3.7.4.2 Secondary Storage Devices. Flashcards
What is a computers primary storage?
Memory such as RAM or ROM.
What is a computers secondary storage?
Use to store files and applications, includes HDD’s, SSD’s and optical disks.
What does a HDD consist of?
A number of circular platters which are made from magnetic material. Above each platter hovers an actuating arm on which is a read/write head.
What does the actuating arm allow for (HDD)?
The actuating arm allows the read/write head, which changes the magnetic polarity of parts of the platter, to access all portions of each platter.
How is data written in HDD’s?
In concentric tracks, each of which is further divided into sectors.
Why do HDD’s have good read/write speeds?
The platter rotates thousands of times per minute, allowing for good read and write speeds.
What is the typical HDD size?
Between 500GB and 5TB.
How can we increase the capacity of a HDD whilst still keeping it the same size?
Adding more platters and decreasing the width of the tracks.
Why are HDD’s susceptible to damage?
Because of the number of moving parts in hard disk drives, they are susceptible to damage from movement. This makes them unsuitable for use in portable devices like phones and tablets.
What does a SSD consist of?
NAND flash memory cells and a controller than manages the structure of data on the drive.
Is NAND flash memory volatile or non-volatile? What does this mean?
Non-volatile, meaning that an SDD’s contents are retained even when there is no power being supplied.
What forms the memory cells of an SSD?
The memory cells are floating gate translators which store information by trapping an electrical charge.
How is data stored on an SDD?
In pages, which are combined to form blocks.
Can SDD’s overwrite data?
No, instead a controller must completely erase the entirety of a page before writing new information to it.
Why do SDD’s have higher read//write speeds than HDD’s?
Because SSDs don’t have any moving parts, they are capable of far higher read and write
speeds than HDDs and are suitable for use in portable devices like phones and tablets.
Define an optical disk.
Store information which can be read optically by a laser. Can be either: Read-only. Recordable. Rewritable. Depending on what they are used for.
Describe the characteristics of an optical disk.
Disks have stripes called pits; the areas surrounding the pits are known as lands.
How are pits created?
Pits are burnt into the disk by a high-power laser which permanently deforms the surface.
How many tracks do optical disks have?
Unlike hard-disk drives which use tracks and sectors, optical disks have just one.
continuous track which spirals from the centre of the disk to the outside edge.
Outline the process of an optical disk.
Low power laser beam is passed over the flat surface of an optical disk. Reflects back onto a photodiode. However, when the laser is incident on a pit, the light from the beam is scattered in different directions rather than reflected back at the photodiode. The resulting pattern of reflections and scatters can be converted into a digital signal of binary
1s and 0s.
What role does opaque dye play in an optical disk?
On recordable and rewritable optical disks, a pattern of reflections and scatters is created not by pits and lands but by an opaque dye on the disk’s surface. Where there is no dye, the disk reader’s laser beam is reflected off of the optical disk’s surface. Where there is dye, the laser beam is absorbed by the dye and not reflected at all.
What purpose does photosensitive dye have in an optical disk?
Recordable optical disks use a special photosensitive dye which changes from opaque to
transparent under a high-power laser which is used to write information to the disk. The dye remains unaffected by the low-power laser used to read the disk.
Capacity:
- HDD
- SDD.
- Optical disk.
- High capacity. Typically
between 500GB and 5TB. - Relatively low capacity.
Typically under 1TB.
3.Very low capacity. Blu-rays
have the highest capacity
at 25GB.
Read/Write speeds:
- HDD
- SDD.
- Optical disk.
- Good speeds.
≈ 100MB/s. - Very high speeds.
≈ 500MB/s - Relatively low
speeds.
≈ 30MB/s
