Memory Unit and types of memory

 Memory Unit

Found on the motherboard

• Short Term

Random Access Memory (RAM)

• Long Term

Read Only Memory (ROM)

Random Access Memory (RAM)

RAM is a read/write memory. Information can be written into and read from a RAM. It is a volatile memory. It retains the stored information as long as it is supplied with power supply is switched off or interrupted, the stored information in the RAM is lost. RAMs of various capacities are available for example, 256MB, 512MB and so on.

Read Only Memory (ROM)

Rom is a permanent type memory. Its content are not lost when power supply is switched off. The user cannot write into a ROM. Its contents are written into at manufacturing time. ROMs store permanent programs and other types of data which are needed by the computer to execute user program.

Types Of ROM

1. PROM (Programmable ROM)
2. EPROM (Erasable PROM)
3. EEPROM (Electronically Erasable PROM)

Cache Memory

The data or contents of the main memory that are used frequently by CPU are stored in the cache memory so that the processor can easily access that data in a shorter time. Whenever the CPU needs to access memory, it first checks the cache memory, then the CPU moves into the main memory. Cache memory is places between the CPU and the main memory. The cache is the fastest component in the memory hierarchy and approaches the speed of CPU components.

The basic operation of a cache memory is as follows:

1. When the CPU needs to access memory, the cache is examined. If the word is found in the cache, it is read from the fast memory. If the world addressed by the CPU is not found in the cache, the main memory is accessed to read the word.
2. The performance of the cache memory is frequently measured in terms of a quality called hit ratio.
3. When the CPU refers to memory and finds the word in cache, it is said to produce a hit.
4. If the word is not found in the cache, it is in main memory and it counts as a miss.

Hard Disk

Hard Disk is a device used for mass storage of data. The data stored on a Hard Disk can be retrieved at a very fast speed, being a direct address device. Unlike Floppies, CDs, etc. You cannot easily remove hard disk from a PC.

Solid State Drive (SSD)

SSD (Solid State Drive) is also a non-volatile storage medium that is used to hold and access data. Unlike a hard drive, it does not have moving components, so it offers many advantages over SSD, such as faster access time, noiseless operation, less power consumption and more.

As the cost of SSD has come down, it has become an ideal replacement for a standard hard drive in desktop and laptop computers. It is also suitable for notebooks and tablets that don't require lots of storage.

Virtual Memory 

Virtual Memory is a storage scheme that provides user an illusion of having a very big main memory. This is done by treating a part of secondary memory as the main memory.

In this scheme, user can load the bigger size processes that the available main memory by having the illusion that the memory is available to load the process.