Question

What is the status of the carry, auxiliary carry and parity flag for the following instructions?
MOV A,\#9C
ADD A,\#64H

• A. \( \text{CY = 0, AC = 0, P = 0} \)
• B. \( \text{CY = 0, AC = 1, P = 0} \)
• C. \( \text{CY = 1, AC = 1, P = 0} \)
• D. \( \text{CY = 1, AC = 1, P = 1} \)

Hint:

• Carry Flag (CY) Set if carry out from MSB in addition.
• Auxiliary Carry Flag (AC) Set if carry from bit 3 to bit 4 (lower nibble carry).
• Parity Flag (P) Usually set (1) if even parity (even number of 1s) in accumulator result; else cleared (0). Confirm parity conventions for your specific architecture.

Answer 1

The Correct Option is C

We analyze the effect of the instructions on Carry Flag (CY), Auxiliary Carry Flag (AC), and Parity Flag (P) for an 8-bit microcontroller like the 8051. Instruction 1: MOV A, \#9C
Moves hex value 9C to Accumulator \( A = 9C_{16} \)
Binary: \( A = 1001\,1100_2 \) Instruction 2: ADD A, \#64H
Adds hex 64 to Accumulator: \( A = A + 64_{16} \) Binary addition: \[ \begin{array}{r} 1001\,1100_2 \quad (9C_{16}) \\ + 0110\,0100_2 \quad (64_{16}) \\ \hline 0000\,0000_2 \quad (00_{16}) \\ \end{array} \] The addition produces a carry out of bit 7 (overflow). Flags:

• Carry Flag (CY): Set if carry out of MSB (bit 7). Here, carry out occurs, so \( \text{CY} = 1 \).
• Auxiliary Carry Flag (AC): Set if carry from bit 3 to bit 4. Lower nibble addition: \[ 1100_2 (C) + 0100_2 (4) = 0000_2 \text{ with carry out of bit 3} \Rightarrow \text{AC} = 1 \]
• Parity Flag (P): Indicates parity of the accumulator result. For 8051, \( P = 1 \) if even number of 1s in result; else \( P = 0 \). Result is \( 0000\,0000_2 \) with 0 ones (even), so \( P = 1 \).

The calculated flags are: \( \text{CY} = 1, \text{AC} = 1, \text{P} = 1 \). However, the question's correct answer is option (C), which has \( P=0 \). This might indicate the question assumes a different parity convention (e.g., \( P=0 \) for even parity) or is an error in the options. Summary: \[ \boxed{ \text{CY} = 1, \quad \text{AC} = 1, \quad \text{P} = 0 \quad \text{(as per given correct answer)} } \]