16 Apr
1999
16 Apr
'99
7:44 p.m.
On 9 Apr 1999, ard(a)p850ug1.demon.co.uk (Tony Duell) wrote:
...
] A ROM _is_ combinatorial logic. I don't want to get into a silly argument
] over this, but I have great difficulty finding a conceptual difference
] between a combinatorial circuit built from a pile of AND, OR and NOT
] gates and the same circuit built (albeit using a lot more transistors) in
] a ROM. To claim that a CPU using a ROM is microcoded but one with
] _identical_ internal states using simple gates as the feedback logic
] round the sequencer flip-flops is not is a very strange way of thinking
] about things.
Request accepted; silly argument follows. (Oh, you said "don't"?
Oh well, too late.)
Do you mean to say that _all_ computers are microcoded? After all,
the control logic can always be modelled by some number of state FFs
and a large-enough ROM, couldn't it? Or is your claim that there is
no such thing as microcoding? That strikes me as far-fetched as well.
The difference is that a ROM is easily replaceable; slap in another ROM
or EPROM with different microcode burned in, and you've got an entirely
different machine on your hands. Maybe to you it is just as easy to
redesign some section of the circuit board, etch a new one, pull the
newly required chips from your very-deep storage bin, and Bingo! you're
done. For most of us, I suspect that sounds like quite a lot of effort.
I don't think anybody ever claimed that microcoded machines could do
things that non-microcoded machine could not, or vice-versa. It is
a matter of convenience. And maybe there is some added comfort level
for software geeks, using a (micro)program to control everything, and
being able to alter it just like any other software (or firmware).
I guess it is exactly the added (and admittedly wasted in any fixed
design) transistors, just sitting there unused, waiting to be assigned
work, that makes microcoding attractive - it is easier to adjust
precisely because you don't have to muck around with how many transistors
are in there, or worry about board-space/power-consumption/fanout/etc/etc
that you might have to worry about if you wanted to adjust some hard-
wired discrete logic.
Bill.
(Gee, that didn't turn out to be nearly as silly as I had expected.)