Auxiliary Memory
The main memory construction is costly.
Therefore, it has to be limited in size. The main
memory is used to store only those instructions and data which are
to be used immediately. However, a computer has to store a large amount of
information. The bulk of information is stored in the auxiliary memory.
This is also called backing storage or secondary storage.
They include hard disk, floppy disks, CD-ROM,
USB flash drives,
etc.
When the electricity supply to the computer
is off, all data stored in the primary storage
is destroyed. On the other hand, this is not true for secondary storage.
The data stored in secondary storage can be stored for the desired time.
The
most common auxiliary memory devices used in computer systems are magnetic
disks and tapes. Other components used, but not as frequently, are magnetic
drums, magnetic bubble memory, and optical disks. To understand fully the
physical mechanism of auxiliary memory devices one must have knowledge of
magnetic, electronics, and electromechanical systems.
Although
the physical properties of these storage devices can be quite complex, their
logical properties can be characterized and compared by a few parameters. The
important characteristics of any device are its access mode, access time,
transfer rate, capacity, and cost.
The
average time required to reach a storage location in memory and obtain its
contents is called the access time. In electromechanical devices with moving
parts such as disks and tapes, the access time consists of a seek time required to position the read-write
head to location and a transfer time required to transfer data to or from the
device. Because the seek time is usually much longer than the transfer time,
auxiliary storage is organized in records or blocks. A record is a specified
number of characters or words. Reading or writing is always done on entire
records. The transfer rate is the number of characters or words that the device
can transfer per second, after it has been positioned at the beginning of the
record.
Magnetic
drums and disks are quite similar in operation. Both consist of high-speed
rotating surfaces coated with a magnetic recording medium. The rotating surface
of the drum is a cylinder and that of the disk, a round flat plate. The
recording surface rotates at uniform speed and is not started or stopped during
access operations. Bits are recorded as magnetic spots on the surface as it
passes a stationary mechanism called a write head. Stored bits are detected by
a change in magnetic field produced by a recorded spot on the surface as it
passes through a read head. The amount of surface available for recording in a
disk is greater than in a drum of equal physical size. Therefore, more
information can be stored on a disk than on a drum of comparable size. For this
reason, disks have replaced drums in more recent computers.
Magnetic Disks
A
magnetic disk is a circular plate constructed of metal or plastic coated with
magnetized material. Often both sides of the disk are used and several disks
may be stacked on one spindle with read/write heads available on each surface.
All disks rotate together at high speed and are not stopped or started for
access purposes. Bits are stored in the magnetized surface in spots along
concentric circles called tracks. The tracks are commonly divided into sections
called sectors. In most systems, the minimum quantity of information which can
be transferred in a sector. The subdivision of one disk surface into tracks and
sectors in shown in Figure 12.5.
Some
units use a single read/write head for each disk surface. In this type of unit,
the track address bits are used by a mechanical assembly to move the head into
the specified track position before reading or writing. In other disk systems,
separate read/write heads are provided for each track in each surface. The
address bits can then select a particular track electronically through a
decoder circuit. This type of unit is more expensive and is found only in very
large computer systems.
Permanent
timing tracks are used in disks to synchronize the bits and recognize the
sectors. A disk system is addressed by address bits that specify the disk
number, the disk surface, the sector number and the track within the sector.
After the read/write heads are positioned in the specified track, the system
has to wait until the rotating disk reaches the specified sector under the
read/write head. Information transfer is very fast once the beginning of a
sector has been reached. Disks may have multiple heads and simultaneous
transfer of bits from several tracks at the same time.
A
track in a given sector near the circumference is longer than a track near the
center of the disk. If bits are recorded with equal density, some tracks will
contain more recorded bits than others. To make all the records in a sector of
equal length, some disks use a variable recording density with higher density
on tracks near the center than on tracks near the circumference. This equalizes
the number of bits on all tracks of a given sector.
Disks
that are permanently attached to the unit assembly and cannot be removed by the
occasional user are called hard disks. A disk drive with removable disks is
called a floppy disk. The disks used with a floppy disk drive are small
removable disks made of plastic coated with magnetic recording material. There
are 2 sizes commonly used, with diameters of 5.25 and 3.5 inches. The 3.5-inch
disks are smaller and can store more data than can the 5.25-inch disks. Floppy
disks are extensively used in personal computers as a medium for distributing
software to computer users.
Magnetic Tape
A magnetic tape transport consists of the electrical,
mechanical, and electronic components to provide the parts and control
mechanism for a magnetic-tape unit. The tape itself is a strip of plastic
coated with a magnetic recording medium. Bits are recorded as magnetic spots on
the tape along several tracks. Usually, 7 or 9 bits are recorded simultaneously
to form a character together with a parity bit. Read/write heads are mounted
one in each track so that data can be recorded and read as a sequence of
characters.
Magnetic tape units can be stopped, started to move forward
or in reverse, or can be rewound. However, they cannot be started or stopped
fast enough between individual characters. For this reason, information is
recorded in blocks referred to as records. Gaps of unrecorded tape are inserted
between records where the tape can be stopped. The tape starts moving while in
a gap and attains its constant speed by the time it reaches the next record.
Each record on tape has an identification bit pattern at the beginning and end.
By reading the bit pattern at the beginning, the tape control identifies the
record number. By reading the bit pattern at the end of the record, the control
recognizes the beginning of a gap. A tape unit is addressed by specifying the
record number and the number of characters in the record. Records may be of
fixed or variable length.
What is
auxiliary memory give an example of auxiliary memory?
The major difference between main memory and auxiliary memory is
that main memory is directly accessed by CPU but the auxiliary memory is not
accessed by the CPU directly. For this the data is first transferred to main
memory from auxiliary memory and then from main memory the data is transferred
to the CPU for further processing.
Distinguish between primary memory and auxiliary memory?
The main memory is the central storage unit of the computer system.
It is relatively large and fast memory used
to store the programs and data during the
computer operation. Main memory communicates directly with the
CPU. Main memory includes RAM and ROMs.
Devices that provide backup storage are known as auxiliary memory. Auxiliary memory is relatively
slow in comparison to the main memory. Auxiliary
memory includes flash memory, magnetic disks and floppy. Main memory is located inside the CPU but auxiliary memory is located out the CPU.
Why is auxiliary memory required?
Auxiliary memory is required so that set of instruction given
first stored in temporary memory which can be edited & then can be stored
Secondary memory is actually the permanent memory/the backing storage of the computer. It is of two types :
- sequential access
- direct access
Auxiliary memory is the computer's internal/temporary memory. It is also of two types:
- random access memory (RAM)
- read only memory (ROM)
The difference between secondary memory and auxiliary memory is that:-
both are same i.e. both perform the same operations.
Written by,
GOH HOOI KUAN
B031210043
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