Chpater 5

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Chpater 5
Main Memory and Backing Storage Devices
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Main memory
The main memory can be thought of as a series of
storage locations.Each location has a unique
address,counting from zero upwards.The address is
called memory address.The main memory is
connected to the CPU by buses.When the CPU wants
to access a particular memory location,it sends
the address to the memory on an address bus.The
data held in that location are returned to the
CPU on the data bus.
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Memory Sizes
The size of the main memory is measured in
kilobytes (KB),which is equal to 1024 bytes. For
convenience,we sometimes round off the value of K
to 1000.For example,640 KB is approximately 640
000 bytes (640 x 1000).To measure memory of large
sizes,we use megabytes (MB) and gigabytes (GB).
1 MB 1024 KB 220 bytes 1 million bytes 1 GB
1024 MB 230 bytes 1 billion bytes
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Semiconductor chips
A semiconductor memory chip is an integrated
circuit (IC) that contains thousands of
transistors.A transistor is an electronic
component that can be either on or off.
A semiconductor chip is very small.This chip
contains 256KB
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Semiconductor chips have several advantages over
previous devices 1. A chip,although small,is
very dense.One chip can therefore store
thousands or even million of bytes. 2. As the
chip is very small,the components are stored very
close together.The amount of time for an
electrical pulse to pass from one component to
another is therefore very small.Memory made from
semiconductor chips is therefore very efficient.
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3. As the chips are so small,the size of the
computer is reduced as well. 4. Chips are cheaper
to manufacture. 5. If the chip has a fault,the
whole chip can be easily replaced.
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Types of memory
There are two main types of semiconductor memory
chips in a computer random access memory (RAM)
and read only memory(ROM).
ROM (top) and RAM (bottom) on a motherboard
(right)
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RAM 1.Data can be read from any word or written
onto any word in the RAM. 2.When a program is
run,the instructions and the working data are
temporarily stored in the RAM. 3.When the
computer is switched off,all the data held in the
RAM are lost.RAM is therefore volatile.
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ROM 1.Data can only be read from the ROM.The
contents cannot be overwritten. 2.ROM is used by
the computer manufacturer to store the systems
programs.These programs control the fundamental
operations. 3.ROM is non-volatile.When the power
is switched off,the contents of the ROM remain
unaffected.
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Backing storage devices
Backing storage devices exist to save programs
and data for future processing.It is not possible
to put all programs and data in to the main
memory for several reasons. 1.Data in the main
memory (except in the ROM)are lost when the
machine is switched off. 2.Main memory is very
expensive. 3.Data for an application program may
be so large that it is physically impossible for
the main memory to hold all the data.
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Backing storage devices provide a more permanent
from of storage than main memory,Backing storage
devices are used to record data and programs onto
various storage media,Commonly used devices are
disk drives and tape drives. Backing storage
devices can be treated as input/output
devices.They function as input devices when the
stored data are transferred to the computer for
processing.They function as output devices when
processed result are written to them.
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In this chapter,we will discuss three storage
mediamagnetic disk,magnetic tape,and optical
disk(CD-ROM). Magnetic diskFloppy disk

Hard disk
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Magnetic tapeReel-to-reel tape
Cartridge tape
Cassette tape
Optical disk
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THE END