Chapter – 5
Central Processing Unit – CPU
The central processing unit (CPU),
also called the microprocessor, the processor or central processor is the
brains of the computer. The CPU is housed on a tiny silicon chip. This chip
contains millions of switches and pathways that help your computer make
important decisions. The switches control the flow of the electricity as it
travels across the miles of pathways. The CPU knows which switches to turn on
and which to turn off because it receives its instructions from computer
programs. Programs are a set of special instructions written by programmers
that control the activities of the computer. Programs are also known as
software.
The CPU has two primary sections:
- Control Unit
- Arithmetic/Logic Unit
Arithmetic/Logic
Unit
Because all computer data is stored
as numbers, a lot of the processing that takes place involves comparing numbers
or carrying out mathematical operations. In addition to establishing ordered
sequences and changing those sequences the computer can perform only two types
of operations: arithmetic operations and logical operations. Arithmetic
operations include addition, subtraction, multiplication and divisions. Logical
operations include comparisons, such as determining whether one number is equal
to, greater than or less than another number. Also, every logical operation has
an opposite. For example, in addition to “equal to” there is “not equal to.”
Many instructions carried out by the control unit involve simply moving data
from one place to another – from memory to storage, from memory to the printer
and so forth. However, when the control unit encounters an instruction that
involves arithmetic or logic, it passes that instruction to the second
component of the CPU, the arithmetic or logic unit, or ALU. The ALU includes a
group of registers – high-speed memory locations built directly into the CPU
that are used to hold the data currently being processed. For example, the
control unit might load two numbers from memory into the registers in the ALU.
Then, it might tell the ALU to divide the two numbers (an arithmetic operation)
or to see whether the numbers are equal (a logical operation).
Secondary Storage
Secondary storage or secondary
memory also referred as backing storage is used to supplement the capacity of
main storage; the memory stores a bulk of information. It is also called
auxiliary storage or mass storage. The information stored in this memory is
used by the CPU by first bringing it to main memory.
Examples
of Secondary Storage Devices
Some Examples of secondary storage
devices are as follows:
1. Magnetic Disks
2. Magnetic Tape
3. Mass Cartridge Systems
Types of
Secondary Storage/Backing Storage Access Methods
There are two types of secondary
storage/backing storage access methods:
Sequential Access Storage
Access through sequential access storage is non-addressable that is why an
operator cannot refer directly to the contents of a particular storage location
such as used with magnetic tape. This involves examining sequentially all
beginning of the tape and continuing to search through all record until the
desired information area is found.
Direct
Access Storage
Direct Access Storage is addressable
that is a given item can be selected from anywhere in storage by simply specifying
the address where it is located direct access devices such as magnetic disks
provide immediate access to individual records and do not require reading from
the beginning of a file to find a particular record.
Computer Bus
In computer the term bus refers that
paths between the components, actually bus, is a group of a wire. The bus is
the common path way through which the processors send/receive data and commands
to/from primary and secondary storage and all can transport 8 bits at a time
whereas a bus with 16 lines can transport 16 bits at a time.
Types of
Computer Bus
There are two main buses in a
computer:
1. Data Bus
2. Address Bus
1. Data
Bus
The data bus is an electrical path
that connects the central processing unit (CPU) memory, and the other hardware
devices on the motherboard. Actually, the bus is a group of parallel wires. The
number of wires in the bus affects, the speed at which data can travel between
components, just as the number of lanes on a highway effects how long it takes
people to get to their destinations. Because each wire can transfer one bit at
a time, an eight-wire bus can move eight bits at a time. A 16-bit bus can
transfer two bytes and a 32-bit bus can transfer four bytes at a time.
2.
Address Bus
The second bus that is found in
every micro computer is the address bus. The address bus is a set of wires
similar to the data bus, but is connects only the CPU and memory and all it
carries are memory addresses. The reason that the address bus is important is
that the number of lines in it determines the maximum number of memory address.
For example, one byte of data is enough to represent 2 = 256 different values,
if the address bus could carry only eight bits at a time, the CPU could address
only 256 bytes of memory. Most of the early PCs had 20-bit addres bus, so the
CPU could address only 2(20) bytes or 1 MB of data.
Computer Ports
A system board with its processor
and memory unit can work only when linked to input/output storage and
communication devices to receive data and communicate results of processing.
Peripheral devices such as a keyboard, mouse, monitor and a printer come with a
cable and a multiple connector. To link a device to the PC, you plug its
connector into a receptacle called a port in much the same way you plug a lamp
cord into electrical outlet. A port is one of the entry lines coming into the
computer. A port provides a direct link to the microcomputer’s common
electrical bus.
Types of
Computer Ports
There are two types of ports used in
computer:
1. Serial Ports
2. Parallel Ports
1. Serial
Ports
A serial port provides a connection
for transmitting data one bit at a time. A serial port connects your computer
to a device such as modem, which requires two-way data transmission, or to a
device such as a mouse, which requires only one-way data transmission.
IBM-compatible computers use either 9-pin or 25-pin connectors for their serial
ports COM1, COM2 etc.
2.
Parallel Ports
A parallel port provides a
connection for transmitting data eight bits at a time over a cable with eight
separate dta lines. Parallel transmission is fast because eight bits travel
simultaneously. Parallel transmission is typically used to send data to the
printer. The cable that connects two parallel ports contains 25 wires, eight
wires carry data and the remaining wires carry control signals that help to
maintain orderly transmission and reception. IBM-compatible computes generally
allow you to use up to three ports which are designated as LPT1, LPT2 etc.
Memory Unit
Memory unit is the place where the
computer program and data are stored during processing. It is the area, through
which all the data which is input into or output of the CPU must pass. It is
monitored by OU which keeps track of every thing in the storage. It is a random
access device, which consists of thousands upon thousands of storage locations,
each of which can be directly reached by the CU. Each storage location is
distinguished by the address.
Types of
Memory Unit
It is divided into two parts:
1. Read Only Memory (ROM)
2. Random Access Memory (RAM)
1. Read
Only Memory (ROM)
This part of memory contains
permanently stored information. When the power is switched off. ROM does not
wash away. This information is available to a computer to read and process but
not to be changed is kept on ROM.
2. Random
Access Memory (RAM)
This part of memory consists of
blank chips and hence the computer can use it to store and retrieve (write and
read) information during its processing. The information stored in RAM is
volatile, that is, when the computer is shut down the stored information is
lost.
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