1. Backing Store

2. Logical Overview of a Computer System
Memory
PROCESS
INPUT
OUTPUT
Backing Store

3. Backing store Place to keep important data Kept on a storage medium
Magnetic, Optical, physical
Generally slower access than Memory
Devices for Storage
Hard Disk Drive (HDD) / Floppy Disk Drive (FDD)
Zip Drive
Tape Drive
CD / DVD (ROM, R, RW)
Memory Chips (USB sticks, Flash memory)
Punched Cards
Barcodes

4. Hard Disk Drives (HDD) Magnetic storage medium
Uses rotating metal disks (platters)
Use read and write ‘heads’ to store and retrieve information
Large storage capacity
Disk Platter
Spindle
Read/Write Heads
Actuator

5. Hard Disk Layout Tracks: Concentric circles
Cylinders
Access
Mechanism
Arms
Read/Write
Heads
Disks
Tracks: Concentric circles
for storing data as magnetized
bits.
Sectors:
Portions of a track
Track

7. Hard disk access Data represented by small magnetized ‘hot spots’
Disk space is divided into sectors (normally 512 bytes)
Operating systems will allocate space to file in clusters
A cluster is one or more sectors (say 4 sectors)
Minimum space taken on disk by a file is therefore one cluster (say 2KB)
Sector
Cluster
Track
Confusingly, you may see a sector defined as a whole “pie slice” of a hard disk!
The 512 bytes definition is more common…

8. Hard drive performance
Hard drive performance is influenced by:
Seek time i.e. time to get read/write head to the right track
Rotation speed i.e. how long till the data arrives at the read/write head
Transfer speed i.e. how quickly the disk’s electronics can move the data off the disk
So a 7200rpm disk is very likely to perform better than a 5400rpm

9. Floppy Disk Drive (FDD)
Magnetic storage medium
Uses rotating single, thin magnetic disk
Requires a drive to read the disk, which has the read/write heads
Small storage capacity
Typically 1.44 Mb (High Density 3.5”)
Portable
Cheap

10. Compact Discs Optical Data Storage 650MB / 700MB Capacity
Data retrieved from disc surface using a laser
One single spiral track from centre to edges
Pits and lands used to represent binary data – think of grooves
Change from pit to land or land to pit = 1
No change = 0

11. Electron Microscopy of Disc Surface
Compact Discs
Disc Track Structure
Electron Microscopy of Disc Surface

12. DVDs Same size media as CDs
Same basic data storage structure as CD but…
Greater capacity e.g. 4.7GB compared to MB for CD
Achieved by higher frequency laser which can be focussed more tightly
Distance between pits and lands is smaller
Tracks more tightly packed