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# Bi-quinary coded decimal

Bi-quinary coded decimal is a numeral encoding scheme used in many abacuses and in some early computers, including the Colossus. The term bi-quinary indicates that the code comprises both a two-state (bi) and a five-state (quinary) component.

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 or five bits. Some examples are:

• IBM 650 – 7 bits (two ‘bi’ bits: 0 5 and five ‘quinary’ bits: 0 1 2 3 4) with error checking (exactly one ‘bi’ bit and one ‘quinary’ bit set in a valid digit); in the picture of the front panel below, the bi-quinary encoding of the internal workings of the machine are evident in the arrangement of the lights (active bits are just visible)
 Value 05-01234 Bits IBM 650 front panel 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
 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 – 4 bits (one ‘bi’ bit: 5 and three ring counter coded ‘quinary’ bits) with 1 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