It is used for storage of data and divided into primary and secondary memory.
Primary Storage Devices
It is primary or internal storage in all computers and consists of cache, register ROM & RAM.
RAM (Random Access Memory)
It provides random selection and usage of memory location to store and retrieve data. It is also called read/write memory because information can be read and can also be written into it. It is a volatile memory; it gets erased as and when the computer is switched off. Whenever a new data is stored, the previous data is erased and new data takes place. It is present in 256/512/1024 MB size.
ROM (Read Only Memory)
A read only memory (ROM) is one in which information is permanently stored. The information from the memory can only be read and it is not possible to write into it. It is non-volatile, i.e. when the power supply is switched off, the contents on ROM do not get erased, and they are permanent and are written only by the manufacturer. The programs, which are always required for running the machine, are stored in ROM like the BIOS (Basic Input & Output System) which checks the hardware when computer is powered and in case of problem it stops booting of computer and audio beeps are given. BIOS also load the Operating System into the memory.
They are located in the CPU to load instructions for execution by the CPU. They function with speed of CPU and hence are the fastest and most expensive memory. It is present in few KB.
It is used to speed up data supply to CPU. It stores the most frequently used data from the RAM. When the CPU needs the particular data item, it access from it which is closely located, instead of accessing the much slower RAM. It is present in KB (256/512).
Secondary Storage Devices
The magnetic tape is similar to an audiocassette. It uses plastic strip coated with the magnetic material as a storage medium in a plastic casing. The strip is a plastic ribbon approximately ½ inch in width and is housed in the cartridge. Magnetic tape is slower in accessing the data, but there input and output speeds are high. They are available up to 5120 GB capacity.
Hard Disk Drive
It is used for mass storage of data needed for direct access and is non-volatile. It is in 2.5” and 3.5” sizes. It has disk platters mounted on a spindle, on which data is read/written by read/write head. It comes in 250-1024 GB capacity and SATA (Serial ATA) or older PATA (Parallel ATA) interface.
Floppy Disk Drive
It is made of flexible plastic material coated with magnetic substance and packaged in a protective plastic envelope. Floppy disks are mainly used, for the purpose of backup and transportability of data, for small data volumes. Floppy comes in different sizes. 5.25”and 3.5”floppies. Floppy have 1.2 MB and 1.44 MB capacity. Flash memory has outdated the use of floppies.
An optical disk uses optics or light for storage and retrieval of information. The disk is made up of polycarbonate and is coated with a reflective material. High power laser beam burns the surface of disk for storing data and low power laser beam for reading data. Most common optical disk is the CD-ROM (Compact Disk – Read Only Memory) which comes with speed rating, which means the amount of data that can be transferred from the disk. 1X refers to 150 Kbps. CD- ROM has speed rating of 52X and 700 MB capacity. DVD is high capacity optical disk to store up to 4.3 GB of data. Blue ray disk is upcoming optical disk technology which can store up to 50 GB data.
The practical application of this type of volatile computer memory can be seen in USB flash drives, memory cards and upcoming SSD hard discs. These allow the data to be written to them and can be detached from the system. They can be attached to another computer system to read and transfer the data to it.