30 Sep
2012
30 Sep
'12
10:16 p.m.
Hmmm... One 'thought experiemnt' goes like
this :
Take your favourite microprocessor and write a
stand-alone program
for it. Burn that program into an EPROM and wire it up [...]. Most
people agree that EPROM cotnains 'firmware'.
But an EPROM is just a combinatorial logic circuit.
So for each of
the 8 data bits of the EPROM, write down the logic equation giving it
as a functionof the address inputs. Now implement that logic
equation using TTL gates. Repeat for the other EPROM data pins.
Wire the mess up, add tri-state buffers to the oputput, and connect
it in place of the EPROM. The microrpccor runs as before.
Well, to be picky, I'm inclined to doubt it. The pile of TTL (quite
aside from drawing enough power to heat your house) is very likely to
have substantially higher propagation delay than the original PROM.
It must do. It gets the same logic levels on its data
pins in the 2
cases for each address it outputs. The microprocessor cann't know
whether it's an EPROM or a mass of 74xx chips.
But the result sure doesn;t _look_ like firmware :-)
I'd say it looks as much like firmware as a PROM implemented as a board
full of diodes, some of which have been cut. :-)
For that matter, It's difficult to distinguish
between 'microcode'
and a 'state machine'.
In theory. In practice, there is usually little doubt - while the two
formalisms may be formally identical, there are, as reflected in the
terminological difference, differences of attitude and style between
the two.
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