Computer Peripherals Part I

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Computer Peripherals Part I
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Plan

• Storage (hierarchy and terminology)
• Magnetic disks

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Storage

• Terminology
• Medium
• The technology or product type that holds the
  data
• Access time
• The time to get to the data
• Specified as an average in seconds (e.g., s, ms,
  µs, ns, etc.)
• Throughput
• The rate of transfer for consecutive bytes of
  data
• Specified in bytes/s (e.g., Kbytes/s, Mbytes/s)

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Storage Hierarchy
You should know this hierarchy
Medium CPU registers Cache memory Conventional
memory Expanded memory Hard disk Floppy
disk CD-ROM Tape
Access Time - 15-30 ns 50-100 ns 75-500 ns 10-50
ms 95 ms 100-600 ms 0.5 s
Throughput - - - - 600-6000 Kbytes/s 100-200
Kbytes/s 150-1000 Kbytes/s 5-20 Kbytes/s
(cartridge) 200-3000 Kbytes/s (reel-to-reel)
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Terminology

• Online storage
• Memory that is accessible to programs without
  human intervention
• Primary storage and secondary storage are
  online
• Primary storage
• Semiconductor technology (e.g., RAM)
• Volatile (contents might be lost when powered off
  )
• Secondary storage
• Magnetic technology (e.g., disk drives)
• Non-volatile (contents are retained in the
  absence of power)

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Terminology

• Offline storage
• Memory that requires human intervention in order
  for it to be accessed by a program (e.g., loading
  a tape)
• Sometimes called archival storage
• Direct Access Storage Device (DASD)
• Pronounced dazz-dee
• Term coined by IBM
• Distinguishes disks (disk head moves directly
  to the data) from tapes (tape reel must wind
  forward or backward to the data sequential
  access)

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Plan

• Storage (hierarchy and terminology)
• Magnetic disks

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Magnetic Disks

• A magnetic substance is coated on a round surface
• The magnetic substance can be polarized in one of
  two directions with an electromagnet (writing
  data)
• The electromagnet can also sense the direction of
  magnetic polarization (reading data)
• Similar to a read/write head on a tape recorder
  (except the information is digital rather than
  analogue)

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Floppy Disks

• Also called flexible disks or diskettes
• The platter is floppy, or flexible (e.g.,
  mylar) (typical 5.25, 3.5)
• Most floppy disk drives can hold one diskette
  (two surfaces)
• The diskette is removable
• Typical rpm 300, 360
• Capacities 180 KB to 1.4 MB ( up to 100 MB
  zip disks, more.)

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Floppy Disk Example
Shutter
Access window
Cutawayshowing disk
Spindle
Case
Writeprotect tab
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Hard Disks

• The platter is hard (e.g., aluminum)
• Most hard disk drives contain more than one
  platter
• On most hard disk drives, the disks are fixed
  (i.e., not removable)
• On some hard disk drives, the disks are in a
  removable pack (hence, disk pack)
• Typical speed of rotation 3600, 5400, 7200 rpm
  (rpm revolutions per minute)
• Capacities 5 MB to 1 TB (terabyte 240 bytes)

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Hard Disk Example
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Winchester Disks

• Invented by IBM
• A type of hard disk drive
• The disk is contained within a sealed unit
• No dust particles
• When powered off, the head is parked at the
  outer edge of the platter and rests on the
  platter surface
• When powered on, the aerodynamics of the head and
  enclosure create a cushion of air between the
  head and the disk surface
• The head floats above the surface (very close!)
  and does not touch the surface
• Thus, head crash (the head touches the surface,
  with damage resulting)

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Hard Disk Layout
Head
Block
Headmotor
Platter
Sector
Track
Cylinder
Track
Drivemotor
Head, onmoving arm
Head assembly
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Terminology

• Platter
• A round surface the disk containing a
  magnetic coating
• Track
• A circle on the disk surface on which data are
  contained
• Head
• A transducer attached to an arm for
  writing/reading data to/from the disk surface
• Head assembly
• A mechanical unit holding the heads and arms
• All the head/arm units move together, via the
  head assembly
• Cylinder
• A set of tracks simultaneously accessible from
  the heads on the head assembly

