The 8051 talks to peripherals like memory: the 74LS138 3-to-8 decoder turns three address lines (A, B, C) into eight active-low outputs, each wired to a chip's CS pin, so one decoder selects eight memory blocks. An I/O chip like the 8255 is wired to the bus as if it were RAM and driven with MOVX; its A1/A0 lines pick Port A, Port B, Port C, or the control register, and a control word configures each port as input or output (with a bit-set/reset mode that can flip single Port C bits). Decoding only part of the address creates aliases — many addresses reaching the same physical port — which must be documented.

— In our words · Source: 8051 Microcontroller Notes