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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Important Questions from Instruction Set of 8085
Calculate the time required to execute the entire instruction cycle if two machine codes, 0011 1110 and 0011 0010, are stored in memory locations 2000H and 2001H, respectively.
If the clock frequency is 2 MHz, the first machine code represents opcode to load data byte in the accumulator and the second code represents data to be loaded in the accumulator.
Following instruction is executed in 8085, LDB 4000H
Identify the correct statement.
In 8085A microprocessor, the operation performed by the instruction LHLD 2100H is
__________ address instruction is supplied to the microprocessor by an external device for INTR signal acknowledgement.
A fetch cycle is the
