29 Nov
2023
29 Nov
'23
1:47 a.m.
On Thu, Nov 23, 2023 at 12:46 AM Sellam Abraham via cctalk <
cctalk(a)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