> > > From: Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt; > <> >Any other AM29xx chips on it, like a 2909 or 2910 sequencer? Or is

> <> >just a demo board for a 4-bit ALU chip? > <> > > <> >It sounds like a really interesting find, though. The 29xx series

> <> >were interesting devices that have been used in all sorts of

> <> > <> > <> Yep, there is a 2909 on board too.... > < > <Ah, so there's a sequencer. And presumably, therefore, there's some

He said _a_ 2909. That implies a 4 bit microcode address, which limits it to 16 locations. 16 micorocode words and a 4 bit ALU is a small processor IMHO. And being an AMD design they won't have simply concatenated an 'instruction' from external memory (which this board probably doesn't have anyway) with those 4 bits. For people who've not come across the AMD sequencer chips, they're quite nice, and even have things like a return stack (about 4 levels IIRC) to allow for subroutines in the microcode. The 2909 and 2911 are very similar. They provide a 4 bit 'slice' of the microcode address logic -- program counter, subroutine stack, etc. You can cascade them to make as wide an address as you like. The difference between these 2 chips is minor -- IIRC, one of them has separate pins for inputs that are combined on the other one, and also has inputs that are ORed with the address output. The 2910 is essentially 3 2909s together with some glue logic in a single chip. If you only need 4K of control store it's a nice chip _but it can't be extended to give more bits without kludging_. Three rivers made that mistake on the PERQ -- the first PERQ had 4K of control store and used a 2910, all later models had 16K and used a 2910 and a '2 bit kludge' (pun intended!). -tony