Chapter 3
Data Storage
Media Storage

Main memory (Electronic Memory):

Secondary Memory
• Stores data currently being used
• Is made of semiconductor chips.
• magnetic (floppy discs, hard disc )
• Optical (CD-ROM, DVD)
Main Memory


Large collection of circuits, each capable of storing a single bit
Arranged in small cells, typically of 8 bits each (a.k.a.: byte)
Arrangement of Memory Cells

Each cell has a unique address

Longer strings stored by using
consecutive cells
value = 01101101

RAM (random access
memory)
Memory cells
n-bit cells
m cells

Can hold
m*n bits
In reality, most electronic memories have 8-bit cells.
Accessing Data in the Main
Memory




Instructions and data are stored in the main memory in a serial
order.
CPU executes instructions one by one top down.
An instruction may tell the CPU
•
•
to jump to particular cell and execute the instruction held in it,
or fetch the data stored is that cell.
How is this done?
System Bus

Main memory and CPU are linked using a set of wire:
• Three wires:
•
•
•
•
address lines,
data lines and
control lines.
Known as
•
•
•
address bus,
data bus and
control bus.
System bus
To read data from each
cell
To issue read or write
signal
To identify each memory
cell
CPU
Main
memory
Add. bus
Data bus
Control bus
Address Bus
Address
Of the cell
To activated
CPU
Main
memory
Address bus
Address
Of the cell
To activated
Binary Address Representation



Each cell has a unique address.
I.e. using 4 digit binary representation we have:
0000 cell 0
0001
cell 1
0010
cell 2
0100
cell 3
How many bits are needed to represent an address?
Address Decoder
Address
Of the cell
To activated
Unique cell
Has a unique
Address.
CPU
Main
memory
Decoder
Address bus
A Simple Address Decoder
2 ad-lines
A1
A0
Q0 00 C0
Q1 01 C1
Q2 10 C2
Q3 11 C3
Decoder is a device between the Main Memory and
the address lines.
4 address
cells
Decoder with N Address Lines
Main Memory
a0
a1
0000…0000
0000…0001
0000…0010
N add.
lines
2N
1111…1111
AN-1
add
cell
Multiplexer




Cells form rows and columns.
Each cell can be identified by a row
address and column address.
Each cells address uses only N/2
address lines.
This can be done using a multiplixed
addresses.
Decoder with 4 Address Lines
(non-multiplexed addresses)
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Decoder with 2 Address Lines
(multiplexed addresses)
00
01
10
11
00
01
10
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
00
Two-Input Multiplexer

A multiplexer is an electronic device that
allows multiple logical signals to be
transmitted simultaneously across a
single physical channel (address line).
Choose the correct answer

A computer’s main memory is linked to
a decoder with 8 address lines. The
maximum number of address that can
be generated is
(a)28
(b) 82
(c)216
Example 1

Suppose computer’s Main Memory is linked to
a decoder with 8 address lines.
1. Can 1000 memory cells be used?
2. If no what is the maximum number of addresses that can
generated?
Answer


Suppose computer’s Main Memory is linked to a decoder
with 8 address lines.
1.
Can 1000 memory cells be used?
2.
If no what is the maximum number of addresses that can generated?
Answer:
1.
2.
NO
With 8 address lines, the maximum number of addresses is
28=256
Example 2


Suppose that a computer’s Main Memory has
1013 cells.
How many address lines are needed in order for
all the cells to be useable? Explain your answer.
Answer

Suppose that a computer’s Main Memory has 1013 cells. How many
address lines are needed in order for all the cells to be useable? Explain
your answer.

Answer:
•
•
•
•
•
With N address lines a computer can have a maximum 2N usable cells. 29 =
512, 210 = 1024.
9 address lines would not generate enough addresses for 1013 cells to be
used. 10 address lines would.
Having more than 10 address lines would lead to too many addresses
wasted. So the desired number of address lines is 10.
N = ⌈log2(1050)⌉ can be used to find the number of address lines.
If multiplexed addresses is used, then 5 address lines would be sufficient
for 1013 cells to be useable.
Address Space

The address space of a computer is the maximum
number of cells a computer can hold.

The address space is determined by the number of
address lines used in a computer.

