Chapter 3
Data Storage
Media Storage
Main memory (Electronic Memory):• Stores data currently being used
• Is made of semiconductor chips.
Secondary Memory • 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
value = 01101101
Each cell has a unique address Longer strings stored by using
consecutive cells
RAM (random access memory)
Memory cells
In reality, most electronic memories have 8-bit cells.
m cells
n-bit cells
Can holdm*n bits
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
CPUMain
memory
Add. bus
Data bus
Control bus
To identify each memory cell
To read data from each cell
To issue read or writesignal
Address Bus
CPUMain
memory
Address bus
Address Of the cellTo activated
Address Of the cellTo activated
Binary Address Representation
Each cell has a unique address. I.e. using 4 digit binary representation we have:
0000 cell 00001 cell 10010 cell 20100 cell 3
How many bits are needed to represent an address?
Address Decoder
CPUMain
memory
Address bus
Address Of the cellTo activated
Unique cellHas a uniqueAddress.
Decoder
A Simple Address Decoder
Q0 00 C0
Q1 01 C1
Q2 10 C2
Q3 11 C3
A1A0
2 ad-lines
4 address cells
Decoder is a device between the Main Memory and the address lines.
Decoder with N Address Lines
Main Memory
0000…00000000…00010000…0010
1111…1111
a0a1
AN-1
N add.lines
2N
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)
0000 0001 0010 0011
0100 0101 0110 0111
1000 1001 1010 1011
1100 1101 1110 1111
00
00
01
10
11
01 10 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. NO
2. 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:
32KB, 256MB and 20GB are used to describe the memory capacity.
Address lines
No of cells Capacity
N 2^N 2^N * 1
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
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.
Coil of wireIron former
Add. bus
Data bus
Control bus
CPU
1
01010
01010
1
1
Read and Write Mechanism (2)
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 landstrong reflectionphoto-sensor generates electrical voltagestore 1s.• Laser: (light Amplification stimulated emission of
radiation). Lightpitslow 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
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