Senin, 22 September 2014
EXPLAIN FUNCTIONAL
UNIT OF A COMPUTER.
A computer consists of five functionally
independent main parts: input, memory, arithmetic and logic, output, and
control units, as shown in Figure 1.1. The input unit accepts coded information
from human operators, from electromechanical devices such as key boards, or
from other computers over digital communication lines. The information received
is either stored in the computer's memory for later reference or immediately
used by the arithmetic and logic circuitry to perform the desired operations. The
processing steps are determined by a program stored in the memory. Finally, the
results are sent back to the outside world through the output unit. All of
these actions are coordinated by the control unit. Figure 1.1 does not show the
connections among the functional units. The arithmetic and logic circuits,
in conjunction with the main control circuits, as the processor, input and
output equipment is often collectively referred to as the input-output (I/O) unit.
Input Unit
Computers accept coded information
through input units, which read data. The well known input device is the
keyboard. Whenever a key is pressed, the corresponding letter or digit is
automatically translated into its corresponding binary code and transmitted over
a cable to either the memory or the processor. Many other kinds of input
devices are available, including joysticks, trackballs, and mouse, which can be used
as pointing device.
Memory Unit
The function of the memory unit is
to store programs and data. There are two classes of storage, called primary
and secondary. Primary storage is a fast memory that operates at electronic
speeds. Programs must be stored in the memory while they are being executed.
The memory contains a large number of semi conductor storage cells, each
capable of storing one bit of information. These cells are rarely read or
written as individual cells but instead are processed in groups of fixed size
called words. The memory is organized so that the contents of one word,
containing n bits, can be stored or retrieved in one basic operation.
To provide easy access to any
word in the memory, a distinct address is associated with each word location.
Addresses are numbers that identify successive locations. A given word is
accessed by specifying its address and issuing a control command that
starts the storage or retrieval process. The number of bits in each word
is often referred to as the word length of the computer .Typical word lengths
range from 16 to 64 bits. The capacity of the memory is one factor that characterizes
the size of a computer.
Programs must reside in the memory
during execution. Instructions and data can be written into the memory or read
out under the controller of the processor. It is essential to be able to access
any word location in the memory as quickly as possible. Memory in which any
location can be reached in a short and fixed amount of time after specifying
its address is called random-access Memory (RAM). The time required to access
one word is called the Memory access time. This time is fixed, independent of
the location of the word being accessed. It typically ranges from a few
nanoseconds (ns) to about 100 ns for modem RAM units. The memory of a computer
is normally implemented as a Memory hierarchy of three or four levels of
semiconductor RAM units with different speeds and sizes. The small, fast,
RAM units are called caches. They are tightly coupled with the processor and
are often contained on the same integrated circuit chip to achieve high
performance. The largest and slowest unit is referred to as the main
Memory.
Although primary storage is
essential, it tends to be expensive. Thus additional, cheaper, secondary storage
is used when large amounts of data and many programs have to be stored, particularly
for information that is access infrequently. A wide selection of secondary
storage devices is available, including magnetic disks and tapes and optical
disks
Arithmetic and Logic Unit.
Most computer operations are
executed in the arithmetic and logic unit (ALU) of the processor. Consider a
typical example: Suppose two numbers located in the memory are to be added.
They are brought into the processor, and the actual addition is carried out by
the ALU. The sum may then be stored in the memory or retained in the processor
for immediate use.
Any other arithmetic or logic
operation, for example, multiplication, division, or comparison of numbers,
is initiated by bringing the required operands into the processor, where the operation
is performed by the ALU. When operands are brought into the processor, they are
stored in high-speed storage elements called registers. Each register can store
one word of data. Access times to registers are somewhat faster than access times
to the fastest cache unit in the memory hierarchy.
The control and the arithmetic and
logic units are many times faster than other devices connected to a computer
system. This enables a single processor to control a number of external devices
such as keyboards, displays, magnetic and optical disks, sensors, and
mechanical controllers.
Output Unit
The output unit is the
counterpart of the input unit. Its function is to send processed
results to the outside world. The most familiar example of such a device is a
printer. Printers employ mechanical impact heads, inkjet streams, or photocopying
techniques, as in laser printers, to perform the printing. It is possible to produce
printers capable of printing as many as 10,000 lines per minute. This is a
tremendous speed for a mechanical device but is still very slow compared to the
electronic speed of a processor unit.
Monitors, Speakers, Headphones and
projectors are also some of the output devices. Some units, such as graphic
displays, provide both an output function and an input function. The dual role
of input and output of such units are referred with single name as I/O
unit in many cases. Speakers, Headphones and projectors are some of the output
devices. Storage devices such as hard disk, floppy disk, flash drives are also
used for input as well as output.
Control Unit
The
memory, arithmetic and logic, and input and output 'units store and process
information and perform input and output operations. The operation of these
units must be coordinated in some way. This is the task of the control unit.
The control unit is effectively the nerve center that sends control signals to
other units and senses their states.
I/O
transfers, consisting of input and output operations, controlled by the
instructions of I/O programs that identify the devices involved and the information
to be transferred. However, the actual timing signals that govern the
transfers are generated by the control circuits. Timing signals are
signals that determine when a given action is to take place. Data transfers
between the processor and the memory are also controlled by the
control unit through timing signals. It is reasonable to think of a
control unit as a well-defined, physically separate unit that interacts
with other parts of the machine. In practice, however, this is seldom the case.
Much of the control circuitry is physically distributed throughout the machine.
A large set of control lines (wires) carries the signals used for timing and
synchronization of events in all units.
The operation of a computer can be summarized as
follows:
1. The computer
accepts information in the form of programs and data through an input
unit and stores it in the memory.
2. Information
stored in the memory is fetched, under program control, into an
arithmetic and logic unit, where it is processed.
3. Processed
information leaves the computer through an output unit.
4. All
activities inside the machine are directed by the control unit.
