Integers: BCD

Since people think in decimal, it may be convenient to use a decimal-like representation.

Binary coded decimal (BCD) uses unsigned binary to represent each decimal digit.

How many bits are needed?

4 bits per decimal digit: ceil (lg 10) = 4

Since 4 bits can represent 16 values, 6 values are unused for every digit.

Advantages

Decimal input/output doesn't require complicated conversion.

Fewer problems with repeating fractions.

Example:

Represent 1739ten in BCD:

decimal

1

7

3

9

BCD

0001

0111

0011

1001

Some early computers used BCD representation, and many calculators still use BCD.

Since calculators can be much slower than computers, they can use hardware which

operates on 4 bits at a time.