12 Apr
2002
12 Apr
'02
4:35 a.m.
On Fri, Apr 12, 2002 at 01:48:12AM +0100, Tony Duell wrote:
Your're right, but the ROM would become _large_ indeed.
If your function requires 16 external inputs and 4 inputs for the
current state you already need a 1MBit ROM. Now you need 4 outputs for
the next state and lets say 4 other outputs. So you would need
a 1MByte ROM to implement your machine...
I'm not sure what you mean by "feedback ROM", if you mean an
additional
ROM between the outputs an the state latch you have and microcoded state
machine again... ;-)
bye
Thilo
So given a processor we can't tell if it's
microcoded or not by looking
at the scehamtic. This seems to make the term 'microcoded' fairly useless :-)
I meant it it's always possible to transform a microcoded state machine
in a not microcoded state machine. So you can design a microcoded control
unit and decide later if you implement it microcoded or not.
No, now you
don't have a seperate ROM (or PROM or any other kind of
linear addressable memory) any more. The main reason to use a microcoded
Of course I do. It's just a somewhat larger ROM. You do realise that
_any_ combinatorial logic circuit can be implemented using a sufficiently
large ROM, right. So all I need to do is pick a large enough ROM to
contain the original microcode + the logic for the counter, etc. That's
now all in one ROM. I connect it to the state latch. It's now a state
machine. It's also, IMHO, the same microcoded system as the original design.
machine
instead of a "pure" state machine is that you can change it
quite easily by putting another "program" in the ROM. If you
And why can't I change the functionality of a state machine by changing
the contents of the 'feedback' ROM? (Hint : I most certainly can do this).