Binary Coded Decimal

Binary Coded Decimal

Binary Coded Decimal
binary coded decimal display

As we naturally live in a decimal (base-10) world we need some way of converting these decimal numbers into a binary (base-2) environment that computers and digital electronic devices understand, and binary coded decimal code allows us to do that.

We have seen previously that an n-bit binary code is a group of “n” bits that assume up to 2n distinct combinations of 1’s and 0’s. The advantage of the Binary Coded Decimal system is that each decimal digit is represented by a group of 4 binary digits or bits in much the same way as Hexadecimal. So for the 10 decimal digits (0-to-9) we need a 4-bit binary code.

But do not get confused, binary coded decimal is not the same as hexadecimal. Whereas a 4-bit hexadecimal number is valid up to F16 representing binary 11112, (decimal 15), binary coded decimal numbers stop at 9 binary 10012. This means that although 16 numbers (24) can be represented using four binary digits, in the BCD numbering system the six binary code combinations of: 1010 (decimal 10), 1011 (decimal 11), 1100 (decimal 12), 1101 (decimal 13), 1110 (decimal 14), and 1111 (decimal 15) are classed as forbidden numbers and can not be used.

The main advantage of binary coded decimal is that it allows easy conversion between decimal (base-10) and binary (base-2) form. However, the disadvantage is that BCD code is wasteful as the states between 1010 (decimal 10), and 1111 (decimal 15) are not used. Nevertheless, binary coded decimal has many important applications especially using digital displays.

In the BCD numbering system, a decimal number is separated into four bits for each decimal digit within the number. Each decimal digit is represented by its weighted binary value performing a direct translation of the number. So a 4-bit group represents each displayed decimal digit from 0000 for a zero to 1001 for a nine.

So for example, 35710 (Three Hundred and Fifty Seven)  in decimal would be presented in Binary Coded Decimal as:

35710 = 0011 0101 0111 (BCD)

Then we can see that BCD uses weighted codification, because the binary bit of each 4-bit group represents a given weight of the final value. In other words, the BCD is a weighted code and the weights used in binary coded decimal code are 8, 4, 2, 1, commonly called the 8421 code as it forms the 4-bit binary representation of the relevant decimal digit.

Binary Coded Decimal Representation of a Decimal Number

Binary Power 23 22 21 20
Binary Weight: 8 4 2 1

The decimal weight of each decimal digit to the left increases by a factor of 10. In the BCD number system, the binary weight of each digit increases by a factor of  2 as shown. Then the first digit has a weight of  120 ), the second digit has a weight of  221 ), the third a weight of  422 ), the fourth a weight of  823 ).

Then the relationship between decimal (denary) numbers and weighted binary coded decimal digits is given below.

Truth Table for Binary Coded Decimal

Decimal Number BCD 8421 Code
0 0000 0000
1 0000 0001
2 0000 0010
3 0000 0011
4 0000 0100
5 0000 0101
6 0000 0110
7 0000 0111
8 0000 1000
9 0000 1001
10 (1+0) 0001 0000
11 (1+1) 0001 0001
12 (1+2) 0001 0010
20 (2+0) 0010 0000
21 (2+1) 0010 0001
22 (2+2) 0010 0010
etc, continuing upwards in groups of four

Then we can see that 8421 BCD code is nothing more than the weights of each binary digit, with each decimal (denary) number expressed as its four-bit pure binary equivalent.

Decimal-to-BCD Conversion

As we have seen above, the conversion of decimal to binary coded decimal is very similar to the conversion of hexadecimal to binary. Firstly, separate the decimal number into its weighted digits and then write down the equivalent 4-bit 8421 BCD code representing each decimal digit as shown.

Binary Coded Decimal Example No1

Using the above table, convert the following decimal (denary) numbers: 8510, 57210 and 857910 into their 8421 BCD equivalents.

8510 = 1000 0101 (BCD)

57210 = 0101 0111 0010 (BCD)

857910 = 1000 0101 0111 1001 (BCD)

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