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The truth table (A and B are inputs and X is an output) for an OR gate is:
A B X
0 0 0
0 1 1
1 0 1
1 1 1
The oraa instruction performs a bit-by-bit ORing of the bits in the destination register with the operand. In this example bit 0 of register A is ORed with bit 0 of the number f0 hex, and bit 1 of A is ORed with bit 1 of the number f0 hex etc.