Introduction - Java Software Solutions: Foundations of Program Design - page 16

device, but nothing moves mechanically. The terms direct access and random access
are often used interchangeably. However, information on a tape can be accessed
only after first getting past the intervening data. A tape must be rewound or fast-for-
warded to get to the appropriate position. A tape is therefore considered a sequential
access device. For these reasons, tapes largely have fallen out of use as a computing
storage device, just as audio cassettes have been supplanted by compact discs.
Two other terms are used to describe memory devices:

random access
memory (RAM) and read-only memory (ROM). It's important to understand
these terms because they are used often and their names can be misleading. The
terms RAM and main memory are basically interchangeable, describing the
memory where active programs and data are stored. ROM can refer to chips
on the computer motherboard or to portable storage such as a compact disc.
ROM chips typically store software called BIOS (basic input/output system)
that provide the preliminary instructions needed when the computer is turned
on initially. After information is stored on ROM, generally it is not altered (as
the term read-only implies) during typical computer use. Both RAM and ROM
are direct (or random) access devices.
A CD-ROM is a portable secondary memory device. CD stands
for compact disc. It is called ROM because information is stored
permanently when the CD is created and cannot be changed. Like
its musical CD counterpart, a CD-ROM stores information in binary
format. When the CD is initially created, a microscopic pit is pressed
into the disc to represent a binary 1, and the disc is left smooth to
represent a binary 0. The bits are read by shining a low-intensity laser beam onto
the spinning disc. The laser beam reflects strongly from a smooth area on the
disc but weakly from a pitted area. A sensor receiving the reflection determines
whether each bit is a 1 or a 0 accordingly. A typical CD-ROM's storage capacity
ranges between 650 and 900 MB.
Variations on basic CD technology emerged quickly. Most personal computers
are equipped with a CD-Recordable (CD-R) drive. A CD-R can be used to cre-
ate a CD for music or for general computer storage. Once created, you can use a
CD-R disc in a standard CD player, but you can't change the information on a
CD-R disc once it has been “burned.” Music CDs that you buy are pressed from
a mold, whereas CD-Rs are burned with a laser.
A CD-Rewritable (CD-RW) disc can be erased and reused. It can be reused because
the pits and flat surfaces of a normal CD are simulated on a CD-RW
by coating the surface of the disc with a material that, when heated
to one temperature becomes amorphous (and therefore nonreflective)
and when heated to a different temperature becomes crystalline (and
therefore reflective). The CD-RW media doesn't work in all players, but
CD-RW drives can create both CD-R and CD-RW discs.

KEY CONCEPT
The surface of a CD has both
smooth areas and small pits. A pit
represents a binary 1 and a smooth
area represents a binary 0.

KEY CONCEPT
A rewritable CD simulates the pits
and smooth areas of a regular CD
by using a coating that can be made
amorphous or crystalline as needed.