DISK THEORY

Disk Theory n How information is stored on disk n How we can take advantage of that when bad things happen

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

Physical Organization n Cylinders n Sectors n Tracks

Capacity n High Density 3.5” floppy = 1.44 Mb n 160 tracks/side x 9 sectors/track x 512 bytes/sector x 2 sides = 1,474,560 bytes = 1.44 Mb

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

Boot Record n Always the first sector on side one n Contains bootstrap and other systems programs if it is a systems disk n contains info about the disk – bytes/sector, sectors/track, sectors/disk

File Allocation Table n Indicates the status of each sector – free, in use, or damaged n FAT tells us where next segment or file can go

Cluster n Inefficient to keep track of each sector n So, FAT keeps track in groups called Clusters n Usually 2 or 4 sectors in a cluster n Each cluster has a FAT entry

Root Directory n Contains name of each file stored on disk n Pointer to first cluster it is stored in n RD is fixed in size and can point to only 112 files – Cheat by creating more directories

Data Storage n New version covers the old version n What if new version is larger (more clusters)? n File extends to next open cluster (see FAT) n New cluster is linked to original clusters – Called a Fragmented File

File Recovery n File info is not erased, – Its name in Root Directory is erased – FAT changes to reflect an unused cluster n So, info exists until it is covered by new file n Utility programs can retrieve this info!