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The kilobyte for 1000 bytes (symbol: kB) or Kilobyte (kibibyte) for 1024 bytes (symbol: KB or KiB or informally KBytes) are two definitions for multiples of the unit byte for digital information. Although the SI-prefixkilo- means 1000, the term kilobyte and the symbol KB have usually been used to refer to 1024 (210) bytes, in the fields of computer science and information technology123 except when where referring to data transfer rates 4 and to disk storage space.5 The symbol kB correctly however refers to 1000 (103) bytes as used in modern times for file sizes. CPU caches always use binary prefixes, where kB is never appropriate, as does random-access memory capacity (not file sizes or flash-based storage) according to JEDEC-standard for memory modules. Informally sometimes the B is dropped, then K (the SI symbol for kelvin not kilo-), given the right context, can been understood as 1024 bytes and k (always the lower case, SI prefix for 1000) can be taken for 1000 bytes. KB and kB are sometimes mistakenly used interchangeably, with or without the B, not following any standard. The B should always be uppercase, because b means bits, kb is kilobit.
The unit kilobyte is commonly used to mean either 1000 or 1024 bytes. The 1024 number originated as compromise technical jargon for the byte multiples that needed to be expressed by the powers of 2 but lacked a convenient name. As 1024 (210) approximates 1000 (103), roughly corresponding SI multiples were used for binary multiples. In 1998 the International Electrotechnical Commission (IEC) proposed standards for binary prefixes, specifying the use of megabyte to strictly denote 10002 bytes and mebibyte to denote 10242 bytes. By the end of 2007, the IEC Standard had been adopted by the IEEE, EU, and NIST. Nevertheless, the term kilobyte continues to be widely used with the following two meanings:
The HP 21MX real-time computer (1974) denoted 196,608 (which is 192×1024) as "196K",10 while the HP 3000 business computer (1973) denoted 131,072 (which is 128×1024) as "128K".11
The Shugart SA-400 51⁄4-inch floppy disk (1976) held 109,375 bytes unformatted,12 and was advertised as "110 Kbyte", using the 1000 convention.13 Likewise, the 8-inch DEC RX01 floppy (1975) held 256,256 bytes formatted, and was advertised as "256k".14 On the other hand, the Tandon 51⁄4-inch DD floppy format (1978) held 368,640 (which is 360×1024) bytes, but was advertised as "360 KB", following the 1024 convention.
In December 1998, the IEC addressed such multiple usages and definitions by creating unique binary prefixes to denote multiples of 1024, such as “kibibyte (KiB)”, which represents 210, or 1024, bytes.17
^Sharma, Kapil; Mohammed J.; Norton, Peter C. Norton; Good, Nathan; Steidler-Dennison, Tony (2005). Professional Red Hat Enterprise Linux 3. John Wiley & Sons. p. 134. "Disk manufacturers sell you their disks saying that a kilobyte is 1,000 bytes, that a megabyte is a thousand of those, and that a gigabyte is another thousand of those, giving you 1,000,000,000 bytes to a gigabyte when you buy a disk. The rest of the computer world, including the programmers who write Linux, thinks of a kilobyte as 1,024 bytes (2^10 bytes), a megabyte as 1,048,576 bytes (2^20), and a gigabyte as 1,073,741,824 bytes (2^30), which means that you're buying just a bit less than you might think."