CP316: Microprocessor Systems and Interfacing

Decimal Arithmetic

Binary Coded Decimal (BCD) Arithmetic

• decimal numbers are coded in binary
• for PIC, arithmetic operations assume packed BCD with 2 decimal digits per 8 bits

e.g.   decimal 1,985 = h'19 85' in packed BCD
                              = 0001 1001 1000 0101 (in binary)

e.g. 19 85      0001 1001 1000 0101
    +34 59      0011 0100 0101 1001
      ------       ------------------------
      54 44      0100 1101 1101 1110   >> the binary sum does not give the decimal result expected
                            0110 0110 0110   >> if a digit is >9 then add 6
                     ------------------------
                    0101 0100 0100 0100   >> result is now in binary coded decimal

In the PIC, use add/addc followed with decimal adjust, daw/dawc.