Memory: 

Storage Unit(Memory Unit): A computer has huge storage capacity. It is used to store data and instructions before starts the processing. Secondly it stores the intermediate results and thirdly it stores information(processed data), that is the final results before send to the output unit(Visual Display Unit, Printer, etc) 

Memory measuring units are given below. 

• 1 bit = 1 or 0(Binary Digit) 
• 4 bits = 1 Nibble 
• 8 bits = 1 Byte 
• 1024 Bytes = 1 KB(Kilo Byte) 
• 1024 KB = 1 MB(MegaByte)
• 1024 MB = 1 GB(Giga Byte) 
• 1024 GB = 1 TB(Tera Byte) 
• 1024 TB = 1 PB(Peta Byte) 

Two Types of storage unit: 

1. Primary Storage alias Main Memory: It is further be classified into Two Random Access Memory (RAM) and Read Only Memory(ROM). The one and only memory that the CPU can directly access is the main memory at a very high speed. 

It is expensive hence storage capacity is less. RAM is volatile(when the power is switched off the content will be erased) in nature but ROM is non volatile(lt is permanent). In ROM a “boot up” program called BIOS(Basic Input Output System) is stored to “boots up” the computer when it switched on. 

Some ROMs are given below. 

1. PROM(Programmable ROM): It is programmed at the time of manufacturing and cannot be erased. 

2. EPROM (Erasable PROM): It can be erased and can be reprogrammed using special electronic circuit. 

3. EEPROM (Electrically EPROM): It can be erased and rewritten electrically 

Cache Memory: The processor is a very high speed memory but comparatively RAM is slower than Processor. So there is a speed mismatch between the RAM and Processor, to resolve this a high speed memory is placed in between these two this memory is called cache memory. Commonly used cache memories are Level(L1) Cache(128 KB), L2(1 MB),L3(8 MB), L4(128MB). 

2. Secondary Storage alias Auxiliary Memory : Because of limited storage capacity of primary memory its need arises. When a user saves a file, it will be stored in this memory hence it is permanent in nature and its capacity is huge. 

Eg: Hard Disc Drive(HDD), Compact Disc(CD), DVD, Pen Drive, Blu Ray Disc etc. 

(a) Magnetic storage device: It uses plastic tape or metal/plastic discs coated with magnetic material. Hard Disk: Instead of flexible or soft disk it uses rigid material hence the name hard disk. Its storage capacity and data transfer rate are high and low access time. 

These are more lasting and less error prone. The accessing mechanism and storage media are combined together in a single unit and connect to the mother board via cable. 

(b) Optical storage device: 

Optical Disk: The high power laser uses a concentrated, narrow beam of light, which is  focuses and directed with lenses, prisms and mirrors for recording data. This beams burns very very small spots in master disk, which is used for making molds and these molds are used for making copies on plastic disks. 

A thin layer of aluminium followed by a transparent plastic layer is deposited on it. The holes made by the laser beam are called pits, interpreted as bit 0 and unburned areas are called lands interpreted as bit 

1. Lower power laser beam is used to retrieve the data. 

1. DVD(Digital Versatile Disc): It is similar to CD but its storage capacity is much higher. The capacity of a DVD starts from 4.7 GB 

2. Blu-ray Disc: It is used to read and write High Definition video data as well as to store very huge amount of data. While Cd and DVD uses red laserto read and write but it uses BlueViolet laser, hence the name Blu ray disc. The blue violet laser has shorter wave length than a red laser so it can pack more data tightly. 

3. Semiconductor storage (Flash memory): It uses EEPROM chips. It is faster and long lasting. 

• USB flash drive: It is also called thumb drive or pen drive. Its capacity varfes from 2 GB to 32 GB. 
• Flash memory cards: It is used in Camera, Mobile phones, tablets etc to store all types of data.