COMPUTER MEMORY

MEMORY ORGANIZATION

A memory unit  is the collection of storages units or devices together. The memory units stores the binary information in the form of bits. Generally memory/storage is classified into two categories.

Volatile Memory  : This loses its data, when power is switched off.
Non Volatile Memory : This is a permanent and does not lose any data when power is switched off.

RAM = Random Access Memory.
ROM = Read Only Memory.
HDD = Hard Disk Drive.
SDD = Solid State Drive.
USB = Universal Serial Bus.
SRAM = Static RAM.
DRAM = Dynamic RAM.
SDRAM = Synchronous Dynamic RAM
RDRAM = Rambus Dynamic RAM.
DDR SDRAM = Double Data Rate Synchronous Dynamic RAM.

MAIN MEMORY
The memory unit that communicates directly within the CPU auxiliary memory and cache memory is  called Main memory.
It is the central storage unit of the computer system. It is a large and fast memory used to store data during computer operations.
The main memory occupies the central position because it is equipped to communicate directly with the CPU and with auxiliary memory devices through input /output processor.

D RAM 
Dynamic RAM, It is made of capacitors and transistors and must be refreshed every 10- 100 ms. It is slower and cheaper than S RAM.

S RAM 
Static RAM, has a six transistor circuit, in each cell and retains data, until powered off

N VRAM
Non Volatile RAM, retains its data, even when powered off.

ROM  Read Only  Memeory.
It is volatile and is more like a permanent storage for information. It also stores the boot strap loader program, to load and start the operating system when computer is turned on.

Characteristics of Main Memory.

• These are Semiconductor memories.
• Usually volatile memory.
• Data is lost in case power is switched off.
• It is working memory of the computer.
• Faster than secondary memories.
• A computer can not run without the main memory.

SECONDARY MEMORY

• This type of memory is also known as external memory or non volatile memory. It is slower than main memory.
• These are used for storing data/information permanently.

Characteristics  of Secondary Memory.

• These are magnetic and optical memories.
• It is known as backup memories.
• It is a Non Volatile memory.
• Data is permanently stored even if power is switched off.
• Storage of data in a computer.
• Computer can run without Secondary Memory. 

AUXILIARY MEMORY 

• Auxiliary memory also known as auxiliary storage, secondary storage, secondary memory or external memory is a non volatile memory that is not directly accessible by the CPU, because it is not accessed via the input/output channel. 
• Auxiliary Memory access time is generally 1000 times that of the main memory, hence it is at the bottom of the hierarchy.
• Devices that provide backup storage are called auxiliary memory. Magnetic Disks and tapes are commonly used auxiliary devices such as optical disks.
• It is not directly accessible to the CPU, and is accessed using the input/output channel.
• The most common form of auxiliary memory are flash memory, optical disks, magnetic disks, and magnetic tape.  The latest addition to the auxiliary memory family is flash memory. 

FLASH MEMORY

An electronic  non volatile computer storage device that can be electrically erased and reprogrammed and work without any moving parts. Examples flash drives, memory cards. 

OPTICAL DISKS

A storage medium from which data is read and written by lasers. Optical disks can store much more data upto 6 gigabytes. more than most portable  magnetic media such as floppies. there are three types of optical disks  CD/DVD ,   WORM (Write once read many), EO (Erasable optical disks).

MAGNETIC DISKS

A magnetic disks is a circular plate constructed of metal or plastic coated with magnetized material. Both side of the disk are used and several disk may be stacked on one spindle. with read/write head available on each surface.  Bits are stored on the surface in spots along concentric circles called tracks. Tracks are commonly divided into sectors.. Examples Floppy Disk Drive, Hard Diak Drive.

CACHE MEMORY

• The Cache memory is used to store program data which is currently being executed in the CPU.
• Approximate time ratio between cache memory and main memory is 1 to 7.
• The data or content of the main memory that are used again and again by the CPU are stored in cache memory so that we can easily access that data in shrter time.

Characteristics of Cache memory

• Cache memory is faster than main memory.
• It consumes less access time as compared to main memory.
•  It stores the program that can be executed within a shorter period of time.
•  It stores data for primary use.
• It has limited capacity.
• Its very expensive.

MEMORY ACCESS METHODS
Each memory type is a collection of numerous memory locations. To access data from any memory first it must be located and then the data is read from the memory location.

Random Access Main memories are the random access memories in which each memory location has a unique address.

Sequential Access This method allows memory access in a sequence or in order.\

Direct Access In this mode information is stored in tracks, with each track having a separate read/write head.


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