Input and Output // IN r, (c); OUT (c), r
The operand n is placed on the bottom half (A0 through A7) of the address bus to select the I/O device at one of 256 possible ports. The contents of the Accumulator (Register A) also appear on the top half (A8 through A15) of the address bus at this time. Then the byte contained in the Accumulator is placed on the data bus and written to the selected peripheral device.
If the Accumulator contains 23h, then upon the execution of an OUT (01h) instruction, byte 23h is written to the peripheral device mapped to I/O port address 01h.