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Module 2: The 8051 Architecture

20202m

Which flag allows to carry out the signed as well as unsigned addition and subtraction operations?
(i) CY
(ii) OV
(iii) AC
(iv) FO

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Answer: (ii) OV

Explanation: OV (Overflow) flag in PSW (PSW.2) indicates signed overflow (carry into sign bit XOR carry out). To correctly detect signed addition/subtraction results, OV is examined along with CY for unsigned. CY alone handles unsigned carry/borrow. AC is auxiliary carry for BCD, FO is user flag (PSW.5). So OV allows both signed and unsigned operations to be distinguished (e.g., ADD vs ADDC with overflow check).**

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