Harvard v. von Neumann

On 12 Dec 2008 at 19:18, Andy Holt wrote: We agree on that one, even "pure" Harvard isn't what it used to mean. Pure Harvard architecture used to mean separate stores (and, by extension, datapaths) for instructions and data. Lower PICs probably qualify, as the data and program areas are very separate (and not even of the same word length). But it's a bit of a stretch to say that a machine with a defeatable protection scheme is "Harvard" in any sense of the word. By that logic, an 80286 running in protected mode is a Harvard machine. By that token upper PICs with software-writable program memory are not Harvard, even though the data books say the contrary. Things get more complicated even on the "pure" architectures if off- line storage (e.g. tape, cards, quipu, guru beads) can be written by a program and read into either data or program (i.e. executed) space. Is a machine with separate code and data spaces still considered to be "Harvard" if the instruction set includes an "execute" instruction that permits modification of the target instruction operand specification? (e.g. the S/360 EX instruction). It's not a simple distinction. Cheers, Chuck