RL78/G1H CHAPTER 29 BCD CORRECTION CIRCUIT
R01UH0575EJ0120 Rev. 1.20 Page 832 of 920
Dec 22, 2016
29.3 BCD Correction Circuit Operation
The basic operation of the BCD correction circuit is as follows.
(1) Addition: Calculating the result of adding a BCD code value and another BCD code value by using a BCD code
value
<1> The BCD code value to which addition is performed is stored in the A register.
<2> By adding the value of the A register and the second operand (value of one more BCD code to be added)
as are in binary, the binary operation result is stored in the A register and the correction value is stored in
the BCD correction result register (BCDADJ).
<3> Decimal correction is performed by adding in binary the value of the A register (addition result in binary)
and the BCDADJ register (correction value), and the correction result is stored in the A register and CY
flag.
Caution The value read from the BCDADJ register varies depending on the value of the A register
when it is read and those of the CY and AC flags. Therefore, execute the instruction <3> after
the instruction <2> instead of executing any other instructions. To perform BCD correction in
the interrupt enabled state, saving and restoring the A register is required within the interrupt
function. PSW (CY flag and AC flag) is restored by the RETI instruction.
An example is shown below.
Examples 1: 99 + 89 = 188
Examples 2: 85 + 15 = 100
Examples 3: 80 + 80 = 160
Instruction A Register CY Flag AC Flag BCDADJ Register
MOV A, #99H ; <1> 99H — — —
ADD A, #89H ; <2> 22H 1 1 66H
ADD A, !BCDADJ ; <3> 88H 1 0 —
Instruction A Register CY Flag AC Flag BCDADJ Register
MOV A, #85H ; <1> 85H — — —
ADD A, #15H ; <2> 9AH 0 0 66H
ADD A, !BCDADJ ; <3> 00H 1 1 —
Instruction A Register CY Flag AC Flag BCDADJ Register
MOV A, #80H ; <1> 80H — — —
ADD A, #80H ; <2> 00H 1 0 60H
ADD A, !BCDADJ ; <3> 60H 1 0 —
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