YOU MUST KNOW THIS
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Terminology

• Drive motor
• The motor that rotates the platters
• Typically a DC motor (DC direct current)
• The disk rotates at a fixed speed (e.g., 3600
  rpm, revolutions per minute)
• Head motion
• A mechanism is required to move the head assembly
  in/out
• Two possibilities
• A stepper motor (digital, head moves in steps, no
  feedback)
• A servo motor (analogue, very precision
  positioning, but requires feedback)

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Terminology

• Sector
• That portion of a track falling along a
  predefined pie-shaped portion of the disk surface
• The number of bytes stored in a sector is the
  same, regardless of where the sector is located
  thus, the density of bits is greater for sectors
  near the centre of the disk
• The rotational speed is constant i.e., constant
  angular velocity
• Thus, the transfer rate is the same for inner
  sectors and outer sectors
• Block
• The smallest unit of data that can be written or
  read to/from the disk (typically 512 bytes)

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Locating a Block of Data
Seek Time
Latency Time
Transfer Rate
Latency
Transfer
Head
Seek
Desiredtrack
Note Access time seek time latency
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Terminology

• Seek time
• The time for the head to move to the correct
  track
• Specified as an average for all tracks on the
  disk surface
• Latency time
• The time for the correct block to arrive at the
  head once the head is positioned at the correct
  track
• Specified as an average, in other words, ½ the
  period of rotation
• Also called rotational delay
• Access time is the time to get to the data
  (remember!)
• Access time seek time latency
• Transfer rate
• Same as throughput

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Latency Example

• A hard disk rotates at 3600 rpm
• What is the average latency?

Period of rotation (1 / 3600)
?minutes (1 / 3600) ? 60
seconds 0.01667 s 16.67
ms Average latency 16.67 / 2 ms 8.33
ms
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Factors Determining Transfer Rate

• Transfer rate can be determined, given
• Rotational speed of the disk platters
• Number of sectors per track
• Number of bytes per sector

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Transfer Rate Example

• Q Determine the transfer rate, in Mbytes/s, for
  a hard disk drive, given
• Rotational speed 7200 rpm
• Sectors per track 30
• Data per sector 512 bytes 0.5 Kbytes
• A
• Transfer rate 7200 x 30 216,000 sectors/min
• 216,000 x 0.5 108,000 Kbytes/min
• 108,000 / 60 1,800 Kbytes/s
• 1,800 / 210 1.76 Mbytes/s

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Exercise - Transfer Rate

• Q Determine the transfer rate, in Mbytes/s, for
  a hard disk drive, given
• Rotational speed 7000 rpm
• Sectors per track 32
• Data per sector 1024 bytes

Skip answer
Answer
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Exercise - Transfer Rate
Answer

• Q Determine the transfer rate, in Mbytes/s, for
  a hard disk drive, given
• Rotational speed 7000 rpm
• Sectors per track 32
• Data per sector 1024 bytes 1 Kb

A Transfer rate 7000 x 32 224,000
sectors/min 224,000 x 1 224,000
Kbytes/min 224,000 / 60 3,733 Kbytes/s
3,733 / 210 3.65 Mbytes/s
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Typical Specs
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Track Format

• Format of each track

data
header
gap
gap
CRC
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Figure 9.7 A single data block
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Figure 9.8 Header for MS-DOS/Windows disk
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Figure 9.9 Disk interleaving
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Disk Formatting

• The track positions, blocks, headers, and gaps
  must be established before a disk can be used
• The process for doing this is called formatting
• The header, at the beginning of each sector,
  uniquely identifies the sector, e.g., by track
  number and sector number

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Disk Controller

• Interface between the disk drive and the system
  is known as a disk controller
• A primary function is to ensure data read/write
  operations are from/to the correct sector
• Since data rate to/from the disk is different
  than data rate to/from system memory, buffering
  is needed

May also require special driver, as in CD-ROM s
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Buffering
Example Reading data from a disk
System
Diskcontroller
Disk
RAM
Buffer (RAM)
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Multi-block Transfers (1 of 2)

