Having read a couple of different takes on this, I see the concern but I’m not sure I agree with Ken’s take.
Basically he’s saying it’s not really a microprocessor because the CROM isn’t on-die. That might indeed be a reasonable definition of microprocessor. However, it’s also the case that other designs of the era did not have an on-board CROM.
An example is the Fairchild F8, which put it on a second chip along with the PC - this seems like a direct analog. I’ve yet to see anyone claim the F8 is not a microprocessor.
This was not common among the systems we are familiar with today, it was relatively common among those processors developed for larger machines, which was the case here. Since the underlying ISA was not specified, and was always intended to change across platforms, putting it on a ROM was a no-brainer.
I’m not going to claim I know the answer one way or the other, but I’m inclined to say that I would not outlaw this because of the location of the CROM.
I think non-overlapping two phase has the same effect.
(I recall that the T9000 was four-phase on the inside, and some blocks used phases 1 and 3 to be truly overlapping, others 1 and 3+4, IIRC. Of course I probably don’t remember correctly, but I do know there were two clocking styles. In fact the whole chip was a great mish-mash of different design styles, one reason for it being late and grossly missing the clock speed target.)
It is very good. I found it interesting that you listed “Address width” as 16 in the information box. I had not seen anything about that and had guessed that addresses would be 8 bits per chips as well (so 24 bits with 3 chips).
Ken’s argument is that the labeled picture of the chip is a lie and that there is nothing like a PC in it. He was able to examine the circuit used in the court case (which I haven’t seen) and claimed an external TTL register chip was used as the PC and that the next address was generated by the ROM. That would make the TTL and ROM a standalone finite state machine - it would be able to follow a sequence of instructions on its own independently (or mostly so) from the AL1 chip. As you noted in the article, other people don’t agree.
At 9 minutes of this 1 hour talk about the TMS9918 there is an explanation about the four phase technology Texas Instruments called “ratioless logic” NMOS but which is better known as “domino logic”.
Excellent - thanks for that! I came across domino in the context of CMOS and didn’t quite understand it (my head was full of NMOS). And yet that presentation tells us that TI were using it in PMOS days - to keep power down.
That’s a terrific video - thanks! The 9918 is course a classic retro chip.
Here’s a link to the book he references in the slide about ratioless logic if you want to go a little deeper (link is to a 350 page PDF).
It would be interesting to build some dynamic logic on a breadboard using larger capacitors. I think you can build pass gates (“transmission gates”) using CD4007 chips, which are still available. I’ve seen reference to an issue with not having access to the “fourth terminal” (“body”) of the FETs but never understood.
I’m afraid we gone off topic on the thread here. But this is good stuff.
As I understood it from that talk, ratioless logic NMOS is all about floating state and transient capacitance and everything is processed on the rising edge. (So, many nodes don’t really have a state and everything is in flux.)