On Thu, Nov 23, 2023 at 12:46 AM Sellam Abraham via cctalk < cctalk@classiccmp.org> wrote:
> The CADC had no program counter: since it was designed from its inception > to be a multi-processing (multi-threading?) system, it made sense to build > a program counter onto each ROM. Therefore, when the CADC switched back to > that ROM to continue executing instructions, the program counter on that > ROM told the CADC where it was supposed to fetch the next instruction. > Once it became clear to Ted that the CADC did not have an integrated > program counter (though it easily could have) he pooh-poohed the entire > thing as not qualifying as a single-chip microprocessor > The 4004, 4040, 8008, and 8080 have program counters, but if one is picking nits, they have so much missing that has to be supplied by additional logic that IMO it's hard to consider them single-chip microprocessors. And if lack of a program counter is sufficient to disqualify a chip from being a single-chip microprocessor, then the Fairchild F8 isn't one either. The F8 CPU (3850) has no program counter or any other address source on-chip. That's all provided by the ROM chip (3851 or 3856 PSU), or by the SMI or DMI (static or dymanic memory interface chips). When the F8 CPU wants to know the value of one of the program counter or data counter registers, it has to ask the other chips for it! Unlike the CADC, though, all of the ROMs and memory interfaces in an F8 system kept track of the same program counter value, so it couldn't do the ultra-fast task switching of the CADC. On the other hand, if you don't consider an on-chip program counter as necessary to be a single-chip microprocessor, then the Motorola MC14500 would qualify as a single-chip microprocessor, and probably the simplest one ever made. Eric
