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

[Johnny Billquist <bqt%update.uu.se@localhost>]

On 2021-07-17 23:09, Mouse wrote:
> > >               C <- {borrow into most significant bit};
>
> > > That looks...implausible.  I believe it should be
>
> > >               C <- {borrow out of most significant bit};
>
> > You're correct; the description you cited is an obvious typo.
>
> A nigh-ubiquitous one!  Turns out EL-00032-00-decStd32_Jan90.txt
> contains much the same language in other places as well.  Here are the
> ones I've managed to find:
>
> - The description of the C bit of the PSL (page 1-27),
> - The description of DEC{B,W,L} (page 3-17),
> - The description of SBWC (page 3-30),
> - The description of SUB{B,W,L}{2,3} (page 3-31),
> - The description of compatibility mode SBC{,B} (page 9-20),
> - The description of compatibility mode SUB{,B} (page 9-28), and
> - The description of compatibility mode CMP{,B} (page 9-29).
>
> I think this may be every case of subtraction affecting a C bit; I
> can't think of any others offhand.

Not sure if it is a typo, or if it is a question on how you interpret it.

With substraction, you actually am borrowing into the most significant 
bit, and that is what the carry indicates. That a borrowing was needed.

Language can be tricky...

  Johnny

--
Johnny Billquist                  || "I'm on a bus
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