[TUHS] Encoding an ISA: Random Logic vs. Control Stores

[Jon Steinhart]

Henry Bent writes:
> On Tue, 30 Nov 2021 at 18:10, Jon Steinhart <jon at fourwinds.com> wrote:
>
> > Humm writes:
> > > Quoth Jon Steinhart:
> > > >arnold at skeeve.com writes:
> > > >> Can someone please explain why it's called "random" logic? Discrete
> > > >> logic I understand (more or less), but I've not heard the term
> > "random"
> > > >> used in this context before now.
> > > >
> > > >Well, as a recovered random logic designer, I think that the name comes
> > > >from there being no particular structure to the logic.  Many parts of
> > > >logic design are very regular, think memory.  But that regularity doesn't
> > > >exist when, for example, decoding irregular instructions.
> > >
> > > Now that's a "random" definition.
> > >
> > > --
> > > Humm
> >
> > OK, I'll try again.  For anybody familiar with Portland, Oregon, it's the
> > difference between driving in Northwest where there's a rectangular grid
> > numbered in one direction and alphabetical in the other, and approaching
> > the Ross Island Bridge from the west side which appears to be designed by
> > someone following a goat while tripping their brains out.  One can address
> > Northwest Portland quite easily unlike getting onto the Ross Island Bridge.
> >
>
> So - and as I say this as someone who has no direct experience with this
> level of logic - everything is directly addressable but the difference is
> in how you get there?

Sorry my analogy didn't work.  Look at a photomicrograph of a chip; at least
to me it's pretty obvious where the random logic is located.

Here's another try.  In a microcoded machine, the same hardware is used for
every microinstruction.  In random logic, custom designed circuitry is used
for each special case.