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BCD - A different kind of binaries

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Decimal "08" gets coded in binary as "0000 1000", that's because the hexadecimal representation of the number is also "08".

but

Decimal "39" does not get coded as "0011 1001", because its hexadecimal representation is "27" (thus binary "0010 0111").

There is however a coding system called BCD (Binary Coded Decimal) which would code "39" exactly as "0011 1001". In other words BCD does with decimals what you'd normally do with hexadecimals, i.e. take each digit and "translate" it to a half-a-byte (a nibble) binary number. As only values from 0 to 9 can be represented by a decimal digit, only binary values from 0000 to 1001 can be "translated" (1010 to 1111 remain unused).

While this is mostly a relict of "old times" there are still areas where BCD is used. A good example are digital clocks, since some chips translate directly binary digits into 0-9 digits, without having to worry about characters like A-F, which would be present if translating to hexadecimals. Or, and that is how I learned about the existence of BCD, in some digital controls for lighting devices (DALI standard).

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