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# Binary Coded Decimal

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Binary coded decimal (BCD) is a way of encoding numbers in computer memory. In BCD, numbers are represented as decimal digits, and each digit is represented by four bits:

> `

>      Digit Bits      Digit Bits

>        0   0000        5   0101
>        1   0001        6   0110
>        2   0010        7   0111
>        3   0011        8   1000
>        4   0100        9   1001

> `

To encode a number such as 127, then, one simply encodes each of the decimal digits as above, giving (0001, 0010, 0111).

Since most computers store data in eight-bit bytes, there are two common ways of storing four-bit BCD digits in those bytes: either one can simply ignore the extra four bits of each byte, usually filling them with zero bits or one bits (as in EBCDIC?); or one can store two digits per byte, called "packed" BCD. Thus the number 127 would be represented as (11110001, 11110010, 11110111) in EBCDIC; or as (00000001, 00100111) in packed BCD (with a leading zero added).

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Last edited June 5, 2001 5:10 pm (diff)
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