P2 T2 L13 - Secondary Storage Devices

Question 1

Why choose secondary storage?

2 points

Answer 1

1. Primary memory is volatile - contents are lost when computer is switched off
2. Capacity of primary storage is limited by width of address bus

Question 2

What does secondary storage refer to? (1 point)

Give 3 examples of the thing it refers to

Answer 2

1. Secondary storage refers to non-volatile storage:

e. g.
1. Magnetic storage such as the hard disk

2. Optical storage that uses laser light such as CD-ROM
3. Solid state devices using Flash memory

Question 3

Different technologies have evolved for saving data
Each of these have their own advantages and disadvantages in terms of:
(6 points)

Answer 3

1. Purpose
2. Durability
3. Read / write speed
4. Capacity
5. Portability
6. Cost

Question 4

In the magnetic hard disk, _________ in the direction of magnetism represent 0s and 1s.

Answer 4

changes

Question 5

What are the principles of a magnetic hard disk working?

6 steps

Answer 5

1. The hard drive consists of metal disks which have been coated in a magnetic material
2. The disks are on a spindle and rotate at a high speed (3,600 – 12,500 rpm)
3. Concentric tracks are created on a magnetic disk
4. Each track is split into sectors that can be individually addressed
5. Sectors are grouped into clusters to manage storage
6. The read/write head moves across the disk surface to access or store data on the disk

Question 6

What is latency? (1 point)

What types of latency are there? (3 types + describe each type)

NOTE: 1 point for each type

Answer 6

1. Latency is the time taken to read/write disk data

This includes:

1. Seek delay - the time the head takes to move across the disk
2. Rotational delay - the time the disk takes to move to the correct sector underneath the read/write head
3. Transfer time to move the actual data

Question 7

How has the HD capacity been increasing despite staying the same physical size?
(3 points)

Answer 7

HDD capacity has been increasing steadily, despite the same physical size due to:

1. More densely packed platters
2. Smaller magnetic parts and read/write heads
3. Perpendicular over longitudinal recording

Question 8

Optical disks include… (3 things)

Answer 8

1. CDs
2. DVDs
3. Blu-Ray

Question 9

Explain how data is read from optical disks (3 steps)

Answer 9

1. Laser technology is used to read and write data
2. Made of a single spiral track, starting from middle and running till the edge of the CD
3. Laser reads the data within the track by measuring how much light is reflected

Question 10

What are the types of optical storage?
(3 types)
typical storage for each type?
is it ROM or/and R or/and RW?

Answer 10

CD (700MB):
ROM - can be written once and read many times
R - Can be written once and read many times
RW - Can be written (erased and rewritten several times) and read many times

DVD(4.7GB):
ROM - can be written once and read many times
R - can be written once and read many times
RW - can be written and read many times

Blu-Ray(25GB-128GB):
RW - can be written and read many times
NOTE for Blu-Ray: Data can be written until you reach the capacity of the disc. You cannot remove data.

NOTE:
In ROM, data is written by making dents in the disk - like in a vinyl
In R, data is written by making dye in the disk opaque OR changing the state of the metal in the disk so that it reflects light differently

Question 11

Explain how data is written and read on CD-ROM (5 steps)

Answer 11

1. On a plastic disk with a mirrored surface (a CD),
2. a high powered laser “burns” pits into the CD surface to write data.
3. A low-powered laser detects the reflection from pits and lands
4. Pit end deflects the laser light and is read as a binary 1
5. Protective layer is then coated on the CD surface

NOTE: With CD-ROM, data can be written once but read many times

Question 12

Explain the CD-R disk format (3 points)

Answer 12

1. Recordable formats use a translucent, photosensitive dye
2. that becomes opaque when heated by a laser at high power.
3. Because the high laser power permanently changes the dye, this format can be written only once.

Question 13

Explain the CD-RW disk format (4 points)

Answer 13

1. These discs feature a layer of reflective phase-change material, for example an aluminium alloy.
2. This material can exist in two different solid states: crystalline or amorphous
3. Re-writable formats use a laser to change the state of the reflective phase-change material
4. and a magnet to set the new state.

Question 14

Explain how data is written and read from CD-R

3 steps

Answer 14

1. Pulsing at high power, the laser in the drive can ablate or “burn” marks in the dye to write data.
2. The disk can then be read using the laser at a lower read power.
3. those marks look like pits to the detectors in the drive.

Question 15

Explain how data is written and read from CD-RW

4 steps

Answer 15

1. The amorphous state reflects less light than the crystalline state does.
2. Therefore, by starting with a disc surface in the crystalline state, heating with the laser can change small spots to the amorphous state, which will appear dark upon playback.
3. To erase or write over recorded data, a higher temperature laser is used, which results in the amorphous form, which can then be reformed by the lower temperature laser.
4. This process of erasing or writing over recorded data can be repeated about 1000 times.

