Книга: Introduction to Microprocessors and Microcontrollers

Binary coded decimal (BCD)

Binary coded decimal numbers are very simple. Each decimal digit is converted to binary and written as a 4-bit or 8-bit binary number. The number 5 would be written as 0101₂ or 00000101₂. So far, this is the same as ‘ordinary’ binary but the change occurs when we have more digits.

Consider the number 25₁₀. In regular binary this would convert to 11001₂. Alternatively, we could convert each digit separately to 4-bit or 8-bit numbers:

2 = 0010₂ or 0000 0010₂

5 = 0101₂ or 0000 0101₂

Putting these together, 25₁₀ could be written using the 4-bit numbers as 0010 0101₂. This uses one byte and is called Packed BCD. Alternatively, we could use the 8-bit formats and express 25₁₀ as 0000 0010 0000 0101₂ and would now use two bytes. This is called Unpacked BCD.

There are two disadvantages. Firstly, many numbers are of increased length after converting to BCD, particularly so if we use unpacked BCD or the numbers are very large like 25?10⁷⁵. In addition, arithmetic is much more difficult although, generally, microprocessors do have the ability to handle them.

The advantage becomes apparent when the microprocessor is controlling an external device like digits on displays at a filling station or accepting inputs from a keyboard. The coding is simple and does not involve the conversion of the numbers to binary.

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