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

On 2021-07-17 23:47, Mouse wrote:
               C <- {borrow into most significant bit};
               C <- {borrow out of most significant bit};
You're correct; the description you cited is an obvious typo.
A nigh-ubiquitous one!  [...]
Not sure if it is a typo, or if it is a question on how you interpret

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

But it's a borrow out of that bit, not into it.

At least, as I learnt to use the terms, and as they are used elsewhere
in the VARM (eg, when describing carries in addition; I see no reason
why the meanings of "into" and "out of" would be switched between
carries and borrows).

Addition is very different than subtraction in what role the carry plays.

In addition it is very simple, since it really just just the overflow from the high bit addition.

With subtraction, you sometimes needs to "borrow" from the next higher bit. But when you are already at the highest bit, there is nothing left from borrow from. So you borrow from the "carry" instead, and that bit you borrow goes into the most significant bit.

And thus, the carry bit itself indicates that this was indeed needed.


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

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