Inside the Intel 4OO4.

The FIM instruction writes the value 0 to the eight-bit register pair 0P, which comprises the two four-bit registers R0 and R1. We used alternate addresses so pairs of adjacent locations could hold eight-bit numbers as two four-bit values. Yet while all the internals were 16 bits wide, to permit it to be used with eight-bit memory, it had an eight-bit external data bus. We're about the use the ADM instruction, which adds the value in the memory location to the value in the accumulator. [Extracted from the article]