Secondary Storage Device

Question 1

SSDs are

Answer 1

Electronic device uses flash memory
No moving parts
Uses NAND flash memory

Question 1

HDDS are

Answer 1

Drive is mechanical device with many moving parts
Data represented on disk surface
Binary data represented by magnetised and non magnetised

Question 2

Principles of operations of a HDD

Answer 2

1. Disk unit has metal platters (these are covered in magnetic coating) Each platter is double sided and number of them is the total storage capacity
2. Binary stored on platter as a polarised state
3. Platter broken into set of tracks. Tracks are broken into sectors. Contiguous group is called a cluster.
4. To read and write data, device has two mechanical arms, for each side of platter. End of arm a read-write head with tiny magnet. Thin gap between head and disk surface. Platter spins and the head sweeps across tracks, while sector pass underneath head as it rotates
5. Location of file recorded in file allocation record. Allows file to be retrieved, and gives free space for new files to be stored
6. When disk is close to full, common for many files to be split across many blocks non-contiguously. Requires a file allocation that keeps track of where segmented files are as well as what order they are in.

Question 3

Principles of operations of SSD

Answer 3

1. SSD made up of controller and millions of NAND ‘flash’ memory cells
2. Each cell has floating gate transistor allowing electrical charge to be trapped. Charged represents 0 and no trapped charge is 1
3. NAND flash cells organised in pages and blocks. Page is cell within a grid. Block is each row of cells
4. Data is read by a page. Can be written a page at a time as long as surrounding cells are empty
5. If file needs updating, cannot be done in original place. Relevant block containing new page must be copied to main memory and updated. Written to a new area and original block(s) will be erased

Question 4

Different optical disks

Answer 4

CD-ROM and DVD-ROM - read only, typically used to store movies

CD-R and DVD-R - WORM (Write-once-read-many). Used to archive

CD-RW and DVD-RW - rewritable. Used to backup temporary files

Question 5

Principles of operations of a Optical disk

Answer 5

1.Made of a set of layers, thin aluminium reflective layer on disc surface. Each disk has continuous track of data in spiral. Data store in pits and lands in structure of disk.

2.Data is read serially, disk rotates at varying speeds.

3. To read data light from laser is shone onto surface of disk. Light reflects of the layer and hits optoelectronic device that detect change in light. Pits and lands reflect different amount of light to interpret the changes as data bits.

4.To write laser in optical disk driver used at higher intensity. Light will burn tiny dots into chemical layer to alter amount of light reflected. Creates readable surface representing binary value

5. With RW disks, chemical dyes used to allow burning process be reversed, so data can be erased