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Computer Peripherals – Part I. Plan. Storage (hierarchy and terminology) Magnetic disks. 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

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Computer Peripherals – Part I

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  • Storage (hierarchy and terminology)
  • Magnetic disks
  • 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)
storage hierarchy




Storage Hierarchy

You should know this hierarchy


CPU registers

Cache memory

Conventional memory

Expanded memory

Hard disk

Floppy disk



Access Time


15-30 ns

50-100 ns

75-500 ns

10-50 ms

95 ms

100-600 ms

0.5+ s






600-6000 Kbytes/s

100-200 Kbytes/s

150-1000 Kbytes/s

5-20 Kbytes/s (cartridge)

200-3000 Kbytes/s (reel-to-reel)

  • 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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  • 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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  • Storage (hierarchy and terminology)
  • Magnetic disks
magnetic disks
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)
floppy disks
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
Floppy Disk Example


Access window

Cutawayshowing disk



Writeprotect tab

hard disks
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)
winchester disks
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)
hard disk layout
Hard Disk Layout










Head, onmoving arm

Head assembly

  • 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


  • 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)
  • 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)
locating a block of data
Locating a Block of Data

Seek Time

Latency Time

Transfer Rate






Note: Access time = seek time + latency

  • 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
latency example
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

factors determining transfer rate
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
transfer rate example
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

exercise transfer rate
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


track format


Next sector

Previous sector






CRC stands for “cyclic redundancy check”. It’s the “footer” at the end of each sector. CRC is a sophisticated form of parity for checking that the data read are accurate.

Track Format
  • Format of each track:






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disk formatting
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
disk controller
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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2. Transfer data from buffer to system RAM (Note: this is a DMA operation)

1. Read data from disk into a buffer in the disk controller


Example: Reading data from a disk







multi block transfers 1 of 2
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
multi block transfers 2 of 2
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

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
interleaving examples
Interleaving Examples

























file system considerations
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
disks capacity in windows
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

alternative technologies 1 of 3
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
alternative technologies 2 of 3
Alternative Technologies (2 of 3)
  • Fixed heads
    • Fewer tracks but eliminates seek time

Moving head



Fixed heads

alternative technologies 3 of 3
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)
r a i d levels 1 of 2
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
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

  • 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
  • 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
  • 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