What does the instruction ORA B do in 8085?
Performs logical OR between accumulator and B
In the 8085 instruction set, ORA r stands for "logical OR the Accumulator with register r", performing a bit-by-bit (bitwise) OR and storing the result back in the accumulator. The operation is:
A ← A OR r
So ORA B means A ← A OR B: each bit of the accumulator is ORed with the corresponding bit of register B, and the answer replaces the contents of A. Recall the OR rule — a result bit is 1 if either input bit is 1, and 0 only when both are 0 (for example, 1010 OR 0110 = 1110). A common use is to set selected bits or to test a register for zero.
Flag effects: ORA is a logical instruction, so it clears the carry flag (CY = 0) and clears the auxiliary carry (AC = 0), while the Sign (S), Zero (Z) and Parity (P) flags are updated according to the result.
Why the other options are wrong: subtracting B from the accumulator is done by SUB B (or SBB with borrow), not ORA. Adding B to the accumulator is the job of ADD B. Performing a logical AND between the accumulator and B is ANA B, which keeps a bit only when both inputs are 1 — a different logic function. ORA B specifically carries out the logical OR operation.
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