COMPUTER MEMORY

       Memory is the most essential part of the computer. Memory is the storage part of the computer. We need to store the data. This storage enables us to use stored data in the future. Computer memory is mainly divided into Primary memory, Secondary memory, and Cache memory. The memory and storage devices are measured in terms of bits and bytes. The measurements used are 0, 1 binary digit.

1. Primary Memory :

Memory units are the storage areas in the computer. The term "memory" usually refers to the main memory of the computer, whereas the word "storage" is used for the memory that exists on the desks, CDs, etc. The main memory is usually called a physical memory which refers to the 'chip'  ( Integrated Circuit) capable of holding data and instructions. 

There are different types of memories as follows :

a. Read-Only Memory ( ROM ) :

ROM chip

    

 In ROM, the information is burnt or pre-recorded into the ROM chips at the manufacturing time only. Once data has been written into the ROM chip, it cannot be erased but you can read it. When we switch off the computer, the contents of the ROM are not erased but remain stored permanently. ROM is a non-volatile memory. ROM stores critical programs such as the programs that boot the computer.

b. Programmable Read-Only Memory ( PROM ) :

PROM chip

 PROM is the memory on which data can be written only once. A variation of the PROM chip is that it is not burnt at the manufacturing time, but can be programmed using PROM programmer or a PROM burner. PROM is also non-volatile memory.

c. Erasable Programmable Read-Only Memory ( EPROM ) :

EPROM chip

In EPROM, the information can be read and re-programmed using a special PROM-programmer. EPROM is a non-volatile memory. An EPROM differs from a PROM in that a PROM can be written only once and cannot be erased. But ultraviolet light is used to erase the contents of the EPROM.

d. Electrically Erasable Programmable Read-Only Memory ( EEPROM ) :

EEPROM chip

   

 EEPROM is a recently developed type of memory. This is equivalent to EEPROM but does not require ultraviolet light to erase its contents. It can be erased by exposing it to an electrical charge. It is also a non-volatile type of memory. EEPROM is not as fast as RAM and other types of ROM. Flash memory is a special type of EEPROM that can be erased and re-programmed.

e. Random Access Memory ( RAM ) : 

RAM chip

   RAM is the most common type of memory found in modern computers. This is the place where programs get stored temporarily. When the CPU runs a program, it fetches the program instructions from the RAM and carries them out. If the CPU needs to store the results of the calculations it can store them in RAM. When we switch off the computer, whatever is stored in the RAM gets erased. It is a volatile form of memory.

 Types of RAM :

*  EDO RAM ( Extended data output RAM ) : 

      EDO RAM is a type of random access memory chip that improves the time to read from memory on faster microprocessors such as the Intel Pentium. EDO RAM was initially optimized for the 66MHz Pentium.

*  SRAM ( Static random-access memory ) :

    It is a type of semiconductor memory that uses bistable latching circuitry to store each bit. The term static differentiates it from dynamic RAM ( DRAM )  which must be periodically refreshed. SRAM exhibits data remanence but is still volatile in the conversational sense that data is eventually lost when the memory is not powered. 

2. Secondary Memory :

        As primary memory is volatile, the computer uses secondary memory to store data, programs, etc. Secondary storage is also called backup storage. Secondary storage can be used to transmit data to another computer either immediately or at a later time. This provides a mechanism for storing a large amount of data for a long period of time. Some of the commonly used secondary storage devices are hard disk, magnetic tapes, floppy disks, and CD-ROM. To understand the physical mechanism of secondary storage devices one must have basic knowledge of magnetism, electronics, and electro-mechanical systems. The average time required to reach a storage location and obtain its contents is called access time.

 ex-

  * Hard disk drive :

     

 The hard disk is a magnetic disk on which you can store computer data. The hard disk is a direct- storage medium. This means you can store and receive the data randomly. Disk storage systems are essentially based on magnetic properties. 

* Floppy disk :

FDD

    The floppy drive uses a thin circular disk for data storage. It is a soft magnetic disk. It is a thin magnetic coated disk contained in flexible or semi-rigid projective jackets. The disk rotates at 360rpm. Floppy disks are slower to access than hard disks and hve less storage capacity. It is less expensive and is portable. It can be assembled randomly.

3. Cache Memory :

       The cache is a small amount of high-speed memory, present between the primary school and CPU. There are two levels of cache memory L1 and L2, L1 cache memory will present inside the data or instructions. Usually with a memory cycle time comparable to the time required by the CPU to fetch one instruction. The cache is usually filled from the 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 memory rapidly.