Types of External Memory
• Magnetic Disk
• Solid State Drive (SSD)
• Optical
• Magnetic Tape
Magnetic Disk
• Disk substrate coated with magnetizable material
• Substrate used to be aluminium
• Now glass is commonly used
Read and Write Mechanisms
-Recording & retrieval via head
-May be single read/write head or separate ones
-Head is stationary platter rotates
-Write through magnetic pulses
-Read (traditional) magnetic field moving relative to head
-Read (contemporary) separate read/write heads better magnetic field operation
Data Organization and Formatting
• Concentric rings or tracks
• Tracks divided into sectors
• Minimum block size read or written is one sector
Disk Velocity
• Bit near center of rotating disk passes fixed point slower than bit on outside of disk
• Rotate disk at constant angular velocity (CAV)
• Can use zones to increase capacity
Finding Sectors
• Must be able to identify start of track and sector
• Accomplished by "formatting" disk
• Gaps between tracks, sectors and fields needed to provide for less precise
tolerances on disk mechanism to rotate disks and position heads
Fixed Read/Write Heads
- One read write head per track
- Heads mounted on fixed ridged arm
- Rotating drum with magnetic surface
- Rarely used today
Movable Read/Write Heads
- One read write head per magnetic surface
- Mounted on a movable arm
- Requires mechanical movement that is very fast and
very precise
Removable Disk
- Can be removed from drive and replaced with another disk
- Provides drive with unlimited storage capacity
- Easy data transfer between systems
- Examples: "floppy disk" or Zip disks
- rarely used today
Non removable Disk
- Permanently mounted in the drive
- Typical hard disk in majority of computing systems
Multiple or Single Platters
