2 Limitations of Primary Storage Limited Capacity: It cannot be used to store the large volumes of data items.Volatility: Looses data when electric power is turned off or interrupted.Data Portability: It cannot be used to port the data from one place to other.These limitations lead to the development of auxiliary memory or secondary storage devices.
4 Magnetic TapeThe first truly mass auxiliary storage device was the magnetic tape driveTape drives have amajor problem; canyou describe it?
5 7 Track;6-bit BCD; Even parity (all tracks must have even number of 1) A frameTrack/Channel numbers123456789ABCDEFGHIJKLMNOPParity bit7654321zoneNumeric6-bit BCD for letter A (Table 4.2 page 36Even parity (all tracks must have even number of 1)Error Checking
6 9 Track;8-bit EBCDIC; Even parity (all tracks must have even number of 1) EBCDIC for the letter ATrack/Channel numbersA frame1123456789ABCDEFGHIJKLMNOP9876543213’s digit2’s digitAdded ZoneZoneParity bitUnit’s digit4’s digitError CheckingOdd parity (all tracks must have odd number of 1)8-bit EBCDIC for letter A (Table 4.3 page 8
7 Terminologies Record: Group of characters separated by blank Inter record gap (IRG)Blocks: Records are grouped into blocksBlocking factor: # of records perblocksInter Block gap (IBG)Storage capacity of tape= Data recording density x LengthData transfer rate (bytes/second to main memoryTape driveTape controllerTape Types (Half-inch tape reel, Half-inch tape cartridge, Quarter-inch streamer tape and 4mm Digital Audio Tape (DAT)
8 Advantages & Limitations of Magnetic Tape Virtually unlimited storage capacity.Low cost.Easy to handle and store.Easy to port.LimitationsThe are not suitably for frequent data access.Must be operated in dustless environment.Must be operated in controlled environment.Must have proper identification labels
9 Uses of Magnetic TapeGood for applications that require sequential data processing.Data backup.Data archiving.Physically large data transferSoftware distribution
10 Magnetic DisksGenerally 8 bit EBCDIC code is used to record the data items.Surface: 2 surfaces per disk.Tracks:Sectors: 8 sectors per track.Disk pack: 200 disks.Cylinder: Each track from all recording surfaces in a disk pack form a cylinder.Storage capacity = # of surfaces × # of tracks per surface x # of sectors per track x # of bytes per sector.Access mechanism: Read/write head moves to an address by specifying the (disk #, track #, sector #, byte #)A platter coated with iron oxide: A magnetizable material
11 Magnetic DisksSeek Time: Time to move the R/W head to the desired track.Latency or Rotational Delay Time: Time to rotate the disk to the desired sector of a track.Average Latency Time?Transfer Rate: Rate at which data to be written/read. Transfer rate = n x r /60 where n is the # of bytes per track and r is the revolution per second.Disk Formatting: Formatting prepares a disk to be used by different disk drives. Formatting creates file allocation table (FAT) that specifies the track and sector of the data items.Important: Magnetic Disks are direct or random access device because data items can be accessed by specifying its address.Disk drives with interchangeable disks.Disk drives with fixed disks.Disk Controller: Controls the movement of R/W heads, and disk rotation.
12 Disk (Portable/Fixed) Magnetic Disk TypeMagnetic DiskFloppyDisk (1.4 MB)HardDiskZip BernoulliDisk (Portable/Fixed)DiskPacksWinchesterDisk
13 Advantages & Limitations Direct Access.Simultaneous sharing.On-line/Off-line access.Unlimited disk may be added.Low cost.PortableFastLimitationsInefficient for sequential data processing.Sharing make data unsecuredDisk failure.Cost of magnetic tape is lowerMust be operated in dustless environment
14 Uses of Magnetic Drives Random access applicationsShared on-line secondary storage.Data backup.Data archivingData and software transportation.Software distribution.
15 OPTICAL DISKIn the optic readable CD-ROM, the data storage consists of millions of indentations burnt into light reflecting silver surface. The burnt dents reflect less light than the shiny surface.
16 TracksThe CD-ROM has only one track, a spiral winding its way from the center to the outer edge. The spiral track is ideal for reading large block of serial data.
17 Access MEchanismA weak laser beam is sent to the disk through a two-way mirror and the sensor registers the difference in light reflection from the burnt and shiny areas as zeros and ones.Access time: millisecond.CD-ROM (Compact disk read only memory).WORM (Write once read memory)R/W CD (Read/Write compact disk)
18 Advantages & Limitations Low costIdeal for reading large volumes of data.No mechanical R/W heads.Long life data storage.Easy to handle, store and port.LimitationsSlower than magnetic disk.Complicated drive than magnetic disk.Uses of optical diskDistribution of large amount of data and Mulltimedia.
19 Mass Storage Disk Array: It uses a set of magnetic disks. Automated tape library: Uses a set of magnetic tapes.CD-ROM jukebox: Uses a set of CDROM
20 Memory Hierarchy Cache Memory Main Memory Hard Disk Smaller Capacity, faster access time and higher costLarger Capacity, slower access time and lower costMain MemoryHard DiskMagnetic tape, Floppy, …Mass StorageMemory Hierarchy