Computer Memory and its Terminologies

WHAT IS COMPUTER MEMORY?

In computing, memory refers to the physical devices used to store programs (sequences of instructions) or data on a temporary or permanent basis for use in a computer.

Memory is what enables your PC to operate. The computer's CPU (central processing unit) is the computer's brain, but a brain is useless without memory.

One of the major advantages that digital systems have over analog systems is the ability to easily store large quantities of digital information and data for short or long period of time. In a digital computer the internal main memory stores instructions that tell the computer what to do so that the computer will do its job with a minimum amount of human intervention.  Magnetic tape and magnetic disk are popular storage devices that are much less expensive than internal memory devices.

MEMORY TERMINOLOGY

1.       Memory cell:

Computer memory is made up of memory cells, where each memory cell contains exactly one number. This is a device or electrical circuit used to store a single bit (0 or 1).  The number contained in a memory cell can be changed over time. When a new number is stored into a memory cell, the old number contained in the memory cell is lost forever. At any time, the computer may examine into a memory cell to read the current contents of the memory cell. A typical computer has millions of memory cells, and every memory cell has a name. The unique number that names a memory cell is called its `address.' No two memory cells have the same address. The address of a memory cell never changes over time. Every memory cell has two numbers -- its address (which never changes) and its current contents (which changes over time.)

2.       Memory Word:

This is a group of bits in a memory that represents information or data of some type. Word size in modern computers typically ranges from 4 to 64 bits, depending on the size of the computer (depend upon the microprocessor). The most popular word size for modern computers is 16, 32, or 64-bits.

3.       Byte:

Computer memory is measured in terms of bytes. Byte is a special term used for 8-bits word i.e. 1 byte= 8 bits. In modern digital computers, the word size is usually a multiple of 8-bits such as 2 bytes or 4 bytes. Byte is a unit of storage capable of holding a single character. A character can be a number, letter or symbol.

The larger units are kilobyte, megabyte and giga byte. The relationship between these units is shown below:

1 byte= 8 bits

I kilobyte=1024 bytes

1 megabyte=1024 kilobytes=1048576 bytes

1 gigabyte=1024 megabytes = 1048576 kilobytes

4.       Memory capacity:

The amount of information a computer can store in memory. This is the way of specifying how many bits can be stored in a memory device. Suppose that we have a memory device which can store 4096 20 bits words. This represents a total capacity of 81,920 bits. We could also represent this memory capacity as 4096×20. When expressed this way, the first number 4096 is the number of words and the second number 20 is the number of bits per word (word size).

5.       Address:

The code that identifies where a piece of information is stored is known as address. This is a number that identifies the location of a word in memory. Each word stored in a memory device or system has a unique address. Addresses are always specified as a binary number, although octal, hexadecimal, and decimal numbers are often used.

Note: Kindly consult the text book at page-21 for further reading of Address and check out the Figure.

6.       Read Operation:

The operation through which the binary word stored in a specified memory location (address) is sensed. For example, if we want to read ‘word 4’ of the memory, we have to perform a read operation on address 100. Read operation is often called a fetch operation, since the word is being fetched from the memory.

7.       Write Operation:

This is the operation whereby a new word is placed into a particular memory location. It is also referred to as a store operation. Whenever a new record is written into a memory location, it replaces the word that was previously stored there.  The old word is lost in the process of writing the new word into this memory location.

8.       Access Operation

This is a measure of a memory device’s operating speed. It is the amount of time required to perform a read operation. It is the time between the memory receiving a read command signal and the data becoming available at the memory output.

9.       Cycle time

It is another measure of a memory device’s speed.  It is the amount of time required for the memory to perform to perform a read or write operation and then return to its original state ready for the next operation. Cycle time is normally longer than access time.

10.   Random Access Memory (RAM)

A type of computer memory that provides direct access to any single location is RAM. Random access memory or RAM most commonly refers to computer chips that temporarily store dynamic data to enhance computer performance.

Random access memory is volatile memory, meaning it loses its contents once power is cut.  This is different from non-volatile memory such as hard disks and flash memory, which do not require a power source to retain data.  The access time is the same for any address in memory because it provides random access to any memory location.

 11.   Sequential Access memory (SAM)

In computing, sequential access memory (SAM) is a class of data storage devices that read their data in sequence. This is in contrast to random access memory (RAM) where data can be accessed in any order. Sequential access devices are usually a form of magnetic memory.

The access time is not constant but varies depending on the address location. A particular stored word is found by sequencing through all address locations until the desired address is reached. This produces access times, which are much longer than those of random access memories. Example of Sequential access memory devices includes magnetic tapes.

12.   Read/Write Memory (RWM)

It is any memory that can be read from or written into with equal ease. Read-write memory is a type of computer memory that may be relatively easily written to as well as read from (unlike ROM or "Read-Only Memory").

13.   Read Only memory (ROM)

A ROM can be written into (Programmed) only once and this operation is performed at the factory. Thereafter, information can only be read from the memory. Other types of ROM are actually Read Mostly Memories (RMM) which can be written into more than once but the write operation is more complicated than the read operation. Therefore, the write operation is not performed is not performed very often.

14.   Static Memory Devices

The Semiconductor memory devices in which the stored data will remain permanently stored as long as power is supplied, without the need for periodically rewriting the data into memory are called Static memory devices. It is type of RAM that is quicker than dynamic RAM and does not need to be refreshed. Because it is more expensive and holds less data than dynamic RAM.

15.   Dynamic Memory Devices

It is type of RAM. These are the Semiconductor memory devices in which the stored data will remain not permanently stored even with power supplied, unless the data is periodically rewritten into memory. This operation is called a refresh operation.