Units Of Computer Memory Measurement


In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term memory is often synonymous with the term primary storage or main memory. An archaic synonym for memory is store.

What is computer memory with example?

It stands for Random access memory.It stands for read only memory.
It is the fastest memory.It is slower memory as compare to RAM.
It is volatile memory.It is non-volatile memory.
In this memory, data will erase when the power is offIn this memory, data will not erase even if the power is off

The key characteristics of memory devices or memory system are as follows:

  1. Location
  2. Capacity
  3. Unit of Transfer
  4. Access Method
  5. Performance
  6. Physical type
  7. Physical characteristics
  8. Organization

1. Location:

It deals with the location of the memory device in the computer system. There are three possible locations:

  • CPU : This is often in the form of CPU registers and small amount of cache
  • Internal or main: This is the main memory like RAM or ROM. The CPU can directly access the main memory.
  • External or secondary: It comprises of secondary storage devices like hard disks, magnetic tapes. The CPU doesn’t access these devices directly. It uses device controllers to access secondary storage devices.

2. Capacity:

The capacity of any memory device is expressed in terms of: i)word size ii)Number of words

  • Word size: Words are expressed in bytes (8 bits). A word can however mean any number of bytes. Commonly used word sizes are 1 byte (8 bits), 2bytes (16 bits) and 4 bytes (32 bits).
  • Number of words: This specifies the number of words available in the particular memory device. For example, if a memory device is given as 4K x 16.This means the device has a word size of 16 bits and a total of 4096(4K) words in memory.

3. Unit of Transfer:

It is the maximum number of bits that can be read or written into the memory at a time. In case of main memory, it is mostly equal to word size. In case of external memory, unit of transfer is not limited to the word size; it is often larger and is referred to as blocks.

4. Access Methods:

It is a fundamental characteristic of memory devices. It is the sequence or order in which memory can be accessed. There are three types of access methods:

  • Random Access: If storage locations in a particular memory device can be accessed in any order and access time is independent of the memory location being accessed. Such memory devices are said to have a random access mechanism. RAM (Random Access Memory) IC’s use this access method.
  • Serial Access: If memory locations can be accessed only in a certain predetermined sequence, this access method is called serial access. Magnetic Tapes, CD-ROMs employ serial access methods.
  • Semi random Access: Memory devices such as Magnetic Hard disks use this access method. Here each track has a read/write head thus each track can be accessed randomly but access within each track is restricted to a serial access.

5. Performance: The performance of the memory system is determined using three parameters

  • Access Time: In random access memories, it is the time taken by memory to complete the read/write operation from the instant that an address is sent to the memory. For non-random access memories, it is the time taken to position the read write head at the desired location. Access time is widely used to measure performance of memory devices.
  • Memory cycle time: It is defined only for Random Access Memories and is the sum of the access time and the additional time required before the second access can commence.
  • Transfer rate: It is defined as the rate at which data can be transferred into or out of a memory unit.

6. Physical type: Memory devices can be either semiconductor memory (like RAM) or magnetic surface memory (like Hard disks).

7.Physical Characteristics:

  • Volatile/Non- Volatile: If a memory devices continues hold data even if power is turned off. The memory device is non-volatile else it is volatile.

8. Organization:

  • Erasable/Non-erasable: The memories in which data once programmed cannot be erased are called Non-erasable memories. Memory devices in which data in the memory can be erased is called erasable memory.E.g. RAM(erasable), ROM(non-erasable).

A bit is a binary digit, the smallest increment of data on a computer. A bit can hold only one of two values: 0 or 1, corresponding to the electrical values of off or on, respectively.

Because bits are so small, you rarely work with information one bit at a time. Bits are usually assembled into a group of eight to form a byte. A byte contains enough information to store a single ASCII character, like “h”.

A kilobyte (KB) is 1,024 bytes, not one thousand bytes as might be expected, because computers use binary (base two) math, instead of a decimal (base ten) system.

