8-bit CPUs have 8 data pins (D0-D7) and anything going in or out of the CPU is doing so 8 bits at a time. This includes all external accesses such as RAM, ROM,

8-bit CPUs have 8 data pins (D0-D7) and anything going in or out of the CPU is doing so 8 bits at a time. This includes all external accesses such as RAM, ROM, and I/O. But 8-bit CPUs have more than 8 address lines, because 256 bytes total for combined RAM, ROM and I/O space is not useful. That number I think is typically 16 although Signetics 2650 had only 12 (with the instruction set only supporting 12-bit addresses), and the Atari 6507 (6502 derivative) had 13 (instruction set still supporting 16-bit addresses but the upper 3 bits of addresses were basically ignored). > Is it just a matter of replacing all instructions/chips/buses to be 32 bit/lane? Depends on the 8-bit CPU really. - The Z80 lets you combine specific register pairs to work with 16 bits and address memory through them. - The 6502 does not, but has the whole "zero page" thing where the first 256 bytes of RAM can contain 16-bit data and pointers. - Both the Z80 and 6502 have a stack pointer register (the 6502's being 8-bit and fixed to point to RAM locations 512-767). But the 2650 had an internal 8-byte stack and stack pointer. - The 8051 (and the 8048 I think) has lots of instructions for manipulating individual bits in registers and RAM, and also has a division between the memory that opcodes are fetched from versus data. None of the above work like that (the F8 might).