Question 16

Why are the capacities of these disks (CDs, DVDs and Blu-Ray) different given they are all the same physical size?
(2 points)

Answer 16

1. Different laser wavelengths ‘burn’ smaller pits

2. The spiral track can therefore be more tightly wound, creating a longer track

Question 17

Explain solid state drive (5 points)

Answer 17

1. Based on non-volatile NAND flash memory with a controller to manage the data
2. The flash memory cells are grouped into a grid that is separated into sections (rows) called ‘pages’ into which data is stored
3. A block consists of many pages (rows)
4. No moving parts, hence there is less chance of mechanical failure
5. More costly than HDD

Question 18

Explain NAND Flash Memory

6 points

Answer 18

1. Data is saved to a grid or ‘pool’ of NAND flash memory
2. NAND itself is made up of what are called floating gate transistors
3. The floating gate transistors trap and store a charge - the charge is retained without power
4. If the floating gate is not charged (neutral), a positive charge in the control gate creates a channel in the p-substrate that carries a current from source to drain.
5. If the floating gate is negatively charged, then this charge blocks the current between source and drain.
6. For this gate, if electrons are trapped inside the middle layer it reads as a 1, if there are no trapped electrons it reads as a 0.

Question 19

Explain NAND Flash Memory in SSD

4 points

Answer 19

1. Data must be read, deleted or written in blocks.
2. Data cannot be overwritten without being erased first.
3. SSDs can only write to empty blocks.
   (as blocks get filled up, overwriting data becomes an issue, because the only way an SSD can update an existing page is to copy the contents of the entire block into memory, erase the block and then write the contents of the old block in addition to the new data)
4. If there are no empty blocks available, the SSD must scan for blocks marked for deletion, but not yet deleted, erase them, and then write the data to the now-erased page.
5. With SSDs, every block of data is accessible at the same speed as every other block, no matter the location. This makes SSDs inherently faster than hard drives, where platters are spinning and drive heads are moving to the right location.

Question 20

Give the types of media paired with typical capacity and access speed
(5 types)

Answer 20

Hard disk - 40GB/8TB - Fast

CD-ROM - 650/700MB - medium

DVD - 4.7/9.4GB - medium

Blu-Ray disk - 25/50GB - medium

Solid state disk - 4GB/1TB - very fast

Question 21

Give the advantages of a hard drive (2 advantages)

Answer 21

Advantages:

1. Hard disks have a high transfer rate
2. High storage capacity (typically 500GB to 10TB)

Question 22

Give the disadvantages of a hard drive (2 disadvantages)

Answer 22

Disadvantages:

1. Where hard disk is fixed inside the computer it can be difficult to transfer data to another computer
2. Care has to be taken handling portable hard disks due to their fragility

Question 23

Give the advantages of an optical disk (3 advantages)

Answer 23

Advantages:

1. Portable device that is cheap to produce - typical blank disks cost between 10p-50p each
2. Reasonable storage capacity for home and faster access time than magnetic tape
3. Optical drivers are compatible with CD and DVD disks so data can be read from either media

Question 24

Give the disadvantages of an optical disk (3 disadvantages)

Answer 24

Disadvantages:

1. Care needs to be taken handling as scratching the shiny data side renders it inoperative
2. Less storage capacity than a hard disk means it unsuitable for large volumes of data
3. Access time is greater than for hard disks and SSD devices

Question 25

Give the advantages of an SSD (2 advantages)

Answer 25

Advantages:

1. SSDs perform faster than hard drives as there are no moving parts in these devices; consequently they can read, write and access data faster
2. Latency is a feature of hard disks as they are slow to access data due to having to spin and position read/write heads over the disk to access data

Question 26

Give the disadvantages of an SSD (2 disadvantages)

Answer 26

Disadvantages:

1. SSDs are significantly more expensive to purchase than hard disks;
2. They tend to be smaller capacity than hard disks

Question 27

Most SSDs are based on ______ ______ memory cells capable of trapping a charge
using ________ ______ _________. The charge remains trapped even with a loss of ______.

Using a string of cells, a page or block is formed. Trapped and ‘untrapped’ charges are read as binary 1s and 0s.

A block consists of several ______.

Pages can be read individually, but writing and erasing data can only be performed in ______. This means that adding data to a page in a block, or editing data within a block requires the existing data to be transferred to a _________ _______ ________, edited there and transferred back again. Since data cannot be simply overwritten, the block must be _______ completely before it is re-written.

Answer 27

NAND Flash

floating gate transistors

power

pages

blocks

temporary memory buffer

erased