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Biquinary code example^[1]

Reflected biquinary code

Japanese abacus. The right side represents 1,234,567,890 in bi-quinary: each column is one digit, with the lower beads representing "ones" and the upper beads "fives".

Bi-quinary coded decimal is a numeral encoding scheme used in many abacuses and in some early computers, notably the Colossus.^[2] The term bi-quinary indicates that the code comprises both a two-state (bi) and a five-state (quinary) component. The encoding resembles that used by many abacuses, with four beads indicating the five values either from 0 through 4 or from 5 through 9 and another bead indicating which of those ranges (which can alternatively be thought of as +5).

Several human languages, most notably Fula and Wolof also use biquinary systems. For example, the Fula word for 6, jowi e go'o, literally means five [plus] one. Roman numerals use a symbolic, rather than positional, bi-quinary base, even though Latin is completely decimal.

The Korean finger counting system Chisanbop uses a bi-quinary system, where each finger represents a one and a thumb represents a five, allowing one to count from 0 to 99 with two hands.

One advantage of one bi-quinary encoding scheme on digital computers is that it must have two bits set (one in the binary field and one in the quinary field), providing a built-in checksum to verify if the number is valid or not. (Stuck bits happened frequently with computers using mechanical relays.)

Examples

Several different representations of bi-quinary coded decimal have been used by different machines. The two-state component is encoded as one or two bits, and the five-state component is encoded using three to five bits. Some examples are:

Roman and Chinese abacuses
Stibitz^[3] relay calculators at Bell Labs from Model II onwards
FACOM 128 relay calculators at Fujitsu

IBM 650

The IBM 650 uses seven bits: two bi bits (0 and 5) and five quinary bits (0, 1, 2, 3, 4), with error checking.

Exactly one bi bit and one quinary bit is set in a valid digit. The bi-quinary encoding of the internal workings of the machine are evident in the arrangement of its lights – the bi bits form the top of a T for each digit, and the quinary bits form the vertical stem.

Value	05-01234 bits^[1]	IBM 650 front panel while running, with active bits just discernible Close-up of IBM 650 indicators while running, with active bits visible
0	10-10000
1	10-01000
2	10-00100
3	10-00010
4	10-00001
5	01-10000
6	01-01000
7	01-00100
8	01-00010
9	01-00001

Remington Rand 409

The Remington Rand 409 has five bits: one quinary bit (tube) for each of 1, 3, 5, and 7 - only one of these would be on at the time. The fifth bi bit represented 9 if none of the others were on; otherwise it added 1 to the value represented by the other quinary bit. The machine was sold in the two models UNIVAC 60 and UNIVAC 120.

Value	1357-9 bits
0	0000-0
1	1000-0
2	1000-1
3	0100-0
4	0100-1
5	0010-0
6	0010-1
7	0001-0
8	0001-1
9	0000-1

UNIVAC Solid State

The UNIVAC Solid State uses four bits: one bi bit (5), three binary coded quinary bits (4 2 1)^[4]^[5]^[6]^[7]^[8]^[9] and one parity check bit

Value	p-5-421 bits
0	1-0-000
1	0-0-001
2	0-0-010
3	1-0-011
4	0-0-100
5	0-1-000
6	1-1-001
7	1-1-010
8	0-1-011
9	1-1-100

UNIVAC LARC

The UNIVAC LARC has four bits:^[9] one bi bit (5), three Johnson counter-coded quinary bits and one parity check bit.

Value	p-5-qqq bits
0	1-0-000
1	0-0-001
2	1-0-011
3	0-0-111
4	1-0-110
5	0-1-000
6	1-1-001
7	0-1-011
8	1-1-111
9	0-1-110

References

^ ^a ^b Ledley, Robert Steven; Rotolo, Louis S.; Wilson, James Bruce (1960). "Part 4. Logical Design of Digital-Computer Circuitry; Chapter 15. Serial Arithmetic Operations; Chapter 15-7. Additional Topics". Digital Computer and Control Engineering (PDF). McGraw-Hill Electrical and Electronic Engineering Series (1 ed.). New York, US: McGraw-Hill Book Company, Inc. (printer: The Maple Press Company, York, Pennsylvania, US). pp. 517–518. ISBN 0-07036981-X. ISSN 2574-7916. LCCN 59015055. OCLC 1033638267. OL 5776493M. SBN 07036981-X. ISBN 978-0-07036981-8. ark:/13960/t72v3b312. Archived (PDF) from the original on 2021-02-19. Retrieved 2021-02-19. p. 518: […] The use of the biquinary code in this respect is typical. The binary part (i.e., the most significant bit) and the quinary part (the other 4 bits) are first added separately; then the quinary carry is added to the binary part. If a binary carry is generated, this is propagated to the quinary part of the next decimal digit to the left. […] [1] (xxiv+835+1 pages)
^ "Why Use Binary? - Computerphile". YouTube. 2015-12-04. Archived from the original on 2021-12-12. Retrieved 2020-12-10.
^ Stibitz, George Robert; Larrivee, Jules A. (1957). Written at Underhill, Vermont, US. Mathematics and Computers (1 ed.). New York, US / Toronto, Canada / London, UK: McGraw-Hill Book Company, Inc. p. 105. LCCN 56-10331. (10+228 pages)
^ Berger, Erich R. (1962). "1.3.3. Die Codierung von Zahlen". Written at Karlsruhe, Germany. In Steinbuch, Karl W. (ed.). Taschenbuch der Nachrichtenverarbeitung (in German) (1 ed.). Berlin / Göttingen / New York: Springer-Verlag OHG. pp. 68–75. LCCN 62-14511.
^ Berger, Erich R.; Händler, Wolfgang (1967) [1962]. Steinbuch, Karl W.; Wagner, Siegfried W. (eds.). Taschenbuch der Nachrichtenverarbeitung (in German) (2 ed.). Berlin, Germany: Springer-Verlag OHG. LCCN 67-21079. Title No. 1036.
^ Steinbuch, Karl W.; Weber, Wolfgang; Heinemann, Traute, eds. (1974) [1967]. Taschenbuch der Informatik - Band II - Struktur und Programmierung von EDV-Systemen (in German). Vol. 2 (3 ed.). Berlin, Germany: Springer-Verlag. ISBN 3-540-06241-6. LCCN 73-80607. {{cite book}}: |work= ignored (help)
^ Dokter, Folkert; Steinhauer, Jürgen (1973-06-18). Digital Electronics. Philips Technical Library (PTL) / Macmillan Education (Reprint of 1st English ed.). Eindhoven, Netherlands: The Macmillan Press Ltd. / N. V. Philips' Gloeilampenfabrieken. doi:10.1007/978-1-349-01417-0. ISBN 978-1-349-01419-4. SBN 333-13360-9. Retrieved 2020-05-11.^{[permanent dead link‍]} (270 pages) (NB. This is based on a translation of volume I of the two-volume German edition.)
^ Dokter, Folkert; Steinhauer, Jürgen (1975) [1969]. Digitale Elektronik in der Meßtechnik und Datenverarbeitung: Theoretische Grundlagen und Schaltungstechnik. Philips Fachbücher (in German). Vol. I (improved and extended 5th ed.). Hamburg, Germany: Deutsche Philips GmbH. p. 50. ISBN 3-87145-272-6. (xii+327+3 pages) (NB. The German edition of volume I was published in 1969, 1971, two editions in 1972, and 1975. Volume II was published in 1970, 1972, 1973, and 1975.)
^ ^a ^b Savard, John J. G. (2018) [2006]. "Decimal Representations". quadibloc. Archived from the original on 2018-07-16. Retrieved 2018-07-16.