30 Sep
2001
30 Sep
'01
5:06 a.m.
On Sun, 30 Sep 2001, Pete Turnbull wrote:
you're assuming (on the left) that
"carry" is the same as "borrow".
Wrong! If you are subtracting, working the columns right-to-left, a
borrow from one column to the previous (on the right) is balanced by
adding the borrowed digit back to the subtrahend, so it gets
subtracted from the result, not added. This is the opposite of a
carry.
This works itself out exactly like that below.
However, you'll get the right answer if you say
that "A - B" is the
same as "A + inv(B) + 1" where "inv(B)" is the ones-complement of B.
Bingo! That fixed it:
* I get correct overflow flag values.
* The correct borrow flag value is the opposite of the carry out of the
addition.
Now what's disturbing is that the same wrong ideas that I was taught were
taught to an entire roomful of college students a few years ago, by a
professor in the School of Electrical and Computer Engineering.
Thanks for the help.
--
Jeffrey S. Sharp
jss(a)subatomix.com