Determinism. Unless you run your software simulator bare-metal - which most aren't - cycle accuracy is always a race. Before you say modern processors are 100,000 times faster than emulated ones - so just spin wait until the next virtual time tick, that is always a moving ratio or opportunity for a context switch right at the threshold. I might want to emulate a vintage i7 on an i19 one day.
In my mind, take the best of both worlds. Emulate at an RTL level to produce as accurate of a functional model possible. Then simulate the design in things like Verilator to create a software runable emulation. Not to mention it's a deeper emersion to recreate the how rather than just the result of an instruction. -Alan On 2015-07-14 13:17, 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. > > 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. > > 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? > > 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. > > --Chuck
