also takes about one wait state.
Transferring the data to the CPU’s storage areas (calledregisters) uses a third wait state.
Even if each wait state only took about 1/400 millionth of a second (based on a 400MHz
Unit Size Description
Bit One binary digit Stores either a binary 0 or 1
Byte Eight bits One character
Word 16 to 64 bits Numeric values and addresses
Kilobyte (KB) 1 thousand bytes About one page of double-spaced text
Megabyte (MB) 1 million bytes About the size of a short book
Gigabyte (GB) 1 billion bytes 1,000 short books
Terabyte (TB) 1 trillion bytes An entire library
Petabyte (PB) 1 quadrillion bytes Just about all the libraries in the U.S.
Table 7-1. RAM Units of Measure