R.H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
COMPUTER MEMORY.
PRIMARY AND SECONDARY STORAGE DEVICES
LESSON NOTES
Lesson/Learning Objectives
At the end of this lesson, students will be able to:
1. Meet the expectations of the Course Curriculum coded as: TFV.01E, TF2.01E, TF2.02E, TF2.03E,
TF2.04E, IC1.02E, SPV.01E;
2. Explain the function and interaction of the primary (RAM, ROM) and secondary storage devices
(magnetic and optical);
3. Describe secondary storage devices: FDD, HD, CD-ROM, DVD;
4. Use precise terminology in relation to all storage devices;
5. Explain how data is stored on the surface of magnetic and optical disks;
6. Uninstall and re-install main and secondary storage devices: RAM, FDD, HD, CD-ROM;
7. Use safe practices in the handling of computer hardware and electronic components.
Introduction
 Computer memory can be divided into two broad categories:
 Primary storage (also called main memory), and
 Secondary storage.
 Primary storage is closely connected to the CPU and is used for temporary storage of data and
instructions during processing.
 Secondary storage is relatively permanent but ``further away'' from the CPU.
 Primary storage is fast; secondary storage is slow (relatively).
 Primary storage is volatile (i.e. contents are lost when power is removed); secondary storage is
non-volatile.
 Primary storage is based on electronic technology; secondary storage is usually based on magnetic
(or optical) and mechanical technologies.
Types of Primary Storage
 RAM (``Random Access Memory'').
Called ``RAM'' because any location in the memory can be directly accessed in the same amount of
time as any other location.
 ROM (``Read Only Memory'').
Is what it says: contains programs and/or data ``burned in'' at the time of manufacture.
Commonly used to contain programs to ``boot up'' the computer when it is switched on.
 PROM (``Programmable Read Only Memory'').
ROM which is ``blank'' and into which programs and/or data can be written, but only once.
 EPROM (``Erasable Programmable Read Only Memory'').
PROM which can be erased, usually by exposure to UV light through a transparent window above
the chip.
1
R.H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
Secondary Storage
 Secondary storage for computers is non-volatile (i.e. does not require electrical power to retain its
contents.
 The basic types of secondary storage are:
- magnetic tape
- magnetic disk (``hard'' and ``floppy'')
- optical disk (CD-ROM, WORM, MO).
Magnetic Tape
 This is oldest, and originally the only, form of secondary storage.
 It is still used, mostly for backup or archival storage, and for batch processing.
 Data is stored on tracks on the tape - most tapes have 9 tracks.
 There are several varieties of magnetic tape:
- 14" reels of 1/2" wide tape (2400' long)
- Cartridges of 1/4" tape
- Cassettes (very similar to audio cassettes)
- Digital Audio Tape (DAT).

Bytes of data (i.e. 8 bits) are stored across the width of the tape.

The 9th bit is a parity bit. It helps to detect storage errors. Two types of parity can be used.

With even parity, the parity bit is set so that the group of 9 bits has an even number of `1' bits.

With odd parity, the parity bit is set so that the group of 9 bits has an odd number of `1' bits.

Data is stored at different densities (e.g. 1600 and 6250 bpi, where ``bpi'' means ``bytes per inch'').
Magnetic tape has the following advantages:
2
R.H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE

It is very inexpensive

It can store large amounts of data

It is relatively robust

And the following disadvantages: 1. It only allows access to data sequentially and is therefore very
slow compared to other secondary storage technologies; 2. May need manual intervention to
mount/dismount.
Magnetic Disks
This is the most common form of secondary storage used today. Magnetic disks come in two forms:
 hard disks
 floppy disks
Hard disks are (usually but not always) permanently mounted inside the computer. All magnetic disks
have a magnetizable iron oxide coating and read/write heads that can move over the surface of the
disk, which is spinning underneath it . The disk head works in a similar way to the head in an audio tape
recorder. The read/write head in a floppy disk drive actually contacts the magnetic disk, but that in a
hard disk ``flies'' a few thousandths of an inch above it.
3
R.H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
Magnetic Hard Disks
 Data on the disk surface is recorded on tracks (magnetic, not physical grooves!) which form
concentric circles on the disk.
 Each disk has a set number of tracks. Depending on type of disk drive, there may be from 100 to
over 10,000 tracks on the disk.
 Tracks with the same number on each disk (recording) surface form a cylinder.
 The disks spin at a fixed speed, typically at 3600 rpm (IDE) or 7500 -15,000 rpm (SCSI).
 Tracks on a disk are organized into sectors.
 To get to a particular piece of data on the disk a track number and a sector number are needed.
 Data is read/written when the required sector on the track rotates into position under the read/write
head (note that data can be accessed from any track of the cylinder).
 The time taken to position the head over the correct track/cylinder is called the seek time.
 The time needed for the sector to arrive (once the head is in position over the correct track/cylinder)
is called the latency time.
 Creating the magnetic tracks on a previously blank disk is called formatting the disk.
 Formatting destroys any data that might have been on the disk previously.
 Typical data access times for modern hard disks (i.e. latency + seek) are about 10-15 milliseconds
 Typical storage capacities today are:
o Microcomputer: 20 Mb - 240 Gb
o Mainframe: 240 - 800 Gb
 Magnetic hard disks are, in a mainframe environment, often referred to as DASD's - ``Direct Access
Storage Devices''.
Magnetic Floppy Disks
 Current size is 3.5" diameter (older floppy disks were 5.25" and 8" in diameter).
 Organized into tracks and sectors, like their hard disk cousins.
 Typically rotate at about 400 rpm,
and only when access to them is
needed (unlike hard disks which
rotate all the time the computer is
on).
 Like magnetic hard disks, floppy
disks must also be formatted
before they can be used.
 Typical storage capacity: 1.4 Mb
 Typical data access time: 200-400
msec.
Optical Disks

