Computer Memory

Computer memory is just like human brain. A memory or Store is required in a computer to store the programs and data. A memory is made up of a large number of cells, with each cell capable of storing one bit. The cells may be organized as a set of addressable words, each word storing a sequence of bits. In one such organization, the time to store or retrieve a word is independent of the address of the word. This organization, called a Random Access Memory (RAM), is used as the main memory of computers. Another organization arranges cell in a linear sequence to form a serial access memory. Cells used to fabricate a random access memory are made (in current technology) with semiconductor flip-flops or capacitors.  Cells used in serial access memories of large size (hundreds of gigabytes) are magnetic dipoles on movable magnetizable surfaces. Magnetic surface recording itself may be organized in different configurations, such as hard disks and tapes. Optical recording with laser beams are now being used to store data on the surface of a coated disk. Such a series access memory is known as CDROM (Compact Disk Read Only Memory).  Better optical recording methods which records data on multiple layers on a disk surface has also been introduced. This storage device is known as Digital Versatile Disk Read Only Memory (DVD ROM).

A computer system needs memory to store the data and instructions for processing. Whenever we talk about the ‘memory’ of a computer system, we usually talk about the main or primary memory. The secondary memory (also called storage device) is used to store data, instructions and results permanently for future use.

{tocify} $title= {Table of Contents}

 

Various memory organizations discussed in this article can be classified on the basis of their storage capacity, the method of storing and retrieving data, and the time needed to store and retrieving data, the time needed to store and retrieve data, the cost per bit of storage and whether it requires continuous supply of power to retain data stored in it. As the speed of data retrieve increases, the cost per cell goes up. In a computer system, a hierarchy of storage systems is provided. At the top of the hierarchy is a very low -speed, low- cost, high-capacity memory which requires no power while it stores data. The speeds and capacities at various levels are chosen so that for a given budget one gets a well-balanced system which meets application requirements.

Computer memory is of two types: Volatile (RAM) and Non-volatile (ROM). The secondary memory (hard disk) is referred as storage not memory.

But, if we categorize memory on behalf of space or location, it is of four types:

•       Register memory
•    Cache memory
•     Primary memory
•     Secondary memory

Storage Evaluation Criteria

Any storage unit of a computer system is characterized and evaluated based on the following properties:

1.     Storage capacity:

It is the amount of data, which can be stored in storage unit. A large capacity is desired. As compared to secondary storage units, primary storage units have less storage capacity.

2.     Access time

This is the time required to locate and retrieved stored data from the storage unit, in response to a program instruction. A fast access time is preferred. As compared to secondary storage units, primary storage units have higher cost per bit of storage.

3.     Cost per bit of storage

This refers to the cost of a storage unit for a given storage capacity. Obviously, a lower cost is desirable. As compared to secondary storage units, primary storage units have higher cost per bit of storage.

4.     Volatile

If the storage unit can retain the data stored in it, even when the power is turned off or interrupted. It is called non-volatile storage. On the other hand, if the data store are lost, when the power is turned off or interrupted, it is called volatile storage. Obviously, a non - volatile storage is desirable. In almost all computer system, the primary storage units are volatile and the secondary storage units are non-volatile.

5.     Random access

If the time taken to access a piece of data from the storage unit is independent of the location of the data in the storage unit, it is called a random access storage or random access memory (RAM). Each separate location of a RAM is as easy to access as any other location, and takes the same amount of time. In almost all computer systems, the primary storage units have random access property, and the secondary storage units have either pseudo-random access (access time is nearly same for all locations but not exactly same), or sequential access (access time directly depends on the location of the data) property. 

Memory Cell

A memory cell may be defined as a device which can store a symbol selected from a set of symbols and may be characterized by the following properties:

1.                    The number of stable states in which it can be placed.
2.                    Whether a cell can store a symbol indefinitely even when power is turned off.
3.               Whether, after reading a symbol from a cell, the stored symbol is retained in the cell or distributed.
4.                    The time taken to read a symbol from a cell and the time taken to write a new symbol in it.
5.                    Whether a symbol, once written, can only be read and not changed.

The number of stable states in which a cell can be placed determines the number of distinct symbols it can store. Each stable state may be assigned to represent a symbol. Thus, if a cell can be placed in 10 stable states, each state may be used to represent one symbol and so the cell can store a decimal digit. If a cell can be placed in only two stable states, then one stable state can be used to represent 0 and the other a 1. Thus the cell may be used to store a binary digit.

·        Volatile cell:

The symbol stored in a cell disappears when no energy is supplied, it is known as a volatile cell.

·        Non-volatile cell:

A symbol can be stored in a cell indefinitely a without continuous supply of energy, it is known as a non-volatile cell.

Normally, if a symbol is read from a cell its content should not be disturbed. Such a cell is known as one where readout is non-destructive. If the symbol is erased, as a consequence of reading, the readout is said destructive. The time taken to read a symbol from a cell is called read-time and the time taken to write a symbol from a cell is called write-time.

If in a memory cell, a symbol is permanently written and can only be read, then it is known as a read-only cell.

Using the computer memory

The computer starts using the memory from the moment the computer is switched on, till the time it is switched off. The list of steps that the computer performs from the time it is switched on are—

·         Turn the computer on.

·         The computer loads data from ROM. It makes sure that all the major components of the computer are functioning properly.

·         The computer loads the BIOS from ROM. The BIOS provides the most basic information about storage devices, boot sequence, security, plug and play capability and other items.

·         The computer loads the OS from the hard drive into the system’s RAM. CPU has immediate access to the OS as the critical parts of the OS are maintained in RAM as long as the computer is on. This enhances the performance and functionality of the overall system.

•          Now the system is ready for use.
•          When you load or open an application it is loaded in the RAM. Since the CPU looks for information in the RAM, any data and instructions that are required for processing (read, write or update) is brought into RAM. To conserve RAM usage, many applications load only the essential parts of the program initially and then load other pieces as needed. Any files that are opened for use in that application are also loaded into RAM.

·         The CPU requests the data it needs from RAM, processes it and writes new data back to RAM in a continuous cycle. The shuffling of data between the CPU and RAM happens millions of times every second.

·         When you save a file and close the application, the file is written to the secondary memory as specified by you. The application and any accompanying files usually get deleted from RAM to make space for new data.

·         If the files are not saved to a storage device before being closed, they are lost.

Sometimes, when you write a program and the power goes off, your program is lost if you have not saved it. This is because your program was in the RAM and was not saved on the secondary memory; the content of the RAM gets erased when the power is switched off.

Summary:

•          A memory or store of a computer is organized using large number of cells. Each cells store a binary digits.
•          A memory cell which does not lose the bit stored in it when no power is supplied to the cell is known as a non-volatile cell.
•          The time taken to store a bit in a cell and the time taken to read it are important parameters of a cell.
•          Using the computer memory turns the computer load data into memory when the power is switched off the content of the RAM of memory gets erased.
•          The memory cell which does not lose the bit stored it when without continuous supply of energy to the cell is known as non-volatile cell.