Encyclopedia of Computer Science and Technology

tape drives 467tape drivesAnyone who has seen computers in old movies is familiarwith the row of large, freestanding tape cabinets with theirspinning reels of tape. The visual cue that the computerwas running consisted of the reels thrashing back and forthvigorously while rows of lights flashed on the computerconsole. Magnetic tape was indeed the mainstay for datastorage in most large computers (see mainframe) in the1950s through the 1970s.In early mainframes the main memory (correspondingto today’s RAM chips) consisted of “core”—thousands oftiny magnetized rings crisscrossed with wires by whichthey could be set or read. Because core memory was limitedto a few thousand bytes (kB), it was used only to holdthe program instructions (see punched cards and papertape) and to store temporary working data while the programwas running.The source data to be processed by the program wasread from a reel of tape on the drive. If the program updatedthe data (rather than just reporting on it), it would generallywrite a new tape with the revised data. In large facilities aperson called a tape librarian was in charge of keeping thereels of tape organized and providing them to the computeroperators as needed.OperationA mainframe tape drive had two reels, the supply reel andthe take-up reel. Because each reel had its own motor, theycould be spun at different speeds. This allowed a specifiedlength of tape to be suspended between the two reels,serving as sort of a buffer and allowing the take-up reel toaccelerate at the start of a read or write operation withoutdanger of breaking the tape. The “buffer” tape was actuallysuspended in a partial vacuum, which both kept thetape taut enough to prevent snarling and allowed for airpressure sensors to activate the appropriate motor whenthe amount of tape in the buffer went above or below presetpoints.Data was read or written by the read and write headsrespectively, in units called frames. In addition to the 1 or0 data bits, each frame included parity bits (see error correction).The frames were combined into blocks, with eachblock having a header in front of the data frames and one ormore frames of check (parity) bits following the data.The two predominant tape formats were the IBM format,which used variable-length data blocks (and thus could notbe rewritten) and the DEC format, which used fixed-lengthblocks, allowing data to be rewritten in place, albeit at somecost in speed and efficiency.During the 1960s, magnetic disks (see hard disk)increasingly came into use, and more of the temporary databeing used by programs began to be stored on disk ratherthan on tape. Eventually, tapes were relegated to storingvery large data sets or archiving old data.However, when the first desktop microcomputers (suchas the Apple II and Radio Shack TRS-80) came along inthe late 1970s and early 1980s, they, like the first mainframes,had very limited main memory and disk driveswere unavailable or expensive. As a result, programs (suchA NASA automated tape library. These facilities can store trillionsof bytes of data. (NASA Photo)as Bill Gates’s Microsoft Basic) often came on tape cassettes,and the computer included an interface allowing it tobe connected to an ordinary audio cassette recorder. However,this use of tapes was quite short-lived, and was soonreplaced by the floppy disk drive and later, hard drives andCD-ROM drives.Tapes as Backup DevicesBy the 1990s, PC users generally used tapes only for makingbackups. A typical backup tape drive uses DAT (digitalaudio tape) cartridges that hold from hundreds of megabytesto several gigabytes of data. Most drives use a rotatingassembly of four heads (two read and two write) that verifydata as it’s being written. As a backup medium, tape has alower cost per gigabyte than disk devices. It is easy to useand can be set up to run unattended (except for periodicallychanging cartridges).However, since tapes are written and read sequentially,they are not convenient for restoring selected files (seebackup and archive systems). Many smaller installationsnow prefer using a second (“mirror”) hard drive as backup,using disk arrays (see raid) or using recordable CDs oroptical drives for smaller amounts of data (see cd-rom anddvd-rom).Many large companies and government agencies havethousands of reels of tape stored away in their vaults sincethe 1960s, including data returned from early NASA spacemissions. As time passes, it becomes increasingly difficult toguarantee that this archived data can be successfully read.This is due both to gradual deterioration of the medium andthe older data formats becoming obsolete (see backup andarchive systems).Further ReadingBrain, Marshall. “How Tape Drives Work.” Available online. URL:http://electronics.howstuffworks.com/cassette.htm. AccessedAugust 22, 2007.