GLOSSARY
LOGICAL ADDRESS
Astoragelocationaddressthatmaynotrelatedirectlytoaphysicallocation.Usually usedtorequestinformationfromacontroller,whichperformsalogicaltophysical addressconversion,andinturn,retrievesthedatafromaphysicallocationinthemass storageperipheral.
LOGICAL BLOCK ADDRESSING
Definestheaddressingofthedevicebythelinearmappingofsectors.
LOGICAL SECTOR
ThelowestunitofspacethatDOScanaccessthroughadevicedriver;oneormore physicalsectors.
LOW FREQUENCY
Thelowestrecordingfrequencyusedinaparticularmagneticrecordingdevice.With FMorMFMchannelcodes,thisfrequencyisalsocalled“IF.”
M
MAIN MEMORY Random-accessmemory(RAM)usedbythecentralprocessingunit(CPU)forstoring programinstructionsanddatacurrentlybeingprocessedbythoseinstructions.(Seealso randomaccessmemory.)
MASS STORAGE
Auxiliarymemoryusedinconjunctionswithmainmemory;generallyhavingalarge, on-linestoragecapacity.
MEGABYTE (MB)
Aunitofmeasureapproximatelyonemillionbytes(actually1,048,576bytes)or106.
MEMORY
Anydeviceorstoragesystemcapableofstoringandretrievinginformation.(Seealso storagedefinitions.)
MICROCOMPUTER
Acomputerwhosecentralprocessingunitisamicroprocessor.Itisusually,butnot necessarily,desktopsize.
MICROPROCESSOR
Acentralprocessingunit(CPU)manufacturedasachiporasmallnumberofchips.
MISSING PULSE
Atermusedinsurfacecertification.Itiswhenaprerecordedsignalisreducedin amplitudebyacertainspecifiedpercentage.
MODIFIED FREQUENCY MODULATION (MFM)
Amethodofencodingdigitaldatasignalsforrecordingonmagneticmedia.Alsocalled “threefrequencyrecording.”Recordingcodethatonlyusessynchronizingclockpulse ifdatabitsarenotpresent.Doublesthelinealbitdensitywithoutincreasingthelineal fluxreversaldensity,comparedtoFrequencyModulation.
MODIFIED MODIFIED FREQUENCY MODULATION (MMFM)
ArecordingcodesimilartoMFMthathasalongerrunlengthlimiteddistance.
MODULATION
1.Readbackvoltagefluctuationusuallyrelatedtotherotationalperiodofadisk.2.A recordingcode,suchasFM,MFM,orRLL,totranslatebetweenfluxreversalsandbits orbytes.
N
NON-RETURN TO ZERO
Aformofdataencodingthatisnotself-clocking,inotherwords,itneedstobe providedwithanexternalbitcellclocksignal.Generallyusedinhigher-performance diskdrives.
O
OFF-LINE processingorperipheraloperationsperformedwhiledisconnectedfromthesystem CPUviathesystembus.
ON-LINE processingorperipheraloperationsperformedwhiledisconnectedfromthesystem CPUviathesystembus.
OPEN LOOP SERVO
Aheadpositioningsystemthatdoesnotusepositionalinformationtoverifyandcorrect theradiallocationoftheheadrelativetothetrack.Thisisusuallyachievedbyuseofa steppermotorwhichhaspredeterminedstoppingpointthatcorrespondstotrack locations.
OPERATING SYSTEM
Asoftwareprogramthatorganizestheactionsofthepartsofthecomputerandits peripheraldevices.(Seediskoperatingsystem.)
OUTSIDE DIAMETER
Thelargestradiusrecordingtrackonadisk.
OVERWRITE
Atestthatmeasurestheresidual1Frecordedfrequencyonatrackafterbeing overwrittenbya2Fsignal.Variationsofthetestexist.
P
PARALLELISM
1.Theconditionoftwoplanesorlinesbeingparallel.Importantindiskdrivesbecause alackofitinmechanicalassembliescanresultinpositioninginaccuracy.More precisely:planes-coplanar;lines-colinear.2.Isthelocalvariationindiskthickness measuredindependentlyofthicknessitself.3.Theabilityofamultiprocessorcomputer toallocatemorethanoneprocessor(CPU)toacomputingproblem,whereeachCPU worksonaseparateproblemorseparatesegmentofthatproblem.Alsoreferredtoas parallelprocessing.
PARITY
Asimplemethodofdataerrordetectionsthatalwaysmakesnumberseitheroddor even,usinganextrabitinwhichthetotalnumberofbinary1s(or0s)inabyteis alwaysoddoralwayseven;thus,inanoddparityscheme,everybytehaseightbitsof dataandoneparitybit.Ifusingoddparityandthenumberof1bitscomprisingthe byteofdataisnotodd,theninthorparitybitissetto1tocreatetheoddparity.Inthis way,abyteofdatacanbecheckedforaccuratetransmissionbysimplycountingthe bitsforanoddparityindication.Ifthecountisevereven,anerrorisindicated.
PARTITION
Alogicalsectionofadiskdrive,eachofwhichbecomesalogicaldevicewithadrive letter.
PEAK SHIFT
Theshiftingintimeofthezero-slopeportionofareadbackvoltagefromthevalues containedinthewritecurrentwaveform.Sometimesincorrectlyusedtodescribebit jitter.
PERIPHERAL EQUIPMENT
Auxiliarymemory,displays,printers,andotherequipmentusuallyattachedtoa computersystem’sCPUbycontrollersandcables.(Theyareoftenpackagedtogetherin adesktopcomputer.)
PHASE LOCKED LOOP (PLL)
Acircuitwhoseoutputlocksontoandtracksthefrequencyofaninputsignal. Sometimesincorrectlycalledadataseparator.
PHASE MARGIN Measureindegreesoftheamountofdifferencebetweenexcursionsfromthewindow centerwherefluxreversalscanoccurandtheedgeofthedatawindow.Similarto windowmargin.
PHYSICAL SECTOR
Thesmallestgroupingofdataontheharddisk;always512bytes.
PIO
ProgrammableInputOutput.Ameansofaccessingdeviceregisters.Alsodescribesone formofdatatransfers.PIOdatatransfersareperformedbythehostprocessorusing PIOregisteraccessestothedataregister.
PLATED THIN FILM MEDIA
Magneticdiskmemorymediahavingitssurfaceplatedwithathincoatingofametallic alloyinsteadofbeingcoatedwithoxide.
PROCESSING
Theprocessofthecomputerhandling,manipulatingandmodifyingdatasuchas arithmeticcalculation,filelookupandupdating,andwordpressing.
PULSE CROWDING
Modificationofplaybackamplitudeduetosuper-positioningofadjacentfluxreversal fieldsbeingsensedbytheread/writegap.
PULSE DETECT
Adigitalpulsetraininwhicheachleadingedgeoreachedgecorrespondstoa magnetictransitionreadfromthedisk.Iftransitionqualificationcircuitryexistsinthe drive,thissignalistheoutputofsame.Alsoknownastransitiondetect.