Backing Store

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

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

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

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

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…

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

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

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

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

DVDs Same size media as CDs Same basic data storage structure as CD but… Greater capacity e.g. 4.7GB compared to 640-800MB for CD Achieved by higher frequency laser which can be focussed more tightly Distance between pits and lands is smaller Tracks more tightly packed