• Effective I/O device – Secondary/auxiliary memory
  • Components
  • One or more platters on disks mounted with common spindle
  • Platter has 2 magnetic surface – top & bottom
  • Oxide coating on aluminum disk surface & currently thin film disks affixed to substrate
  • R/W head flies over disk
  • Earlier ferrite heads & now thin film heads

Working

  • Data recorded on tracks 
  • Cylinders – tracks on a specific circle
  • No. of cylinders = no. of tracks in a surface
  • Data – MFM form & serial through singe wire (DATA + CLOCK)
  • Control signals inform exact location of storage 
    • Cylinder number, surface number, etc.
  • Disk rotation: 2400, 4800, 7200 rpm

Types I

1. Removable Disk Drive

  • Disks can be removed when not used
  • So, Any number of disk packs
  • Transfer of data from one disk to another

2. Fixed Disk Drive

  • Disk cannot be removed from disk drive

Types II

1. Single head assembly Drive

  • One head for each surface
  • Heads mounted on one spindle & move together
  • All head positioned in same cylinder

2. Dual head assembly Drive

  • Two heads for each surface
  • One set covers 1st half & 2nd set covers 2nd half of cylinder

Types III

1. Winchester Disk Drive

  • R/W heads in a sealed enclosure
  • Head flies close to hard disk less than 19 micro inches
  • heads park on parking / landing zone – no data written
  • Surface of disk is lubricated – prevent damage to head or track
  • Higher track densities
  • No preventive maintenance
  • Built-in preventive system against head crash – back up

2. Non – winchester Disk Drive

  • Obsolete
  • used till 1973

Types IV

1. Open loop Disk Drive

  • Stepper motor for head movement
  • Track-0 is reference point
  • STEP Pulse issued to stepper motor for head move on track
  • No. of step pulses decide total distance of head movement
  • No feed back system to verify head movement
  • S/w tracks formatting info to verify head correctness
  • Direction signal – direction of movement of stepper motor (forward & backward)
  • When head is on track0, HDD sends TRACK0
  • Present track number is with ref to track0 &  Low cost

Open loop Disk Drive

Closed loop disk drive

  • Hard disk info – reference info positioning

Ways for storing head positioning information

  • Servo tracks
  • Servo surface
  • Linear voice coil mechanism – control linear motor position using feedback control – moves head at different velocities
  • Head acceleration based on distance
  • Automatic reposition 

Closed loop positioning system

Advantage

  • Accurate head positioning on desired track
  • Less access time
  • Cost twice or thrice of open loop system
  • Suitable for high performance system

Size & capacity

  • Inches – platters diameter
  • Sizes – 20, 14, 101/2, 8, 51/4, 31/2, 21/2 inches
  • Capacity – size & technology
  • 14 inch HDD – 20MB & 21/2 inch HDD – 40GB

To have more data capacity,

  • Higher track density
  • Higher recording density
  • Better Packaging methods

Hard disk drive organization

  • Both electronic & electromechanical subsystems, viz.,
    • R/W Head
    • Disks
    • Spindle motor 
    • Positioning mechanism
    • Air Circulation System
    • Air Filters
  • In Winchester HD – sealed head disk assembly (HAD) with R/W head, disks, band actuator assembly & air circulation system

Head Types

  • Manganese/Zinc ferrite head
  • Thin film head

Standard interface for hard disk connection

  • SMD, IDE, EIDE, SCSI, IPI, ESDI, ST506/412
  • In ST506/412 interface,
  • Radial/Data Cable - Data Sent/received on separate cable

Daisy Chain Cable 

  • Control sgls sent through to 1st HDD
  • Same sgls extended to 2nd HDD by another cable

Data organization on hard disk

  • Magnetic tracks – into sectors
  • Currently, Soft sector format 
  • In ID Field,
    • Track Number
    • Head Number
    • Sector Number