Java Reference
In-Depth Information
Logical unit
Description
Arithmetic
and logic unit
(ALU)
This “manufacturing” section performs
calculations
, such as addition, sub-
traction, multiplication and division. It also contains the
decision
mecha-
nisms that allow the computer, for example, to compare two items from the
memory unit to determine whether they're equal. In today's systems, the
ALU is implemented as part of the next logical unit, the CPU.
Central
processing
unit (CPU)
This “administrative” section coordinates and supervises the operation of
the other sections. The CPU tells the input unit when information should
be read into the memory unit, tells the ALU when information from the
memory unit should be used in calculations and tells the output unit when
to send information from the memory unit to certain output devices. Many
of today's computers have multiple CPUs and, hence, can perform many
operations simultaneously. A
multi-core processor
implements multiple
processors on a single integrated-circuit chip—a
dual-core processor
has two
CPUs and a
quad-core processor
has four CPUs. Today's desktop computers
have processors that can execute billions of instructions per second.
Secondary
storage unit
This is the long-term, high-capacity “warehousing” section. Programs or
data not actively being used by the other units normally are placed on sec-
ondary storage devices (e.g., your
hard drive
) until they're again needed,
possibly hours, days, months or even years later. Information on secondary
storage devices is
persistent
—it's preserved even when the computer's power
is turned off. Secondary storage information takes much longer to access
than information in primary memory, but its cost per unit is much less.
Examples of secondary storage devices include hard drives, DVD drives and
USB flash drives, some of which can hold over 2 TB (TB stands for tera-
bytes; a terabyte is approximately one trillion bytes). Typical hard drives on
desktop and notebook computers hold up to 2 TB, and some desktop hard
drives can hold up to 4 TB.
Fig. 1.2
|
Logical units of a computer. (Part 2 of 2.)
Data items processed by computers form a
data hierarchy
that becomes larger and more
complex in structure as we progress from the simplest data items (called “bits”) to richer
ones, such as characters and fields. Figure 1.3 illustrates a portion of the data hierarchy.
Bits
The smallest data item in a computer can assume the value
0
or the value
1
. It's called a
bit
(short for “binary digit”—a digit that can assume one of
two
values). Remarkably, the
impressive functions performed by computers involve only the simplest manipulations of
0
s and
1
s—
examining a bit's value
,
setting a bit's value
and
reversing a bit's value
(from
1
to
0
or from
0
to
1
).
Characters
It's tedious for people to work with data in the low-level form of bits. Instead, they prefer
to work with
decimal digits
(0-9),
letters
(A-Z and a-z), and
special symbols
(e.g., $, @, %,
