Lecture 8 - External Memory Of a computer architecture

The summary for external memory of a computer architecture

1. Lecture 8
External Memory
1

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):

H
s
= the time for the I/ O head to start moving

H
t = 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)= H
s+ H
t*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 S
t sectors per track, the time to
transfer one sector would be 1/ S
t 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:
S
t = the total number of sectors per track
the transfer time for n contiguous sectors on the same track
is:
Transfer Time =(n/S
t)*(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 blocksInterblock gap
(for acceleration &
deceleration of tape)
EOT
marker