[Hardware] language neutral processors

Jecel Assumpcao Jr jecel at merlintec.com
Tue Jul 31 21:46:24 UTC 2007

I am back from a nice vacation (my first since 1982...) and see that you
have all been waiting patiently for me to return before starting any
interesting discussions ;-)

One interesting side effect of this trip is that I now have a new FPGA
development kit - the ML401 from Xilinx. It has some limitations (like
the 50MHz video and soldered 64MB of SDRAM) compared to newer kits but
makes a far nicer Smalltalk computer than what I had been using up to
now. The FPGA is about twice as large as what I plan to use in the
Merlin 6 board I am designing and the Virtex 4 FPGAs allow designs to
run at twice the clock of what the low end Spartan 3 can deliver. For
the next couple of months I will focus on this board so I will return
the Spartan 3 Starter Kit that I had borrowed the my university.

Besides a FAQ I should probably set up a "Frequent Suggestions" page for
my project so people don't waste their time giving my advice I have
heard many times before. Certainly the number one on that page would be
"some people have already tried to make a language specific computer...
using Lisp or something like that... and it was a horrible failure, so
you should do a language neutral processor instead or just use what is
commercially available".

The problem with this advice is that it assumes that a "language neutral
processor" exists and is a good thing. If we define that to mean "a
processor so primitive and horrible that no high level language is
practical on it so you limit yourself to assembly" then I have seen a
few. Other than that, I have never seen such a beast since the 8 bit
microprocessor days (or early mainframes or early minis - each
generation recapitulated what the previous one had done so it would be
redudant to tell the same story three times).

The 8086 was a mix of features to support the assembly programmer and to
deal with Pascal. The segments, the frame pointer and similar stuff made
it trivial to implement the UCSD virtual machine on it or have Turbo
Pascal generate code for it. Meanwhile, Intel's 32 bit iAPX432 started
life as a PL/I+objects machine and was converted into an Ada+objects
processor when that language started to make headlines (even before it
was defined!) and some of what was learned in that design was applied to
the 80286 (which was even renamed iAPX286 to reflect this).
Unfortunately for Intel the C/Unix virus was taking over the market and
their pride and joy couldn't run C. Oh, it could run something very much
like C with near and far pointers and other horrors or it could run real
C if you contented yourself with 128KB or less, but you couldn't just
take some application running on the VAX and recompile it for the 286.

Motorola had done something halfway between a PDP-11 and a VAX in its
68000 processor. So even though it put a lot of effort into Pascal (see
LINK and UNLINK instructions, for example) it naturally did well with
the "portable PDP-11 assembly" that is C. But they got scared with all
the Ada/Modula-2 noise Intel and National were making and made the 68020
a Modula-2 machine to compete with National's 32032. Except that the
market was interested in C, so Motorola actually removed the extra
features from the 68030. And nobody noticed :-)

Intel decided that "if you can't beat them, join them" with its 386
processor. With some very well thought out hacks it was a hybrid capable
of dealing with C and Unix as well as its competitors while at the same
time running all the old code, including what really mattered through
the new "virtual 8086" mode. Meanwhile, some university people figured
out that these C and assembly processors were more expensive and slower
than what a pure C processor would be and soon their RISCs were putting
commercial cpus to shame. Intel split its 486 into a pure C part and an
extras part in order to keep up, and followed NextGen's (bought by AMD)
lead into the core RISC plus translation style with its Pentium Pro
(also released as the Pentium II, Pentium II and Pentium M not to
mention Celerons and stuff like that). The Pascal and iAPX432 stuff is
still in our x86 processors today (as is the BCD math from the 8080
days) but the designers don't put any effort on them and so it is far
faster to do the same thing with lots of C instructions instead.

Now that we have a world where every OS is Unix (yes, I even count
Windows NT in that crowd), every language is C or a front end for it and
every PC has a x86 processor it is no wonder that people can't imagine
alternatives, nor even imagine why you might want to imagine
alternatives. I have heard many times in the past couple of years that
"C is the actually language of the machine, so even if you do stuff in
other languages it has to be translated into C or you need short
fragments of C code". I normally point out "just try clearing the cache
from C" but in a way they are right. But I don't agree that this is how
things have to be.

Oh, and about those Lisp Machines: they died off but so did the VAX
which were as "language neutral" as anything we have today.

-- Jecel

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