• The smallest transfer is one block (e.g., 512
  bytes)
• However, often multi-block transfers are required
• The inter-block gap provides time for the
  controller electronics to adjust from the end of
  one sector to the beginning of the next
• time may be needed for a few reasons
• Compute and/or verify the CRC bytes
• Switch circuits from read mode to write mode
• During a write operation the header is read but
  the data are written
• (Remember, the header is only written during
  formatting.)
• Perform a DMA operation

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Multi-block Transfers (2 of 2)

• Sometimes, sectors simply cannot be read or
  written consecutively
• There is not enough time (see preceding slide)
• The result is lost performance because the disk
  must undergo a full revolution to read the next
  sector
• The solution interleaving

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Interleaving
A must know item

• Rather than numbering blocks consecutively, the
  system skips one or more blocks in its numbering
• This allows multi-block transfers to occur as
  fast as possible
• Interleaving minimizes lost time due to latency
• Interleaving factor (see next slide) is
  established when the disk is formatted
• Can have a major impact on system performance

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Interleaving Examples
Factor
2
1
3
5
4
6
8
7
9
11
etc.
1
2
3
4
5
21
etc.
1
2
3
31
etc.
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21 Interleaving
2
6
1
7
5
3
9
8
4
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File System Considerations

• There is no direct relationship between the size
  and physical layout of blocks on a disk drive and
  the size and organization of files on a system
• File system
• Determines the organization of information on a
  computer
• Performs logical-to-physical mapping of
  information
• A file system is part of each and every operating
  system
• Logical mapping
• The way information is perceived to be stored
• Physical mapping
• The way information is actually stored

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Disks Capacity in Windows

• To determine the capacity of the C hard disk on
  Windows
• From the Desktop, double click on My Computer
• Right click on C and select Properties

Next slide
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NoteVaries on different systems
Demo
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Alternative Technologies (1 of 3)

• Removable hard disks
• Also called disk packs
• A stack of hard disks enclosed in a metal or
  plastic removable cartridge
• Advantages
• High capacity and fast, like hard disk drives
• Portable, like floppy disks
• Disadvantage
• Expensive

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Alternative Technologies (2 of 3)

• Fixed heads
• Fewer tracks but eliminates seek time

Moving head
Disk
Spindle
Fixed heads
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Alternative Technologies (3 of 3)

• R.A.I.D. Redundant array of inexpensive disks
• A category of disk drive that employs two or more
  drives in combination for fault tolerance and
  performance
• Frequently used on servers, but not generally
  used on PCs
• There are a number of different R.A.I.D. levels
  (next slide)

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R.A.I.D. Levels (1 of 2)

• Level 0
• Provides data striping (spreading out blocks of
  each file across multiple disks)
• No redundancy
• Improves performance, but does not deliver fault
  tolerance
• Level 1
• Provides data mirroring (a.k.a. shadowing)
• Data are written to two duplicate disks
  simultaneously
• If one drive fails, the system can switch to the
  other without loss of data or service
• Delivers fault tolerance

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R.A.I.D. Levels (2 of 2)

• Level 3
• Same as level 0, but also reserves one dedicated
  disk for error correction data
• Good performance, and some level of fault
  tolerance
• Level 5
• Data striping at the byte level and stripe error
  correction information
• Excellent performance, good fault tolerance

Rev. text p 255 - 256
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Terminology

• Fault tolerance
• The ability of a computer system to respond
  gracefully to unexpected hardware or software
  failure
• Many levels of fault tolerance
• E.g., the ability to continue operating in the
  event of a power failure
• Some systems mirror all operations
• Every operation is performed on two or more
  duplicate systems, so if one fails, another can
  take over

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Terminology

• Data mirroring (also shadowing)
• A technique in which data are written to two
  duplicate disks simultaneously
• If one disk fails, the system can instantly
  switch to the other disk without loss of data or
  service
• Used commonly in on-line database systems where
  it is critical that data are accessible at all
  times

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Terminology

• Data striping
• A technique for spreading data over multiple
  disks
• Speeds operations that retrieve data from disk
  storage
• Data are broken into units (blocks) and these are
  spread across the available disks
• Implementations allow selection of data units
  size, or stripe width

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Thank you