UPD78F1502AGK-GAK-AX Datasheet, PDF (872/1031 Pages) Renesas Technology Corp – 16-Bit Single-Chip Microcontrollers

78K0R/Lx3
CHAPTER 29 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
BCDADJ register.
<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
register.
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 Register AC Flag
MOV A, #91H
; <1>
91H
−
−
SUB A, #52H
; <2>
3FH
0
1
SUB A, !BCDADJ
; <3>
39H
0
0
BCDADJ
Register
−
06H
−
R01UH0004EJ0501 Rev.5.01
856
Jun 20, 2011