FPGA VAX update

[der Mouse]

> And of course, with a VAX, you have the bloody instruction fetch and
> decode stuff which really is a pain.  [...]  Worst instructions take
> five arguments if I remember correctly.

Eight instructions take six.  Five of them (addp6, subp6, mulp, divp,
ashp) are packed-decimal and thus may not be in silicon or microcode,
and two (movtc, movtuc) are relatively rare string operations, but the
other (index) is simple and (for some languages) heavily used; I'd
expect at least that one to be done in silicon/microcode.  (And for the
other seven, even if they're just traps to macrocode as they are on
(eg) the uVII, I *think* you need to do at least rudimentary argument
parsing before taking the trap.)

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