[J-core] VHDL programming resources.

[Rob Landley]

Unfortunately there isn't a good "how to program VHDL the way j-core
uses it" book or tutorial online that we can point people at yet. Just
as a lot of assembly programmers who learned C basically wrote assembly
language in C, a lot of verilog programmers who learned VHDL write
verilog in VHDL. Doing that is _not_ making proper use of the langauge,
and leads people to say "what's the point of VHDL, just use verilog".

The j-core VHDL source code uses VHDL in a manner pioneered by the
European space Agency for Project Leon (their SPARC clone, which they
sent up in some space probes). (As we mentioned in our presentation,
before cloning superh the j-core developers first tried to use Leon, but
were disappointed by the memory bandwith requirements of Sparc's verbose
instruction set.)

Jiri Gaisler's paper on the ESA's VHDL programming techniques is a
valuable resource if you're trying to learn j-core's style of VHDL
development:

  http://gaisler.com/doc/vhdl2proc.pdf

And here's a set of slides describing the technique:

  http://www.gaisler.com/doc/structdes.pdf

Another excellent resource for vhdl development used to be vhdl.org,
which unfortunately went down (the company that hosted it now redirects
the page to their corporate website). Luckily most of the material can
be salvaged out of archive.org:

  https://web.archive.org/web/20150214082011/http://www.vhdl.org/

(We're trying to get permission to either take over the website or
rehost the material ourselves somewhere.)

VHDL has "libraries", I.E. standard sets of predefined functionality you
can #include for things like floating point registers. (It's a bit like
libc.a in C programming.) One of the things vhdl.org had was the
standard library reference.

Rob