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Bytes, bits, words:: deciphering computer capacity

Library Systems Newsletter [March 1982]

A reader has asked us to explain the relationship between computer capacity quoted in "bytes" and in "words." In order to make the comparison, it is necessary to restate some basics. Computer capacity-both that of primary memory and secondary storage devices-- is measured in bytes and expressed as a number followed by the symbol K or KB. 1KB stands for 1,024 bytes. The convention of using K to represent a count of 1,024 units is common in discussions of computer storage. This practice is not limited to the description of bytes, though these are usually intended when the notation K is used. A computer described as having 128K of main memory can store 128 times 1,024 bytes or characters. In practice, the value of K is rounded to the nearest 1,000 and the memory capacity is described in kilobytes or thousands of characters. It is, therefore, common to hear the expressions 128 Kilobytes, 128K and 128KB. Very large CPUs and virtually all secondary storage devices have capacities measured in millions of bytes or Megabytes (MB).

While the byte is the most commonly used expression of memory and storage capacity, computer capacity is sometimes described in "words," a measure which denotes the number of bits which the control unit can retrieve from the primary memory at one time. An 8 bit-word computer can access 8 bits, or one byte, at a time. The word lengths of available computing devices range from 8 to 64 bits, although most of the machines installed in libraries have 8 or 16 bit-word capacities. When capacity is expressed in words, the same convention of using K for 1,024 (words in this case) is followed, as in a 64K 16 bit-word computer. Word length is directly related to a computer's operating speed-the longer the word, the faster the machine.

To convert a statement of primary memory capacity which has been expressed in words to one expressed in bytes, multiply the number of words by the number of bits in each and divide by 8. Thus a computer with 64K 16 bit-words of main memory can store 64 x 1,024 x 16 bits, or 1,048,576 (1,024K) bits. Dividing by 8, the equivalent character capacity is 131,072 bytes (nominally 128K).

View Citation
Publication Year:1982
Type of Material:Article
Language English
Published in: Library Systems Newsletter
Publication Info:Volume 2 Number 03
Issue:March 1982
Publisher:American Library Association
Place of Publication:Chicago, IL
Notes:Howard S. White, Editor-in-Chief; Richard W. Boss, Contributing Editor
Subject: Computer terminology
Record Number:3768
Last Update:2024-05-20 07:27:06
Date Created:0000-00-00 00:00:00