8051 instruction “DA A” (Decimal Adjust Accumulator): “In an 8051 program, the instruction DA A adjusts the accumulator after addition so that the result becomes a valid packed BCD value.” Decide if this is correct.

Introduction / Context:
Many embedded applications need decimal arithmetic for displays or human interfaces. The 8051 includes the DA A instruction to facilitate packed BCD operations after binary addition, ensuring the accumulator holds a valid BCD result.

Given Data / Assumptions:

• Packed BCD format: two decimal digits per byte (nibbles 0–9).
• DA A is applied after ADD/ADDC on BCD operands.
• Auxiliary carry (AC) and carry (C) flags affect the correction.

Concept / Approach:
BCD correction adjusts the lower and/or upper nibble if they exceed 9 or if AC/C indicate a carry out of a nibble. The DA A instruction adds 6 to the lower nibble if needed, and conditionally adds 6 to the upper nibble, producing a valid packed BCD result in A and an updated carry if the adjusted value exceeds 99 decimal.

Step-by-Step Solution:

Verification / Alternative check:
Work an example: 49 (0x49) + 58 (0x58) = 0xA1 (invalid BCD). DA A transforms 0xA1 into 0x07 with carry set, representing 107 decimal as a packed BCD across carry and A.

Why Other Options Are Wrong:

• Incorrect: DA A is explicitly defined for BCD adjustment after addition.
• Only after subtraction / ASCII: DA A is not intended for subtraction or ASCII-coded arithmetic; different techniques are used there.

Common Pitfalls:
Forgetting to use ADDC when adding multi-byte BCD; applying DA A to pure binary arithmetic; misunderstanding that DA A relies on AC/C and current nibble values.

Final Answer:
Correct