Introduction - Java Software Solutions: Foundations of Program Design - page 17

CDs were initially a popular format for music; they later evolved to be used
as a general computer storage device. Similarly, the DVD format was originally
created for video and is now making headway as a general format for computer
data. DVD once stood for digital video disc or digital versatile disc, but now the
acronym generally stands on its own. A DVD has a tighter format (more bits per
square inch) than a CD and can therefore store much more information. DVD-
ROMs eventually may replace CD-ROMs completely because there is a compat-
ible migration path, meaning that a DVD drive can read a CD-ROM. Similar to
CD-R and CD-RW, there are DVD-R and DVD-RW discs. The drive listed in
Figure 1.8 allows the user to read and write CD-RW discs and read DVD-ROMs,
including the ability to play music CDs and watch DVD videos.
The speed of a CD or DVD is expressed in multiples of x , which represents
a data transfer speed of 153,600 bytes of data per second for a CD; nine times
that speed, or about 1.5 megabytes of data per second, for a DVD; and three
times the speed of a DVD, or 4.5 megabytes of data per second, for a Blu-ray
disc. The drive described in Figure 1.8 has a maximum data access speed of
16 x , or about 72 MB of data per second. A dual-layer Blu-ray disc has a stor-
age capacity of 50 GB.
The capacity of storage devices changes continually as technology improves.
A general rule in the computer industry suggests that storage capacity approxi-
mately doubles every 18 months. However, this progress eventually will slow
down as capacities approach absolute physical limits.
The Central Processing Unit
The central processing unit (CPU) interacts with main memory to perform all
fundamental processing in a computer. The CPU interprets and executes instruc-
tions, one after another, in a continuous cycle. It is made up of three important
components, as shown in Figure 1.13. The control unit coordinates the processing
steps, the registers provide a small amount of storage space in the CPU itself, and
the arithmetic/logic unit performs calculations and makes decisions. The registers
are the smallest, fastest cache in the system.
The control unit coordinates the transfer of data and instructions between
main memory and the registers in the CPU. It also coordinates the execution of
the circuitry in the arithmetic/logic unit to perform operations on data stored in
particular registers.
In most CPUs, some registers are reserved for special purposes. For example,
the instruction register holds the current instruction being executed. The pro-
gram counter is a register that holds the address of the next instruction to be
executed. In addition to these and other special-purpose registers, the CPU also