Systems Reference Library - All about the IBM 1130 Computing ...

120255A1Each track is divided into four equal segmentscalled sectors. Sectors are numbered from 0 through7. As shown in the following illustration, sectors 0through 3 divide the upper surface tracks; sectors 4through 7 divide the lower surface tracks. A sectorcontains 321 data words and is the largest segment ofdata that can be read or written with a single instruction.BitsData WordsSectorsTracksCylindersWord Sector Track Cylinder Disk16 5,120 20,480 40,960 8,192,000320 1,280 2,560 512, 00048 1,6002400200Sectors 4-7(Lower Disk Surface)Sectors 0-3(Upper Disk Surface)Timingr 291451The magnetic disk revolves at 1500 rpm, making therevolution time 40 ms. The word rate is 36 kHz.There is no timing consideration for switchingread/write heads. The shortest time available betweenthe end of a 321 word sector and the beginningof the next sector is approximately 235 ps. Forevery word not used, 27.8 As may be added.The sector is the basic addressable unit forreading and writing. Each XIO initialize read or XIOinitialize write must address one of the eight sectorsavailable to the heads in each cylinder.Although the capacity of the sector is 321 words,it is recommended that one word be written as asector address, leaving 320 data words. A commandmay specify any number of words equal to orless than 321. A zero word count will force an operationcomplete interrupt with no reading or writingoccurring. The 321-word capacity, then, refersonly to a maximum number of words that may betransferred with one command and still ensure atleast 235 ius before the next sector starts. Thisallows sufficient time for the issuance of the nextXIO.No circuitry is provided in the 1810 to limitreading or writing to 321 words. Therefore, theprogram must limit the word count. A 1810 word iscomposed of 16 data bits, 3 check bits, and 1 spacebit. (The check and space bits do not appear on thein or out bus. ) The following illustration shows theorganizational components of disk storage. Notethat the capacities are based upon the 320-wordsector.Access Time: Models Al , A2, and A3The incremental access mechanism used with A-models of the 1810 requires 15 ms to move one ortwo cylinders. Therefore, the number of cylinders(n) must be even when calculating access time. (Thenext higher even number is used if an odd number ofcylinders is specified.)In addition to the time required to move from onecylinder to another, 20-25 ms must be allowed forcarriage stabilization before reading or writing canoccur. This delay occurs automatically; any XIOinitialize read or XIO initialize write issued is executedimmediately after the 20-25 ms delay. However,20-25 ms is also required between accesses;this delay is usually accomplished by a read operationbetween accesses.Figure 85 shows access time, including carriagestabilization, for A-models of the 1810.Access Time: Models 81, B2, and 83The direct access mechanism used with B-models ofthe 1810 moves in one continuous motion to the cylinderaddress. Access time, including carriage stabilization,is shown in Figure 86.164