14 Jul
2015
14 Jul
'15
4:49 p.m.
On 7/14/2015 12:17 PM, Chuck Guzis wrote:
I'm missing something in this discussion, I
think.
HDL's (take your pick) are just programming languages like FORTRAN or C
with different constraints. What's the point of going to all the
trouble of doing an FPGA implementation of a slow old architecture, when
pretty much the same result could be obtained by running a software
emulator? Neither accurately reflects the details of the real
thing--and there will always be the aspect of missing peripherals.
Not necessarily. For example, it is impossible to find an IBM 1410, as
far as I know. But there ARE 1415 consoles I knew of a while back, and
there are certainly 729s and 1403 printers and 1402 card read/punch
units up and running.
And it would at least reflect how the original hardware worked. There
is a continuum here:
Software "just make it work" emulator. (Most of SimH stuff seems to be
at this level).
Software "make it use the same cycles" simulator. (This is what I write
simulators to).
A logic model which has the same behavior as the original
(this would be sort of like a 360/50 does the same thing as a 360/65
kind of comparison).
A logic model which is structurally the same as the original, and thus
provides a portable and verifiable model (if you have the software) of
the original design.
Perhaps the worst aspect of using FPGA is that this is a rapidly moving
field, so that the part you used to do your implementation 10 years ago
will no longer be available. I've done a few designs using 5V CPLDs
(XC95xx series) not *that* long ago. Now they themselves are quaint
examples of obsolete hardware. You can't win.
That is why I use VHDL (or Verilog is fine to). So that those models
are portable into the future. The FPGA part doesn't matter so much,
but the model future portability does matter.
You can move software-only simulators quite easily, but I'm not as
sanguine about FPGA designs.
And you still don't have the peripherals. I suppose one could emulate a
Univac Solid State machine in FPGA, but what would one do about the
all-important drum coupled to the card reader and printer. Has anyone
rolled out a design for a DIY 1403 printer?
1403's and IBM 729's and 1402 card read/punch still exist. I seem to
recall the CHM doing something like building a 729 tape drive tester, too.
I've run the Cyber emulator as well as various SIMH emulators from time
to time, but it's just not the same as the real thing--it's not even
remotely the same.
But something like the SBG 6120 PDP-8 is closer, potentially with real
lights and switches. As another I example, I can envision an FPGA
sitting inside a real IBM 1415 console, running it's lights, responding
to it's switches and interacting with it's selectric typewriter.
Probably more than I will accomplish, but it is good to have goals.
--Chuck