Port-vax archive

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Old Index]

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

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 Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

Home | Main Index | Thread Index | Old Index