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PDP-11 compat mode [was Re: New Vax - future directions :-)]

>>> If we ignore the whole problem that the PDP-11 compatibility mode
>>> does not include split I/D space to start with, [...]
>> That's fixable, since I'll be running on an emulated VAX to begin
>> with.
> I'd be interested in hearing you you will fix that. :-)

It's just a little code in the instruction-stream code paths.

But the major reason I'm writing now is, I'm implementing PDP-11 mode
and I've run into something I want to check.

The VARM I have (EL-00032-00-decStd32_Jan90.txt), describing the PDP-11
SBC and SBCB instructions, says (page 9-20)

              N <- dst LSS 0;
              Z <- dst EQL 0;
              V <- {integer overflow};
              C <- {borrow into most significant bit};

That looks...implausible.  I believe it should be

              C <- {borrow out of most significant bit};

because multiprecision subtraction is a major pain if the text is
accurate, and ADC(B) does say "carry from most significant bit".  Does
anyone here know for sure how real -11s work in this regard?

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