A Guide to PC Hardware Maintenance and Repair

1
Chapter 12 Hard Disk Drive Structure and
Geometry
2
Objectives

• To learn how hard disks are constructed
• Examine some of the key parts of the hard disk
• Learn about the file system in more detail
• To learn how geometry affects how much your hard
  disk can hold
• To examine some encoding methods used

3
The Basics of Hard Disk Geometry

• Like floppies, hard disks are laid out in sectors
  and tracks
• Hard disks have more than one platter. Each
  surface needs a separate R/W head
• All the tracks that are vertically aligned make
  up a cylinder

4
CHS determines capacity

• Multiplying the number of cylinders times the
  number of heads times the number of sectors per
  track gives total capacity.
• A BIOS call known as Int13h dictated the address
  space for all hard disks

5
Int13h Addressing

• Provides 10 bits for tracking cylinders
• 210 1024
• Provides 8 bits for tracking R/W heads
• 28 256
• Provides a 6-bit address for sectors per track
• 26 64

6
Int13h Limitations

• 1024 x 256 x 63 16,515,072
• Thats the maximum number of sectors allowed
• Why 63 instead of 64 for sectors per track? The
  very first sector is reserved for the boot sector
  and not available to the file system
• 16,515,072 sectors x 512 bytes per sector allows
  for a maximum of 8,455,716,864 bytes (just over
  8GB)

7
Getting around Int13h

• Drive Translation Methods
• Different methods will be discussed later
• They are introduced to the system in the Int13
  Extensions
• Drive Translation works by remapping the drive
  and/or reallocating address bits

8
Mapping Sectors

• In the old days, all tracks had to have the same
  number of sectors, whether on the inside or
  outside of disk.
• Zone Bit Recording allows for the outside tracks
  to have more sectors than inside tracks.

9
Hard Disk Construction The Parts

• Platters
• Read/Write heads
• Actuator Arms
• Actuator Mechanism
• Stepper motors
• Voice Coils

10
Head Parking

• The R/W head cannot come in contact with any part
  of the platter that contains data, even when the
  drive is stopped.
• Manufacturers employ different methods of
  retracting the actuator arms to a safe haven when
  drive spins down.

11
Hard Disk I/O Operations

• Rotational Speed
• Average Access Time
• Data Transfer Rate

12
Rotational Speed

• Determines how quickly the R/W heads can locate
  and lock onto data
• Dictates the maximum theoretical speed of
  transferring data from drive to buffer memory

13
Average Access Time

• A measure of the time (in milliseconds) it takes
  from the instant a command is issued to the
  moment the first bit of requested data is located
• Access time is a function of the speed and
  efficiency of the actuator mechanism and of
  rotational speed

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Data Transfer Rate

• A function of rotational speed
• Also dependent on the number of sectors per track
  on the platters
• Overall transfer rate can be affected by the
  amount of buffer memory on the hard disk
  controller

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Overview of an I/O operation

• The Queuing Phase This is where all the commands
  required by the hard disk controller are issued
  and, when possible, lined up in the correct order
  for execution
• The Command Phase The commands are executed in
  the order in which they exist in the controllers
  cache memory
• The Access Phase The R/W heads locate and lock
  on to the first sector containing the requested
  data
• The Data Transfer Phase Data is copied from the
  surface of the drive, moved to the controllers
  cache RAM, and then to system RAM

16
Data Encoding Mechanisms

• Data is stored on magnetic surface with either
  negative polarity or positive polarity (flux
  reversals)
• Different methods of storing digital information
  as magnetic pulses
• MFM
• RLL
• PRML
• EPRML

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MFM

• Modified Frequency Modulation
• Works like a cassette recorder
• Each end of a flux reversal acts as either a 1 or
  a 0
• Required a separate controller
• Obsolete

18
RLL

• Run Length Limited
• A clocking mechanism on the controller measure
  how many bits are recorded in a specific time
  interval
• Run Length is how many bits can be written per
  clock cycle
• Data can be packed tighter on the surface and
  data transfer rates are faster

19
PMRL

• Partial Response/Mechanism Likelihood
• A burst of electrical signals records a series of
  bits
• On read operations, not every single bit is read.
  Obvious bits are assumed by the controller using
  a maximum likelihood algorithm
• Increase maximum density by up to 35

20
EPRML

• Extended PRML
• Faster data transfers
• More efficient algorithms
• Is used on most drives in production today

21
File Systems

• Provides a map of the hard disk that the OS and
  disk controller can use to store and retrieve
  data
• This virtual map is stored in a portion of the
  disk located outside of the formatted data area

22
Different File Systems in Use

• FAT
• FAT12
• FAT16
• FAT32
• HPFS (obsolete)
• NTFS
• NTFS4
• NTFS5

23
FAT12

• As with all FAT systems, drive mapping is stored
  in the file allocation tables (bet youre
  wondering where the name came from!)
• Also with all FAT systems, file names are limited
  to 8-character names with 3-character extensions
• Each FAT entry in FAT12 is 12 bits long
• Is used with floppy disks and on FAT16 formatted
  hard disks with partitions under 128MB

24
FAT16

• Each FAT entry is 16 bits long
• Supports a maximum partition size of 2GB
• Date is stored in file allocation units (FAU)
  instead of individual sectors
• Number of sectors required for each FAU is
  dependent on the size of the partition

25
FAT16 and Disk Slack

• A FAU in FAT16 is a minimum of 4 sectors (2K) and
  as big as 64 sectors (32K)
• Each FAU can hold data from only one file
• If a 640-byte file is stored in a 32K FAU, 31,360
  bytes of potential storage go wasted

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FAT16 Partitions and FAU Size
 
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FAT32

• Each FAT entry is 32 bits long
• Allows for much larger partitions with smaller
  FAUs
• Introduced Long File Names (LFN)

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FAT32 Partitions and FAU Size
 
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Long File Names

• Allows for file names up to 255 characters
  (including extension)
• Automatically generates an 8.3 compatible file
  name for backward compatibility

30
NTFS4

• Replaces FAT with a relational database of
  information concerning each file including sector
  location, security settings and a number of other
  factors
• Allows files and directories to be selectively
  compressed on the fly
• Permits file-level security settings

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NTFS5

• In addition to NTFS4 features, NTFS5 adds
• Native file encryption
• The Distributed File System
• Drive usage limitations can be set for individual
  users

32
HPFS

• An obsolete file system used by IBM for their
  now-defunct OS-2 operating system
• Introduced most of the features found in the
  initial release of NFTS4

33
The Master Boot Record

• The MBR is the most important sector on your hard
  disk
• It contains an executable string that introduces
  the file system
• Partitions are mapped and identified
• An OS pointer directs the boot process to the
  first line of OS code.