[Oberon] [Fwd: Re: question: project oberon on Spartan 3E Starter Board?]

[Paul Reed]

Hi Frans-Pieter,

> You ask me why I want oberon on a fpga, as they are far more complex than
> the arm processors. What can I say, from your website it just looked easy.
> At least a lot less complex than the datasheet of the cortex.

Right - this is exactly why the FPGA is useful, to implement a simple
design of processor so that it can be explained and understood (but also
played with!).

But there is a high hidden cost, namely the enormously complex tools and
the variety of external hardware on today's experimental FPGA boards. 
Very much a moving target, and I certainly don't recommend it! :)  (Unless
you want to make configurable logic your career, in which case you will be
dealing with stuff a great deal more complex than the RISC Oberon
system...)

As I forgot (and then was thankfully reminded) to mention, I think Chris
Burrows' Astrobe is a fantastic way to get you going with Oberon on a nice
cheap ARM board.  Like I said, it depends what you want to do.


> I hoped that the RISC5 core was easily transfered to other fpga's but that
> seems not the case however.

The core runs fine on a variety of FPGAs.  It's actually an amazingly
portable processor - I've had the pleasant experience of porting it to a
new chip, expecting a long debugging session, and it's worked first time
(after removing my own mistakes)! ;-)

However, since the RISC is simple (no cache or pipeline) it requires very
fast asynchronous static RAM, which does not seem to be common on FPGA
boards these days.  The Digilent Spartan 3 Starter board seems to have
been unique in this respect.

Any port of the design to a board with dynamic RAM will inevitably be much
more complicated.  And it's probably not worth it: internal block RAM on
FPGAs has traditionally been rather small (since it's more flexible to use
the silicon for configurable logic blocks, and external RAM is available
if needed), but good old Moore's law is making some inroads here now,
finally.

I just looked at Digilent's recommended replacement for the Spartan 3
Board, which is the Nexys 4, with a huge increase of complexity for the
same price; however, it does claim to have 600KB of on-chip block RAM. 
Still expensive and complicated compared to today's ARM boards though...

Hope that helps,
Paul