Optical disks are becoming more common. They come in several varieties:

CD-ROM (Compact Disk Read Only Memory)

CD-R (Compact Disk Recordable)
4
R.H. KING ACADEMY
INFORMATION TECHNOLOGY

CD-RW (compact Disk Re-Writable)

DVD (Digital Versatile/Video Disk)

DVD-R (Digital Versatile Disk Recodable)

DVD-RW (Digital Versatile Disk Re-Writable)

MO (magneto-optical).
TEE2O COURSE
CD-ROM

Stands for ``Compact Disk - Read Only Memory''.

Comes with data already stored on it - like a music compact disk.

Useful for distributing information that should not be changed (e.g. encyclopaedias) and programs
(e.g. new versions of software) and, inevitably, games.

Typical storage capacity: 650 Mb.

New technology, known as DVD, will store up to 4G per disk. DVD disk drives are required to read
these disks.

Typical data access time: 30-50 msec, although this is steadily improving and should shortly be
comparable with magnetic hard disks.

Easy to handle, and relatively robust..
WORM

Stands for ``Write Once, Read Many''.

Data can be written onto the initially blank optical disk.

Once written, the data cannot be erased.

Useful for storing data that needs to be permanently retained.

Similar in other respects to CD-ROM disks.
Magneto-Optical Disks (CD-Rewriteable)

These are the optical version of magnetic hard disks - that is, data can be written to them and
erased from them.

Similar in storage capacity and access time to the other types of optical disks.

Data is written by heating a spot on the disk with a laser beam, in the presence of a magnetic field,
the direction of which determines whether a `0' or `1' is written.

Data is read by shining a lower power laser beam onto the disk spot (without the magnetic field)
and detecting the polarization of the light that is reflected.
5
R.H. KING ACADEMY
INFORMATION TECHNOLOGY
TEE2O COURSE
Lesson Review
•
List four types of magnetic and four types of optical storage devices.
•
Identify three common uses for floppy disks.
•
Name the four areas created on a magnetic disk during formatting.
•
Explain how data is stored on the surface of magnetic and optical disks.
•
List three variations on optical disk technology.
Evaluation – Short Test
1. Define the following acronyms:
RAM
___________________________
EPROM _____________________________
ROM
___________________________
EEPROM____________________________
SRAM
___________________________
CD-ROM ____________________________
DRAM
___________________________
FDD ________________________________
SDRAM
___________________________
HD _________________________________
EDRAM
___________________________
DVD ________________________________
PROM
___________________________
CD-RW _____________________________
2. Cache memory has the fastest speed:
T
F
3. A TB has 1024 MB?
T
F
4. EPROM is a volatile memory?
T
F
5. SRAM needs to be refreshed?
T
F
6. EEPROM use ultraviolet light to erase data? T
F
7. A land reflects light back to its source, representing a bit value of 1 (on).
T
F
8. A pit, does not reflect the light back and this represents a bit value of 0 (off).
T
F
Sources:
http://pcwebopedia.com
http://www.dis.unimelb.edu.au/staff/tanya/hwtute/Peripheral_devices/storage.htm
http://courses.wccnet.org/computer/mod/g05c.htm
http://en.wikipedia.org/wiki/Computer_memory
http://ioc.unesco.org/oceanteacher/resourcekit/Module2/Computers/Hardware/storage.html
http://www.ib-computing.com/primsecmem.htm
6