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Lecture 8 - External Memory Of a computer architecture
- 2. The time to access a sector in a track on a surface is divided into 3 components: Time Component Action Seek Time Time to move the read/write arm to the correct cylinder Rotational delay (or latency) Time it takes for the disk to rotate so that the desired sector is under the read/write head Transfer time Once the read/write head is positioned over the data, this is the time it takes for transferring data
- 4. Seek time is the time required to move the arm to the correct cylinder. Largest in cost. Difficult quantity to pin down Startup time Time taken to traverse the tracks Settling time (positioning head over track until track identification) The smaller the disk the lesser the seek time 31/2 inch (8.9 cm) in diameter common size
- 5. It is usually impossible to know exactly how many tracks will be traversed in every seek Manufacturer’s specifications for disk drives often list this figure as the average seek time for the drives. Most hard disks today have s under 10 ms, and high-performance disks have s as low as 7.5 ms.
- 6. Seek time depends on the speed with which the head rack moves, and the number of tracks that the head must move across to reach its target. Given the following (which are constant for a particular disk): Hs = the time for the I/ O head to start moving Ht = the time for the I/ O head to move from one track to the next Then the time for the head to move n tracks is: Seek(n)= Hs+ Ht*n
- 7. The rotational delay is the time required for the addressed area of the disk to rotate into a position where it is accessible by the read/write head. Maximum rotational delay is the time it takes to do a full rotation (as the relevant part of the disk may have just passed the head when the request arrived). Most rotating storage devices rotate at a constant angular rate (constant number of revolutions per second). The maximum rotational delay is simply the reciprocal of the rotational speed
- 8. Latency is the time needed for the disk to rotate so the sector wanted is under the read/write head. Hard disks usually rotate at about 5000-7000 rpm, 12-9 msec per revolution. Note: Min latency = 0 Max latency = Time for one disk revolution Average latency (r) = (min + max) / 2 = max / 2 = time for ½ disk revolution Typically 6 – 4.5 ms, at average
- 9. Given the following: R = the rotational speed of the spindle (in rotations per second) = the number of radians through which the track must rotate then the rotational latency radians is: Latency= (/2)*(1000/R), in ms
- 10. The transfer time is given by the formula: number of sectors Transfer time = --------------------------------------- x rotation time track capacity in number of sectors e.g. if there are St sectors per track, the time to transfer one sector would be 1/ St of a revolution.
- 11. The transfer time depends only on the speed at which the spindle rotates, and the number of sectors that must be read. Given: St = the total number of sectors per track the transfer time for n contiguous sectors on the same track is: Transfer Time =(n/St)*(1000/R), in ms
- 13. Optical storage CD-ROM CD-Recordable (CD-R) CD-R/W DVD
- 14. Compact disk originally for audio, but now used as computer storage device. Disk formed from polycarbonate Digital information is imprinted as a series of microscopic pits on the surface. Done by high-intensity laser to create a master disk. Master used to create die to stamp out copies. Then coated with highly reflective coat, usually aluminium or gold. Clear acrylic – top coat to protect against dust and scratches. Finally label on acrylic.
- 15. Read by low –powered laser reflected through clear polycarbonate. Intensity is different for pits and lands detected and converted into digital signal. Pits rough – low intensity Lands smooth – high intensity
- 17. Contains single spiral track, from near centre to outer edge of the disk. Same length sectors Constant packing density
- 18. for Easy to mass produce inexpensively Removable against Expensive for small runs Slower than magnetic disk Read only
- 19. CD-Recordable (CD-R) Write Once Read Many (WORM) Medium includes dye layer. Dye used to change reflectivity and is activated by a high-intensity laser. Compatible with CD-ROM drives or CD drives CD-RW Erasable Mostly CD-ROM drive compatible Phase change Disk uses material with two different reflectivities in different phase states A beam of laser light can change the material from one phase to another
- 20. Digital Versatile Disk Will read computer disks and play video disks Multi-layer Very high capacity
- 22. Magnetic Tape
- 23. No direct access, but very fast sequential access. Resistant to different environmental conditions. Easy to transport, store, cheaper than disk. Before it was widely used to store application data; nowadays, it’s mostly used for backups or archives.
- 24. A sequence of bits are stored on magnetic tape. For storage, the tape is wound on a reel. To access the data, the tape is unwound from one reel to another. As the tape passes the head, bits of data are read from or written onto the tape.
- 25. Reel 1 Reel 2 tape Read/write head
- 26. Typically data on tape is stored in 9 separate bit streams, or tracks. Each track is a sequence of bits. Recording density = # of bits per inch (bpi). Typically 800 or 1600 bpi. 30000 bpi on some recent devices.
- 27. ½” 0 1 1 0 1 1 0 1 0 0 1 1 0 1 1 0 1 0 0 1 1 0 1 1 0 1 0 0 1 1 0 1 1 0 1 0 … … … … … … parity bit 8 bits = 1 byte
- 28. … 2400’ logical record BOT marker Header block (describes data blocks) Data blocks Interblock gap (for acceleration & deceleration of tape) EOT marker