1.3.5 Computer Memory

Computer memory is any physical device capable of storing information temporarily or permanently. Computer memory is used for storing your files on the computer. Without computer memory, you would't be able to save any files.

Three types of Memory :

  • Cache memory
  • Primary memory/main memory
  • Secondary memory

Cache memory

The cache is a small amount of high-speed memory, usually with a memory cycle time comparable to the time required by the CPU to fetch one instruction. The cache is usually filled from main memory when instructions or data are fetched into the CPU. Often the main memory will supply a wider data word to the cache than the CPU requires, to fill the cache more rapidly.


  • Cache memory is faster than main memory.
  • It consumes less access time as compared to main memory.


  • Cache memory has limited capacity
  • It is very expensive.

Primary Memory / Main Memory

Primary memory is where information is stored after it is first obtained. The primary memory is also referred to as short-term memory. Information sent to primary memory storage is deciphered and either discarded or retained for later retrieval. Although the information can be stored for a time in the primary memory, it is subject to interference and information can be easily forgotten. Over time, information decays and is lost from the primary memory storage.


  • It is known as main memory.
  • It is working memory of the computer.
  • Faster than secondary memories.


Secondary memory is where programs and data are kept on a long-term basis. Common secondary storage devices are the hard disk and optical disks. secondary memory include disc drives for compact discs (CDs) or digital versatile discs (DVDs), as well as removable flash memory.


  • Slower than primary memories.
  • It is used for storage of data in a computer.
  • Computer may run without secondary memory.

Hard Disk

The disk described so far is the type of disks which can be removed from the disk drive and carried from place to place. Some disks however, are built into the computer or a special disk drive. These are fixed disks. They are used in just the same way as removable disks, but tend to be used for storing programs or data which have to be permanently available. Hard disks can store anywhere from 20 MB to more than 40GB. Hard disks are also from 10 to 100 times faster than floppy disk.


The CD-ROM (Compact Disk Read Only Memory) is a direct extension of audio CD. CD-ROM players are more rugged and have error correction facility. This ensures proper data transfer from CD-ROM to the RAM of the computer. CD-ROM is written using a CD writer. Information is retrieved from a CD-ROM using a low power laser, which ingenerate in an on optical disk drive unit. In CD-ROMs the information is stored evenly across the disk in segments of the same size.