Computer storage and memory is often measured in megabytes (MB) and gigabytes (GB). A medium-sized novel contains about 1 MB of information. 1 MB is 1,024 kilobytes, or 1,048,576 (1024×1024) bytes, not one million bytes.

Similarly, one 1 GB is 1,024 MB, or 1,073,741,824 (1024x1024x1024) bytes. A terabyte (TB) is 1,024 GB; 1 TB is about the same amount of information as all of the books in a large library, or roughly 1,610 CDs worth of data. A petabyte (PB) is 1,024 TB. 1 PB of data, if written on DVDs, would create roughly 223,100 DVDs, i.e., a stack about 878 feet tall, or a stack of CDs a mile high. Indiana University is now building storage systems capable of holding petabytes of data. An exabyte (EB) is 1,024 PB. A zettabyte (ZB) is 1,024 EB. Finally, a yottabyte (YB) is 1,024 ZB.

Many hard drive manufacturers use a decimal number system to define amounts of storage space. As a result, 1 MB is defined as one million bytes, 1 GB is defined as one billion bytes, and so on. Since your computer uses a binary system as mentioned above, you may notice a discrepancy between your hard drive’s published capacity and the capacity acknowledged by your computer. For example, a hard drive that is said to contain 10 GB of storage space using a decimal system is actually capable of storing 10,000,000,000 bytes. However, in a binary system, 10 GB is 10,737,418,240 bytes. As a result, instead of acknowledging 10 GB, your computer will acknowledge 9.31 GB. This is not a malfunction but a matter of different definitions.

We count in base 10 by powers of 10:

  101 = 10
  102 = 10*10 =  100
  103 = 10*10*10 = 1,000
  106 = 1,000,000

Computers count by base 2:

  21 = 2
  22 = 2*2 = 4
  23 = 2*2*2 = 8
  210 = 1,024
  220 = 1,048,576

So in computer jargon, the following units are used:

1 kilobyte (KB)1,024 bytes
1 megabyte (MB)1,048,576 bytes
1 gigabyte (GB)1,073,741,824 bytes
1 terabyte (TB)1,099,511,627,776 bytes
1 petabyte (PB)1,125,899,906,842,624 bytes

Note: The names and abbreviations for numbers of bytes are easily confused with the notations for bits. The abbreviations for numbers of bits use a lower-case “b” instead of an upper-case “B”. Since one byte is made up of eight bits, this difference can be significant. For example, if a broadband Internet connection is advertised with a download speed of 3.0 Mbps, its speed is 3.0 megabits per second, or 0.375 megabytes per second (which would be abbreviated as 0.375 MBps). Bits and bit rates (bits over time, as in bits per second [bps]) are most commonly used to describe connection speeds, so pay particular attention when comparing Internet connection providers and services


Memory unit is the amount of data that can be stored in the storage unit. This storage capacity is expressed in terms of Bytes.

The following table explains the main memory storage units −

S.No.Unit & Description
1Bit (Binary Digit)A binary digit is logical 0 and 1 representing a passive or an active state of a component in an electric circuit.
2NibbleA group of 4 bits is called nibble.
3ByteA group of 8 bits is called byte. A byte is the smallest unit, which can represent a data item or a character.
4WordA computer word, like a byte, is a group of fixed number of bits processed as a unit, which varies from computer to computer but is fixed for each computer.The length of a computer word is called word-size or word length. It may be as small as 8 bits or may be as long as 96 bits. A computer stores the information in the form of computer words.

The following table lists some higher storage units −

S.No.Unit & Description
1Kilobyte (KB)1 KB = 1024 Bytes
2Megabyte (MB)1 MB = 1024 KB
3GigaByte (GB)1 GB = 1024 MB
4TeraByte (TB)1 TB = 1024 GB
5PetaByte (PB)1 PB = 1024 TB

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