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STC89LE53RC bảng dữ liệu(HTML) 50 Page - STCmicro Technology Co,.Ltd |
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50 / 165 page  DA A Function: Decimal-adjust Accumulator for Addition Description: DA A adjusts the eight-bit value in the Accumulator resulting from the earlier addition of two variables (each in packed-BCD format), producing two four-bit digits.Any ADD or ADDC instruction may have been used to perform the addition. If Accumulator bits 3-0 are greater than nine (xxxx1010-xxxx1111), or if the AC flag is one, six is added to the Accumulator producing the proper BCD digit in the low-order nibble. This internal addition would set the carry flag if a carry-out of the low-order four-bit field propagated through all high-order bits, but it would not clear the carry flag otherwise. If the carry flag is now set or if the four high-order bits now exceed nine(1010xxxx- 111xxxx), these high-order bits are incremented by six, producing the proper BCD digit in the high-order nibble. Again, this would set the carry flag if there was a carry-out of the high-order bits, but wouldn’t clear the carry. The carry flag thus indicates if the sum of the original two BCD variables is greater than 100, allowing multiple precision decimal addition. OV is not affected. All of this occurs during the one instruction cycle. Essentially, this instruction performs the decimal conversion by adding 00H, 06H, 60H, or 66H to the Accumulator, depending on initial Accumulator and PSW conditions. Note: DA A cannot simply convert a hexadecimal number in the Accumulator to BCD notation, nor does DA A apply to decimal subtraction. Example: The Accumulator holds the value 56H(01010110B) representing the packed BCD digits of the decimal number 56. Register 3 contains the value 67H (01100111B) representing the packed BCD digits of the decimal number 67.The carry flag is set. The instruction sequence. ADDC A,R3 DA A will first perform a standard twos-complement binary addition, resulting in the value 0BEH (10111110) in the Accumulator. The carry and auxiliary carry flags will be cleared. The Decimal Adjust instruction will then alter the Accumulator to the value 24H (00100100B), indicating the packed BCD digits of the decimal number 24, the low-order two digits of the decimal sum of 56,67, and the carry-in. The carry flag will be set by the Decimal Adjust instruction, indicating that a decimal overflow occurred. The true sum 56, 67, and 1 is 124. BCD variables can be incremented or decremented by adding 01H or 99H. If the Accumula- tor initially holds 30H (representing the digits of 30 decimal), then the instruction sequence, ADD A,#99H DA A will leave the carry set and 29H in the Accumulator, since 30+99=129. The low-order byte of the sum can be interpreted to mean 30 – 1 = 29. 50 |
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