FPGA VAX update

[Peter C. Wallace]

On Mon, 24 Oct 2005, Johnny Billquist wrote:

> On Sun, 23 Oct 2005, "Peter C. Wallace" <pcw at mesanet.com> wrote:
>
>> On Sun, 23 Oct 2005, Tim Shoppa wrote:
>>
>>> Getting back to FPGA's, I know of a couple FPGA implementations of
>>> PDP-11's.  (They are mostly KDJ11 clones, but they differ in a couple
>>> of tiny respects.)  With extreme effort in the late 90's, they managed
>>> to make 4 FPGA's be about a factor of 2 factor on most benchmarks than
>>> a 11/93.
>
> And the PDP-11 is much easier to implement than a VAX.
>
>>> By the time the FPGA implementations made it to market the PC-based
>>> emulators were so much more cost-effective for most applications
>>> (despite their warts of running under a host OS...)
>>>
>>> I suspect that a FPGA implementation of a VAX would have a
>>> performance about equal to a 11/780 if done by an average Joe.
>>> Someone with much experience in caching/pipelining could probably
>>> eek out a factor of 2x or 3x by pulling out all the tricks in the
>>> book.
>>>
>>> Tim.
>>>
>>
>> Well since its pretty easy to get microcode to run at 75-100 MHz or so in
>> current cheap FPGAs, I'd say that 10 X a 780 should be trivial...
>
> I'd say that's wrong. Considering that the VAX8650 microcode engine runs
> at 68 MHz and manages about 7 times the 11/780, you're optimistic.
> The 8650 have a very large microcode word, actually have three (or was
> it four?) microcode engines running in parallell, and some very advanced
> cacheing and pipelining to speed it up to get even that far.


 	Not sure, but my guess is that a lot of that sophisticated caching and 
pipelining was needed to get the microengine to run at that 68 MHz because of 
the chip-chip delays in a large multi-chip design.

 	Current cheap FPGAs can manage 75-100 MHz with no pipelining. They can 
also do 32 bit adds/subtracts in < 7 or so nS. I doubt if the 8650s hardware 
could manage that. Would be interesting to know the average number of 
microinstructions per macroinstruction on the 8650...


>
> 	Johnny
>
> Johnny Billquist                  || "I'm on a bus
>                                  ||  on a psychedelic trip
> email: bqt at update.uu.se           ||  Reading murder books
> pdp is alive!                     ||  tryin' to stay hip" - B. Idol
>

Peter Wallace