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 > > >
