Section 5 Chapter 30 - Secondary Storage Devices Flashcards
Secondary storage
A slow but large storage that doesn’t lose it’s content when the computer is turned off
How a hard disk stores data
Multiple disks (platters) spin very fast and have a track of iron particles that are polarised to either north or south, representing 1 and 0. A drive head is then used to read or write
Optical disk
A disk that uses a laser, such as a CD or DVD
How an optical disk works
A high powered laser is used to burn sections of the surface and make them less reflective. A laser at a lower power is then used to read the disk by detecting whether light bounces off. At the beginning or end of a pit the light is scattered and not detected, this is a 0.
There is a single track arranged in a tight spiral.
How rewriteable compact disks work
A laser and magnet is used to heat a spot on the disk and then set its state with the magnet
How an SSD works
They are made of millions of NAND flash memory cells and a controller that manages pages and blocks of memory. A current is run along the word line which forces some electrons across the oxide layer onto the floating gate. The state of the gate is determined by the charge in the floating gate
How data on an SSD is organised
Data is stored in pages of roughly 4kb.
Pages are grouped into blocks of roughly 512kb.
How SSDs handle overwriting
The whole block of pages must be wiped so a copy of the data is made to copy from
Advantages (2) and Disadvantage of a Hard Disk
A - Very large capacity and can be read and written to
D - Not very portable
Advantage and Disadvantages (2) of an Optical Disk
A - Portable
D - Small capacity, cannot be written to (unless its a rewriteable disk)
Essence of how an SSD stores data
Floating gate transistors that trap and store charge
Advantages of an SSD (4)
- Low latency
- Faster transfer speeds
- Consume less power
- Quiet