Hardware Reference
In-Depth Information
in memory is not saved, the original copy of the file on the hard disk remains unaltered.
This is like saying that any changes made to files left on the desktop are discarded when
the office is closed, although the original files are still preserved in the cabinet.
Memory temporarily stores programs when they are running, along with the data being
usedbythose programs. RAM chips are sometimes termed
volatile storage
because when
you turn off your computer or an electrical outage occurs, whatever is stored in RAM is
lost unless you saved it to your hard drive. Because of the volatile nature of RAM, many
computerusersmakeitahabittosavetheirworkfrequently—ahabitIrecommend.Many
software applications perform periodic saves automatically to minimize the potential for
data loss.
Physically, the
main memory
in a system is a collection of chips or modules containing
chips that are usually plugged into the motherboard. These chips or modules vary in their
electrical and physical designs and must be compatible with the system into which they
are being installed to function properly. This chapter discusses the various types of chips
and modules that can be installed in different systems.
To better understand physical memory in a system, you should understand what types of
memory are found in a typical PC and what the role of each type is. Three main types
of physical memory are used in modern PCs. (Remember, I'm talking about the type of
memory chip, not the type of module that memory is stored on.)
•
ROM
—Read-only memory
•
DRAM
—Dynamic random access memory
•
SRAM
—Static RAM
TheonlytypeofmemoryyounormallyneedtopurchaseandinstallinasystemisDRAM.
The other types are built in to the motherboard (ROM), processor (SRAM), and other
components such as the video card, hard drives, and so on.
ROM
Read-only memory, or ROM, is a type of memory that can permanently or semiperman-
ently store data. It is called read-only because it is either impossible or difficult to write
to. ROM also is often referred to as nonvolatile memory because any data stored in ROM
remains there, even if the power is turned off. As such, ROM is an ideal place to put the
PC's startup instructions—that is, the software that boots the system.
Note that ROM and RAM are not opposites, as some people seem to believe. Both are
simply types of memory. In fact, ROM technically could be classified as a subset of the
system's RAM. In other words, a portion of the system's random access memory address
space is mapped into one or more ROM chips. This is necessary to contain the software
