UGC NET CSE | November 2017 | Part 3 | Question: 1

Question

In $8085$ microprocessor which of the following flag(s) is (are) affected by an arithmetic operation?

• A. AC flag only
• B. CY flag Only
• C. Z flag Only
• D. AC, CY, Z flags

Comments

All flags are affected.

Answer 1

Ans: d

Flags in 8085 are zero flag, carry flag, sign flag, parity flag and auxiliary carry flag. All these gets affected by an arithmetic operation.

Answer 2

ALU of 8085 have five flip flops whose states (set/reset) are determined by the result data of other registers and accumulator.

They are called as Zero, Carry, Sign, Parity and Auxiliary-Carry flags.

• Zero Flag (Z): When an arithmetic operation results in zero , the flip-flop called the Zero flag - which is set to one.
• Carry flag (CY): After an addition of two numbers, if the sum in the accumulator is larger than eight bits, then the flip-flop uses to indicate a carry called the Carry flag – which is set to one.
• Sign (S): It is set to 1, if bit D7 of the result = 1; otherwise reset. D7 is the first digit of a binary number.
• Parity (P): If the result has an even number of 1s, the flag is set to 1; for an odd number of 1s the flag is reset.
• Auxiliary Carry (AC): In an arithmetic operation, when a carry is generated by digit D3 and passed to digit D4, the AC flag is set. Generally this flag is used internally for Binary Coded Decimals (BCD).

Figure shows a 8-bit flag register, adjacent to the accumulator. It is not used as a register. Out of eight bit-positions, five positions are used to store the outputs of five flip-flops.

Some of the arithmetic operations are -

1. ADD : Add
2. ADI : Add Immediate
3. SUB : Subtract
4. SUI : Subtract Immediate

 

So all of the given options will be correct.