If each cell in a memory is 8-bit, then the memory is
called byte addressable: 1 byte long has a unique
address
Features of the Main Memory




Memory Capacity.
Access of information
Access time
Transfer rate
Memory Capacity



Most computer’s memory have 8-bit (1-byte)
cells.
In this case we have:
Address
lines
No of cells
Capacity
N
2^N
2^N * 1
32KB, 256MB and 20GB are used to describe
the memory capacity.
Capacity Units
1kB = 210 = 1024 Byte.
1MB =1024 KB = 220 Bytes= 1, 048,576 B.
1GB =1024 MB = 230 kB=1, 073,741,824 Bytes.
Access Time



Access time is taken between the moment
when the CPU wants the read/write from/into
a cell and the moment when the cell is
activated.
It is the moment that the CPU takes to activate
a cell.
60ns (10-9 sec)
Transfer Rate

Is the amount of information per second
exchanged between the CPU and main
memory.


Main memory
• electronic signals
• Implies fast transfer rate in the scale about
100MB/sec
Random Access

If the CPU wants to activate particular
cell.
• It does not search for the target cell from top
•
•
to bottom.
It does put the address of the target cell in
the address line, then the cell will be
activated.
This type of accessing information is called
Random Access
The need for other type of
memories.

Main memory
• Fast as all the exchange between CPU and
•
Main memory is done electronically.
However, it is volatile.
• Information lost when the machine is turned off.
• The need for non-volatile memory:
• Hold information when the machine is off.
• i.e. Magnetic disk, optical disk, magnetic tape
A Magnetic Disk Storage
System
• Each track contains same number of sectors
• Location of tracks and sectors not permanent (formatting)
• Examples: hard disks, floppy disks, ...
Magnetic Disk Terminology





Platter:
•
•
rigid metal or glass platter Coated with magnetic material.
rotating at constant angular velocity
Arm:
•
With movable magnetic read/write heads
Track:
•
•
A complete ring of data
The disk surface is divided into tracks
Sectors:
•
Each track is subdivided into sectors
Cylinder (see slides 71-72):
•
A vertical collection of tracks at the same radial position
Read/write Head

Coil of wire
Iron former



A coil of wire wound onto an iron former.
gap.
If a spot on the magnetic memory passes
under the gap then an electrical current is
induced in the coil. And the read/write head
will know that there is a 1 stored on that
spot. Otherwise it is 0.
By passing an electric current on the wire
we can magnetise and demagnetise spots.
Read and Write Mechanism (2)
1
CPU
01010
Add. bus
Data bus
Control bus
01010
1
1
Maximum data transfer rate

It is the rate at which data passes under
the read/write head (bytes/sec).
• Number of bytes / track
* Number of rev / sec
Multiple Platters (2)
• Disk platters speed (3600 to 10 000 rpm
(rev/min).
•floppy (360rpm).
•The read data we need to specify cylinder,
head, and sector numbers.
Each cylinder represents a track number.
Cylinders
Magnetic Tape (1)





Serial access
Slow
Very cheap
High capacity
Backup
Optical Storage CD-ROM







Originally for audio
650 Mbytes giving over 70 minutes audio
Polycarbonate coated with highly reflective coat, usually
aluminium
Data stored as pits
Read by reflecting laser
Constant packing density (data/surface= constant)
•
•
More data in outer edges
Less data towards the centre of the disc
Constant linear velocity
•
The drive must adjust the disc speed (495 to 212 rev/m) edges
•
•
Faster when reading data closer to the centre
Slower when reading data in outer edges
Optical Storage – CD-ROM




Is a disc with highly reflective surface.
Tiny areas flat and depressed:
• Flat (land)  strong reflection.
• Depressed (pits)  low reflection.
Laser  landstrong reflectionphoto-sensor generates
electrical voltagestore 1s.
• Laser: (light Amplification stimulated emission of
radiation).
Lightpitslow reflection no electrical voltage 
stores 0s.
Summary



Main memory
• RAM
• Low storage capacity
• Fast (electrical signals)
• Volatile.
Magnetic memory
• Floppy disk
• Hard disk
• Magnetic tape
Optical memory
• CD_ROM disk
• DVD