What are the really unusual or weird computers?

[Tim Shoppa]

"Chuck Guzis" <cclist at sydex.com> wrote:
> On 23 Jun 2007 at 0:40, Sridhar Ayengar wrote:
> > How does one implement a 1-bit program counter?  Seems to me being able 
> > to distinguish between two program instructions doesn't seem *all* that 
> > useful.  8-)
>
> The program counter (like memory and its addressing as well as 
> registers to hold a jump target) was to be provided externally.

Some of the more clever designs avoided a program counter by having
the program cycle around sequentially and repeatedly. Why put the
concept of a PC into the engine if the engine doesn't need the concept
of a PC?

And the most clever designs avoided jump targets too by never having
the program take a branch. They would use the conditional-skip
codes to skip the instructions they didn't want, in the never ending
repeating instruction stream.

For industrial controllers, the above is really a very good match.
If you want to be really really sure you can't smash the stack or
start executing data, not only not having a stack but not having a
PC in the traditional sense was a huge advantage.

Now the minimalist industrial-controller mindset is not out there
very much today. Everyone carries around ipods and cellphones and all sorts
of junk sold by having a bazillion features and submenus.

Tim.