3.1 Storage Devices
Classification of storage devices:
|a) Processer Component||Register||<See chapter 5>|
|b) Primary Storage*||Random-Access Memory (RAM)||DRAM|
|Read-Only Memory (ROM)||ROM|
|c) Secondary Storage||Magnetical Media||HDD|
* Another way to categorize Primary storage is “Cache memory” and “Main memory”. Cache memory stores data most likely to be used again by the computer.
Going down the table from registers to secondary storages:
- Access time: Longer
- Capacity: Higher
- Storage size: Bigger
- Cost: Lower
The fastest components have limited capacity, are expensive, and they are volatile. Therefore a combination of all sections of memories are used. It’s a compromise.
3.1.b Primary Storage
Consists of two types: RAM and ROM.
Primary memory have the characteristics of random access / direct access, meaning memory can be access at any location, not restricted by previous access location.
Random-access Memory: Volatile memory, can be read/write for any number of times.
|Used In||Cache Memory||Main Memory|
|Energy||Less energy||More energy|
Requires regular recharging
* There are contradictions between the textbook and past papers
on which chip consumes more energy.
The table follows the answers from past paper.
Read-only Memory: Non-volatile memory, can not be written to, but can be read from for any number of times.
- PROM: Programmable ROM
- EPROM: Erasable Programmable ROM
- EEPROM: Electronically Erasable Programable ROM
3.1.c Secondary Storage
i) Magnetic Media – HDD
Direct-access, read-write storage.
Structure of the Hard Disc Drive:
- The alternative states on the discs are interpreted by 1/0.
- All discs spin in union.
- Read / Write Head
- Each disk has a Read / write head for each side.
- The head never touch the surface of the plate.
- An actuator move the Read / write head.
- All actuators are synchronized
- Data is stored in concentric tracks.
- The track is divided into bits, then formatted into sectors, consisting a defined number of bytes. The sector is the smallest unit of storage.
- Data has to be read sequentially in sectors.
ii) Optical Media – Discs
|Read / Write||Read Only||Read & Write|
Writing laser melts the surface.
The surface reverts into the two states,
Depending on the intensity of the laser.
Other previous models: CD(Compact Disk), DVD (Digital Versatile Disc)
- A single spiral track from the center to periphery.
- By formatting the track, direct-access can be done.
- Storage capability
- How close different binary digits can get
- Wavelength of light
- Blue-ray discs
iii) Solid-State Media
Based on flash memory (A form of EEPROM), where the programmable part is for normal use.
Uses transistors, which has no moving parts. NAND gates are usually used.
Read / Write operation had to be conducted on entire block of memory as a whole. e.g. Before writing any data on the block, the whole block has to be removed first, then re-write the new data into the block.
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