DISK THEORY

Disk Theory

How information is stored on disk How we can take advantage of that when

bad things happen

Writing

Disk covered with iron oxide (rust) Write Head magnetizes/demagnetizes areas Similar to audio tape Information is stored digitally

Physical Organization

Cylinders Sectors Tracks

Capacity

High Density 3.5” floppy = 1.44 Mb

160 tracks/side x 9 sectors/track x 512 bytes/sector x 2 sides

= 1,474,560 bytes = 1.44 Mb

Logical Organization

Boot Record File Allocation Table (FAT) Root Directory Data Area

Boot Record

Always the first sector on side one Contains bootstrap and other systems

programs if it is a systems disk contains info about the disk

– bytes/sector, sectors/track, sectors/disk

File Allocation Table

Indicates the status of each sector– free, in use, or damaged

FAT tells us where next segment or file can go

Cluster

Inefficient to keep track of each sector So, FAT keeps track in groups called

Clusters Usually 2 or 4 sectors in a cluster Each cluster has a FAT entry

Root Directory

Contains name of each file stored on disk Pointer to first cluster it is stored in RD is fixed in size and can point to only

112 files– Cheat by creating more directories

Data Storage

New version covers the old version What if new version is larger (more

clusters)? File extends to next open cluster (see FAT) New cluster is linked to original clusters

– Called a Fragmented File

File Recovery

File info is not erased, – Its name in Root Directory is erased– FAT changes to reflect an unused cluster

So, info exists until it is covered by new file Utility programs can retrieve this info!