13.1 through 13.5, 13.10 and 13.11

13.2 Secondary Storage Devicesi\ (rl{rrl i\ -\nderr ll:t-ll thr'\ ll lllFigure 13.2(a), calls a portion of a track that subtends a fixed angle at the center asector. Several other sector organizations are possible, one of which is to have thesectors subtend smaller irngles irt the center as one moves away, thus n-raintaining auniform density of recording, as shown in Figure 13.2(b). A technique called ZBR(ZoneBit Recording) allows a range of cylinders to have the same number of sectorsper arc. For example, cylinders 0-99 may have one sector per track, 100-i99 mayhave two per track, and so on. Not all disks have their tracks divided ir-rto sectors.l:icl-llllr)The division of a track into eqr.ral-sized diskblocks (or pages) is set bv tl.re operatingsystem during disk formatting (or initialization). Block size is flxed during initiirlizationand cannot be changed dynamically.'Iypical disk block sizes ranse fror.r.r512 to 8192 bytes. A disk with hard-coded sectors often has the sectors subdividedinto blocks during initialization. Blocks irre separated by fixed-size interblock gaps,rvhich include specially coded control iufbrmation written during disk initialization.This information is used to determine which block on the track fbllows eachinterblock gap. Table l3.l representspecifications of a typical disk.There is continuous imprrovement in the storage capacity and transfel rates associatedwith disks; they are also progressivelv getting cheaper-currently costing only afraction r.r[ a dollar per r-negabyte of disk storage. Costs are going dorvn so rapi{11'that costs as low 0.1 cent/MB which translates to $l/GB and $lK/TB are not too fhrarvay.Adiskis arandotn cccessaddressabledevice.Transf'erof databetweenmain menroryand disk takes place in units of disk blocks. The hardware address of a block-acombination of a cylinder nunrber, track nun'rber (surfirce number within the cylinderon which the track is located), and block number (within the track) is suppliedto the disk I/O hardware. In many modern disk tlrives, a single number called LBA(Logical Block Address), which is a number between 0 and rr (assunting the totalcapacity of the disk is rr + I blocks), is mapped automatically to the right block bythe disk drive controller. The acldress of a buffer-a contiguons reserved areir inmain storaqe that holds one block-is also provided. For a read cornn.rand, theSector (arc of track) Figure 13.2Different seciororganizations on disk.(a) Sectors subtendinga fixed angle.(b) Sectors maintaininga uniformThree sectorsTwo sectorsOne sectorraeardinndonciirr