Actually, for most of the calculators mentioned, the address path *is* 4 bits. And it's the same 4 bits as the data path. The Saturn sends the 20-bit address on five consecutive clock cycles. The nice thing is that they have two different data pointers in each device [*], and separate memory read and write commands that use them. So one data pointer can track the PC, and the CPU only has to output a new value to that data pointer over the bus when branching. Operand accesses use the other data pointer. Reads and writes using one data pointer have no effect on the other. The Saturn bus was designed back when it was considered impractical for compact plug-in modules for a handheld device to have much more than a dozen contacts. This replaced the earlier HP-41 bus, which operated in a bit serial fashion. In addition to a two-phase clock, there was a sync line to indicate the start of a 56-bit frame. The ISA line provided a 16-bit address from the CPU to the ROMS, and a 10-bit data value from the ROMs back to the CPU. (Other devices had to watch the data value as well.) RAM data transfers used a separate signal that generally transferred 56 bits in a frame. Going to a four-bit bus increased the available bandwidth considerably. And by only using a minimum of clock cycles for a transfer (rather than always 56 bits), they made much more efficient use of the bandwidth. Eric [*] Keeping address pointers in the memory and peripheral chips has a long history. The Intel 4004 and 4040, the Fairchild F8, and other processors have done this. IIRC, the F8 also used the dual data pointer concept. Whereas on the Saturn bus, neither data pointers is "special", with the F8 one of them is explicitly designated to be the program counter.