Chapter 7: Computer Memory^133
must fast forward over the first two songs on the tape. This is calledsequentialorserial
access. Everything is accessed in its physical sequence or in series. To listen to the third
song on a music CD, however, you merely indicate that you wish to move to track 3, and
bingo—there you are. This is calleddirectorrandom access. You pick where you’d like to
gorandomlyandthengodirectlythere.Accessingsomething,aprogramordata,inRAM
is very much like the music CD, except that your choices are millions of individual storage
locations (bytes), each of which can be addressed directly by your programs.Volatile versus Nonvolatile
ROM was described earlier as beingnonvolatile, meaning that it holds its contents without
a power source. The opposite of nonvolatile isvolatile. Volatile memory cannot hold its
contents, the data, or programs placed on it without an active power source, such as a
wall socket or battery. RAM is a volatile form or memory and when it loses its power, it
loses its contents. If you have ever lost everything you were working on when a power
failure hit, someone tripped over the power cord, or you had to reboot the PC, then
you’ve experienced the downside of volatile memory.
So, why is volatile memory used in the PC? Why not just use nonvolatile memory? If
you were to use EEPROMs or any of the newer types of SRAM (see the section “RAM
Types” later in this section), the cost for the amount of memory you need to run the
high-graphic and feature-rich software of today would exceed that of the entire rest of the
PC, including all of the options and bells and whistles you could add. Volatile RAM is in-
expensive, readily available, easily expanded, and, as long as you protect your system
against power problems (see Chapter 14), it is error- and trouble-free for the most part.Bits, Bytes, and Words
Nearly everything the PC connects to is measured in bits these days, especially modems
and Internet connections, but RAM is still measured in bytes—actually, kilobytes, mega-
bytes,orgigabytes.Table7-1liststhevariousdataunitscommonlyassociatedwithRAM.Memory Speeds
RAM is much faster than a hard disk, floppy disk, CD-ROM, or any other form of second-
ary storage. On the average, accessing data from a hard disk drive takes from 8 to 16milli-
seconds(ms). Accessing the same data from RAM takes from 50 to 80 nanoseconds (ns).
There are 1,000ms and 1 billion nanoseconds in a second. What this works out to is that
RAM at 50ns is over a million times faster than a hard disk. Other secondary storage
devices, such as the CD-ROM or floppy disk, are even slower.Clock Speeds
Most,butnotall,oftheactionstakingplaceinsidethePCaresynchronizedtooneormore
“clocks.” These clocks provide electronic timings to which the components of the PC can
synchronize their actions to those of the CPU and other devices. For example, the proces-
sor’sinternal clock speedprovides the tempo at which electronic signals and data are sent
around the PC.