This might be a good time to point out that the presence of PROMs doesn't
automatically imply the presence of "microcode" since PROMs were often used
for instruction decoding and other simple logic functions. Microcode implies
the presence of a low-level processor, and I saw little of that up to the
point at which bit-slice, e.g. AM2900-series or Intel 3000-series, became
popular, though PROMs did appear from time to time in functions other than
simple microcode.
I've built CPU's using standard TTL MSI/SSI devices, without any sort of
microcode, and, since that was in the days before PALs, bipolar PROMs were the
only handy programmable device that allowed simple localized decoding and
steering without discarding resources. The fact they can easily provide a
preset to a cycle counter, based on their inputs even though there might be a
"sparse" lookup table for that function, saved both time and real estate,
often rendered PROMs the vehicle of choice.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, April 09, 2002 12:34 PM
Subject: Re: TTL computing
> >
> > Playing aroud a bit with TTL using SSI parts, I get about 8-10 chips per
> > bit on the average for the alu. For 16 bit computer that is about 160
> > chips. I would say 1/3 more for control or about 250 chips total. Does
> > this sound right people with 16 bit TTL computers?
>
> IF by 'SSI' you mean just simple gates and flipflips ('74, '76, etc) then
> it sounds far too few.
>
> The PDP11/05 was 2 full hex-height boards just for the CPU, so around 200
> chips. It used mostly TTL, but also some PROMs containing the microcode.
> And the TTL included chips rather more complicated than just gates --
> things like 16*4 RAMs, 4 bit latches, multiplexers, etc.
>
> The Philips P850 was about the same number of chips, but with no
> microcode PROMs. Mostly simple gate chips, but also '181 ALUs, 16*4 RAMs,
> decoders, etc. And it's really an 8 bit machine at the hardware level. It
> appears to be 16 bit to the programmer, but the ALU is only 8 bits wide
> (yes, every ALU operation takes 2 cycles, the instruction word is fetched
> 8 bits at a time, and so on).
>
> -tony
>
>
Received on Tue Apr 09 2002 - 15:02:10 BST