A computer needs storage devices, i.e. memory units to store data. All data are not of the same priority to the CPU, some data is needs frequently, and some rarely. That is why there are different types of memory units.
Primary Memory Unit
Primary memory unit provides the fastest access to the CPU for necessary information. There are two types of memory – Random Access Memory (RAM) and Read Only Memory (ROM).
Random Access Memory (RAM)
Random Access Memory or RAM is used by the CPU to fulfil any task that is given by the user. RAM is a volatile memory, the data is stored in the RAM as long as the computer is running, once the computer is shut down, all the data from RAM is deleted. RAM is a very fast memory unit and that is why it is used by the CPU to store information needed to run any program.
Read Only Memory (ROM)
Read Only Memory or ROM is only readable, data stored in ROM cannot be altered without special programs. Unlike RAM, data stored in ROM is not deleted even after the computer is shut down. For example, the BIOS is a ROM, data stored in it is the first necessary ones after the computer is turned on.
Cache Memory is a very fast memory unit which acts as a buffer between the CPU and primary memory unit. The cache memory is also known as Static RAM (SRAM).
Secondary Memory Unit
Secondary Memory Units, i.e. storage devices are used to store large amount of information. There are different types of storage devices.
Floppy Disks were small plastic disks covered with Magnetic Oxide that was then put in a square cartridge. Floppy disks were very popular as small portable storage devices, like the pendrives of this age. But a floppy disk could only store data of 2 MB.
Hard Disk Drive
Hard Disk Drive or HDD is a storage device that is used to store large amount of data. It is a spinning disk, and a head reads data off that spinning disk. Capacity of a HDD spans from a few GB to a few TB. For example, 1TB HDD is common in modern day consumer laptops, whereas one can buy 6TB HDD if necessary.
Solid State Drive
Solid State Drive or SSD is also a storage device that can store large amount of data. But unlike a HDD, SSD has no spinning parts, SSD uses flash memory to store data, and that makes SSD a very fast and more reliable storage. From the outside it looks just like the common Hard Disk Drive.
CD, DVD and Blu-RAy Disc
CD (Compact Disks) and DVD (Digital Versatile Disks) are optical disks that usually store data that is read-only. CD and DVD were very popular a few years ago, mainly to store movies, songs, games, etc. But with time, digital distribution of content was on the rise, making the use of CD and DVD unnecessary. Now-a-days, Blu-Ray disks have come up which are able to store a lot more information than DVDs could. The difference between all of them was based on two factors – which type of laser they used and the storage disc was made of different material.
Apart from these, there are pen drives, punch cards, cassette tapes etc. Pen drives use flash storage, like SSD, but the quality of the flash storage is significantly lower. Punched card is a paper media, where holes are punched to store data as 0 and 1. Cassette tapes use thin plastic strips to store data.
For Wikipedia entry on Computer Memory, click here.
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