Lecture 10: secondary storage

1
Lecture 10 secondary storage

• In this lecture we will cover
• variety of technologies that can be used to
  provide secondary storage of data and programs
  for a computer
• magnetic storage disks - floppy and hard disk
• disk caches
• optical storage CD-ROMs, CD-Rs, CD-RW, DVD, etc

2
Disk Controller Card

• IDE (intelligent drive electronics) - old
  standard - limited to 528MB size
• EIDE (Enhanced IDE) - IDE without the limits
• SCSI (small computer systems interface) - can
  attach variety of different devices - not just
  disks

3
The disk
Magnetic Oxide Coating
4
Bit encoding in magnetic media

• Bits are encoded onto surface of disk by creating
  magnetic domains (magnetised areas) in the
  magnetic coating. Each magnetic domain has a
  specific polarity (orientation of magnetic
  north-south).
• 2 domains next to each other encode 1 bit
• if domains have the same polarity then the bit is
  0
• if domains have opposite polarity then bit is 1

5
Bit encoding
0
1
0
1

• Polarity of first domain of bit is always reverse
  of previous domain so that start of new bit is
  indicated
• So it is alignment of 2 domains - same for 0,
  different for 1 - that encodes bit NOT the
  polarity itself.
• Density number of bits that can be encoded on
  the magnetic surface per square inch

6
PC Floppy disks

• 3.5
• Double sided, Double Density 720K
• Double Sided, High Density 1.44Mb
• Features
• Enclosed when not in use
• Rotates only when being read from/written to
• Cheap
• Write protectable

7
The disk
Heads move in and out (linear movement)
Rotation Floppy disks 300rpm
8
Hard disks

• Typical disk set up in 1984
• 1 floppy disk(160k) hard disk (10Mb)
• low capacity
• slow - access time for hard disk 40 ms
• Now - typical set up
• 1 floppy disk(1.44Mb)
• CD ROM(650Mb) or DVD (8.5 Gbyte)
• 100 Gbyte Hard disk - access time lt6 ms

9

• Fixed sealed storage.
• Drives have number of individual disks.
• Stacked on common spindle.
• Each disk has two Read / Write heads.
• Heads are not in contact with disk surface
• Landing areas provided (parking).
• Head flies above surface.
• Higher density than floppies more bits per
  square inch

10
The hard disk
Heads move across tracks by arm rotation (angular
movement)
Rotation Hard disks 5400-7200rpm
11
Basic Disk Organisation
Typical numbers Hard disk 3,711 3.5 Floppy
disks 80
Tracks

• Note that they are concentric - i.e. NOT a spiral

12
Structure of the disk
Typical numbers Hard disk 146 Floppy disk 18
BLOCK data held in a single sector -
typically 512 bytes per block
Sectors
Magnetic domains

• cluster is a sequence of sectors and is smallest
  unit for operating system to access data

13
Cylinders

• A pile of tracks in the same position on each
  surface is called a cylinder

There are 10 tracks in each cylinder on this disk
14
Head size
15
Data capacity of floppy disk

• 512 bytes per block (sector), 18 sectors per
  track, 80 tracks per surface, 2 surfaces (so 2
  tracks per cylinder)
• So
• bytes per sector sectors per track bytes per
  track
• bytes per track tracks per surface bytes per
  surface
• bytes per surface number of surfaces bytes
  per disk
• Thus
• capacity of disk (bytes per sector) (sectors
  per track) (tracks per surface) (number of
  surfaces)
• 512 bytes 18 80 2 9k 80 2 1440
  Kbytes
• this is called 1.44 Mbytes by convention but
  technically wrong 1440 K actually 1.40625 Mbytes

16
Whats on a disk?
0
1
boot sector contains details about the disk If it
is a boot disk contains small program that loads
system files into computer memory
File allocation table holds information
about sequence of clusters that make up a file 2
copies
17
Disk cache

• Same principle as processor cache - a faster
  local copy
• a copy of part of disk (slower access) is held in
  main memory(faster access)

I WANT track 47 sector 3 head 6 please
Disk Controller Card
Microprocessor
SLOW
Disk Cache
18
(No Transcript)
19
CD-ROM

• Compact Disk Read Only Memory
• Size
• 650 Mbytes
• Oxford English Dictionary
• 74 minutes of video (MPEG COMPRESSED)
• slow
• minimum standard 300Kbytes per second 2x speed
  drives, but 52x speed drives these days (7800kps
  approx 7.7MBps)

20
CD-ROM - structure

• A single spiral track
• Disk rotates at constant linear velocity
• i.e. changes speed depending on position of read
  head
• Faster in centre, slower at edges
• Table of contents in centre of disk
• Handling area on outside
• Clamping area on inside

21
CD-ROM
Reading area - single spiral track
Clamping area
Handling area
Lead in area containing TOC
22
CD-ROM
laser head moves to position
focussed laser beam shone at track
Disk rotates at a speed depending on position
of the laser - so that the laser is always
looking at a surface moving at a constant linear
velocity
23

• Pits are small depressions in reflective layer -
  represents 0
• Lands are part of surface of reflective layer -
  represents a 1

24
CD-R

• CD-Recordable - can record once only using
  special equipment

Read laser sees reflecting gold or non reflecting
dye
Write laser bleaches dye and allows light to
reflect off gold
Dye Layer
Gold Layer
Polymer Base
25
DVD

• Digital Versatile Disk
• similar operation as CD ROM - laser reflecting
  from pits and lands held in a spiral
• difference - holds more data - approx. 8.5 GB
  because
• size of pits and lands much smaller
• in DVD pits and lands implemented using 2 layers
  of coating so there are 2 sets of pits and lands
  with one set of pits and lands lying under
  another set, the focal length of the lens is
  adjusted to read from the different sets of pits
  and lands
• 2 spiral tracks - one goes from outside to centre
  of disk and the other from centre to outside

26
CD-RW, DVD-RW

• CD-RW and DVD-RW use a crystaline surface which
  reflects light when unheated
• Data is encoded by reflective or non-reflective
  areas as normal
• Data is written with laser at highest power.
• Melts crystal, when crystal cools - it is no
  longer reflective - it defuses light
• Data is erased with laser at mid power.
• Warms crystal to less than melting point.
• Crystal settles back into highly reflective form