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 it.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
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