read-only memory (ROM), programmable read-only
memory (PROM), and erasable programmable read-
only memory (EPROM).
RANDOM-ACCESS MEMORY (RAM).
RAM, also called read/write memory, is like a
chalkboard. You can write notes, read them, and erase
them when you no longer need them. In the computer,
RAM is the working memory. Data can be read
(retrieved) from or written (stored) into RAM just by
giving the computer the address of the RAM location
where the data is stored or is to be stored. When the data
is no longer needed, you can simply write over it. This
allows you to use the storage again for something else.
Core, semiconductor, and bubble storage have random-
access memory (RAM) capabilities.
READ-ONLY MEMORY (ROM). In most
computers, it is useful to have often used instructions,
such as those used to bootstrap (initial system load) the
computer or other specialized programs, permanently
stored inside the computer. The memory that enables us
to do this without the programs and data being lost even
when the computer is powered down is read-only
memory (ROM). Only the computer manufacturer can
install these programs into ROM, and, once installed,
they cannot be changed. Consequently, you cannot put
any of your own data or programs into ROM. Many
complex functions, such as routines to calculate square
root, translators for high-level programming languages,
and operating systems, can be stored into ROM.
Because the instructions are permanently stored, they
are quickly performed with accuracy. Also, your
computer facility can order programs designed for its
needs and have them permanently installed into ROM
by the manufacturer. To describe these permanently
installed programs, the term microprogram, or
firmware, is used.
PROGRAMMABLE READ-ONLY MEMORY
(PROM). Your computer facility can also buy
programmable read-only memory (PROM) already
programmed by the manufacturer or in a blank state.
Using a blank PROM and a device designed to write
(burn) a program into PROM, you can enter any
program into the memory. However, you cannot make
any changes to the program once it has been written into
PROM. But, PROM does provide flexibility not
available with ROM. Of course, you must be sure the
program is error free before it is written into PROM.
ERASABLE PROGRAMMABLE READ-
ONLY MEMORY (EPROM). The erasable
programmable read-only memory (EPROM) was
developed to overcome the drawback of PROM. Your
facility can buy blank EPROMs from the manufacturer,
and you can write programs developed at your
command/activity using a special device. The big
difference with EPROM is that you can erase it if and
when the need arises. The data and programs can be
retrieved many times.
If you want to reprogram
memory, you first erase the EPROM with a burst of
This not only enables you to
reprogram when requirements change, but also, you can
erase and write the program again if a mistake is made
while programming the EPROM. In other words, a
mistake is not fatal, as it is when using PROM. You have
the flexibility to change programs to include
improvements or modifications in the future.
The last type of memory we briefly introduce here
is called secondary storage or auxiliary storage. This is
memory outside the main body of the computer where
we store programs and data for future use. When the
computer is ready to use these programs and data, it
reads them into primary storage. Secondary (auxiliary)
storage media extends the storage capabilities of the
computer. We need secondary storage for two reasons.
First, because the computers working memory
(primary storage) is finite and limited in size, it cannot
always hold all the data we need. Second, in secondary
storage, data and programs do not disappear when
power is turned off as they do when semiconductor
memories are used.
Secondary storage media are
nonvolatile memories. This means the information is
lost only if you or the users intentionally erase it. The
three types of secondary storage we most commonly
use are magnetic tape, magnetic disk/diskette, and
Peripheral devices include all the I/O devices used
with a computer system. When these devices are under
control of the CPU, we say they are online. When they
perform their functions independently, not under direct
control of the CPU, we say they are offline. The
peripheral devices described in the following
paragraphs are the ones we commonly use: magnetic
ink character readers, scanners, bar-code readers, key-
to-online data entry terminals, magnetic tape units,
magnetic disk drive units, floppy disk drive units,