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RL78/G1A CHAPTER 27 BCD CORRECTION CIRCUIT
(2) Subtraction: Calculating the result of subtracting a BCD code value from another BCD code value by
using a BCD code value
<1> The BCD code value from which subtraction is performed is stored in the A register.
<2> By subtracting the value of the second operand (value of BCD code to be subtracted) from the A register as is
in binary, the calculation result in binary 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 subtracting the value of the BCDADJ register (correction value) from the A
register (subtraction result in binary) in binary, 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.
Example: 91 − 52 = 39
Instruction A Register CY Flag AC Flag BCDADJ
Register
MOV A, #91H ; <1>
− − −
91H
SUB A, #52H ; <2>
3FH 0 1 06H
SUB A, !BCDADJ ; <3>
−
39H 0 0
R01UH0305EJ0200 Rev.2.00 829
Jul 04, 2013