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The smallest unit of data in a computer is called Bit (Binary Digit). A bit has a single binary value, either 0 or 1. In most computer systems, there are eight bits in a byte. The value of a bit is usually stored as either above or below a designated level of electrical charge in a single capacitor within a memory device.
Half a byte (four bits) is called a nibble.
In most computer systems, a byte is a unit of data that is eight binary digits long. A byte is the unit most computers use to represent a character such as a letter, number or typographic symbol (for example, “g”, “5”, or “?”). A byte can also hold a string of bits that need to be used in some larger unit of application purposes (for example, the stream of bits that constitute a visual image for a program that displays images or the string of bits that constitutes the machine code of a computer program).
In some computer systems, four bytes constitute a word, a unit that a computer processor can be designed to handle efficiently as it reads and processes each instruction. Some computer processors can handle two-byte or single-byte instructions.
A byte is abbreviated with a “B”. (A bit is abbreviated with a small “b”). Computer storage is usually measured in byte multiples. For example, an 820 MB hard drive holds a nominal 820 million bytes – or megabytes – of data. Byte multiples are based on powers of 2 and commonly expressed as a “rounded off” decimal number. For example, one megabyte (“one million bytes”) is actually 1,048,576 (decimal) bytes.
In some systems, the term octet is used for an eight-bit unit instead of byte. In many systems, four eight-bit bytes or octets form a 32-bit word. In such systems, instructions lengths are sometimes expressed as full-word (32 bits in length) or half-word (16 bits in length).
A Kilobyte (kb or Kbyte) is approximately a thousand bytes (actually, 2 to the 10th power, or decimal 1,024 bytes).
As a measure of computer processor storage and real and virtual memory, a megabyte (abbreviated MB) is 2 to the 20th power byte, or 1,048,576 bytes in decimal notation.
A Gigabyte (pronounced Gig-a-bite with hard G’s) is a measure of computer data storage capacity and is “roughly” a billion bytes. A gigabyte is two to the 30th power, or 1,073,741,824 in decimal notation.
A Terabyte is a measure of computer storage capacity and is 2 to the 40th power of 1024 gigabytes.
A Petabyte (PB) is a measure of memory or storage capacity and is 2 to the 50th power bytes or, in decimal, approximately a thousand terabytes (1024 terabytes).
An Exabyte (EB) is a large unit of computer data storage, two to the sixtieth power bytes. The prefix exa means one billion billion, or on quintillion, which is a decimal term. Two to the sixtieth power is actually 1,152,921,504,606,846,976 bytes in decimal, or somewhat over a quintillion (or ten to the eighteenth power) bytes. It is common to say that an Exabyte is approximately one quintillion bytes. In decimal terms, an Exabyte is a billion gigabytes.
A Zettabyte (ZB) is equal to one sextillion bytes. It is commonly abbreviated ZB. At this time, no computer has one Zettabyte of storage. It has 1024 Exabytes.
A Yottabyte is equal to one septillion bytes. It is commonly abbreviated YB. At this time, no computer has one Zettabyte of storage. It has 1024 Zettabytes.