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Usually n decimal digits are represented by 4n bits in computers. Actually, two BCD digits can be compressed optimally and reversibly into 7 bits, and three digits into 10 bits, by a very simple algorithm based on the fixedlength combination of two variable field-length encodings. In over half of the cases the compressed code results from the conventional BCD code by simple removal of redundant 0 bits. A long decimal message can be subdivided into three-digit blocks, and separately compressed; the result differs from the asymptotic minimum length by only 0.34 percent. The hardware requirement is small, and the mappings can be done manually.Four bits are needed to represent a single decimal digit in binary technology; the BCD (binary-coded decimal) scheme is most commonly used, in which the digit A is rendered as a string of 4 bits (abed), such that A = Sa + 4b + 2c + d. Similarly, a pair of decimal digits A,B is rendered as {abcd){efgh), and a triplet A,B,C, as {abcd){efgh){ijkl), and so on.

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