[yuubi]

the 6502 has a single 16-bit address space with some parts (zero page, stack) addressable by means other than full 16-bit addresses. the 8051 has 16-bit read-only code space, 16-bit read/write external memory space, and 8-bit internal memory space, except half of it is special: if you use indirect access (address in an 8-bit register), you get memory. but if you encode that same address literally in an instruction, you get a peripheral register.

at least that's the part I remember