(^130) PC Hardware: A Beginner’s Guide
I
n spite of all appearances to the contrary, computers, including personal computers,
cannot think and cannot remember. This may seem contradictory when you consider
that a computer’s memory is one of its most important components. Everything a
computer does and all of the data it processes are stored in its memory before and after
they are passed to the CPU.
A PC’s memory is made up of electronic components in which the PC temporarily
stores data and instructions. Technically, any device that stores data or instructions on
the PC can be called memory, including the hard disk, floppy disks, ROM, CMOS, RAM,
and cache. However, what is commonly referred to asmemoryon the PC is its primary
storage, which is also known as system memory, temporary storage, or RAM. With the
exception of ROM, which is discussed in this chapter and in Chapter 6, the other forms of
storage (hard disk, floppy disk, CD-ROM, and the like) are known assecondary storage
(see Chapter 9 for more information on secondary storage devices).
A Brief Overview of ROM
In order to change data stored on the computer, you must be able to write to it. If you cannot
write to a memory, you cannot change it. It’s logical then that data stored onread-onlymemory
(ROM)cannot be changed, as its name implies. ROM also has the added feature of beingnon-
volatile, which means that it can kee pits contents even without a power source. This makes it
ideal for storing the PC’s startup instructions and system BIOS (Basic Input/Output System)
(see Chapter 4). Figure 7-1 shows a ROM chip. While virtually all ROM chips are packaged in
a DIP (dual inline packaging) form, there are three types of ROM used in a PC: PROM (programmable read-only memory) This type of ROM chip is
programmed using a special type of programming device called aPROM
burner(a.k.a. PROM programmer), which permanently stores machine
language (binary instructions) code on the PROM chip. A PROM chip
is also referred to asOTP(One Time Programmable)memory.
EPROM (erasable programmable read-only memory) This type of ROM,
pronounced “e-prom,” is erasable and can be reprogrammed. Unlike a PROM
chi pthat cannot be reused and can only be thrown out when it becomes obsolete,
an EPROM chip can be reprogrammed and reused. As shown in Figure 7-2, an
EPROM has a quartz window on the face of the chi pthat ex poses the chi p’s
interior circuits. When ultraviolet (UV) light is shined through this window,
it causes a chemical reaction that erases the EPROM. In order to reprogram
an EPROM, it must be removed from the computer, erased with UV light,
and then reprogrammed on a PROM programmer.
EEPROM (electronically erasable programmable read-only memory) Most
newer PCs now include an EEPROM (pronounced “e-e-prom”) that can be re-
programmed like an EPROM, but, unlike the EPROM, doesn’t need to be removed