27 Oct
2017
27 Oct
'17
10:47 a.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Paul
Koning
via cctalk
Sent: 27 October 2017 18:28
To: Dave Wade <dave.g4ugm at gmail.com>; General Discussion: On-Topic and
Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: Which Dec Emulation is the MOST useful and Versatile?
> On Oct 27, 2017, at 4:54 AM, Dave Wade via cctalk
<cctalk at
classiccmp.org>
wrote:
>
> Kip,
> I think "emulation" and "simulation" get used pretty much
interchangeable.
SIMH is touted
a simulator, Hercules/390 as an emulator yet they are
both programs that provide a "bare metal" machine via software on
which an operating system can be installed. Neither make any attempt
to reproduce the speed of the original CPU.
True. And by some argument, an FPGA implementation (from an HDL
behavioral model) is also a software implementation, just written in a programming language.
Recently I commented to an old colleague that there are many different
levels
of emulation possible, and any one of those may make
sense -- it's just a
question of what you're after. So you can emulate in a conventional
programming language, as SIMH does, reproducing the programmer-visible
behavior of the machine but not its timing. Bugs from the original might
appear if those bugs are known to be important, but probably not
otherwise.
This kind is (nowadays) likely to run faster than the
original; certainly
it won't
usually mimic the original timing, neither for
computation nor I/O.
You can make timing-accurate software emulators, with lots of work. SIMH,
in
paced mode, and provided the I/O waits are reasonably
accurately expressed
in
units of machine cycles, isn't quite timing
accurate but is somewhat
similar.
You can build a behavioral simulator (SIMH style, basically) in an FPGA.
That
isn't necessarily any more capable or accurate
than a software simulator.
PDP-
2011 is an example I know of, and I've see
articles about other PDP
emulations
of this kind. Since the design is new, created from a
behavioral
description
(data book, functional spec, architecture spec) it
will be about as
accurate as
SIMH.
You can also, if the data exists, build a lower level (gate level or
thereabouts)
FPGA model. Given schematics and wire lists, it
should be possible to
build an
implementation that's an exact copy of how the
original machine worked
(assuming of course the documentation is accurate, which is not
necessarily the
case). Such an emulation would replicate strange and
undocumented
behavior
of the original -- and allow you to find out where
that came from. I've
been
working on such a thing for the CDC 6600, which is
surprisingly hard given
that
the design lives right on the hairy edge of not
working at all
timing-wise. But it
does accuarately model the peripheral processors right
now, and indeed it
shows and explains some undocumented oddities that are part of that
machine's folklore.
So it's a question of what you're after. If you want to run the software,
or teach
the machine at the programmer level, SIMH or
equivalent is quite adequate.
If
you want to teach FPGA skills, an FPGA behavioral
model emulation is a
good
project, especially for a small machine like a PDP-8.
As for the gate
level
model, I'm not sure what argument to make for that
other than "paul is a
bit
crazy" and "because the data exists to do
it". :-)
If I had the skill, data and time, I would always go for a gate level model.
However, I do most (sim/em)ulation in SIMH instead, like I have been doing
for MU5 where I lack the data and the time and probably the skill as well,
but I can always acquire the skill, the other two are harder to find.
Regards
Rob