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Message
From: Victor<victor.lopez@a...>
Date: Thu May 13 14:34:18 CEST 2004
Subject: [openrisc] 1 bit condition code register
Hello Ben I am not an authority in this subject but if I am not mistaken the
condition codes that you mention are Z for Zero, N for Negative, C for
Carry and V? I guess it stands for Overflow, anyways the thing is that
maybe this 1 bit condition code is a bit uneffective as many
comparisons in a software program involve checking at least two of the
condition codes. Let's just take a look at comparisons between numbers:
We have number A and number B both are 2's complement signed integers.
The comparisons in the lowest level are usually done by substracting
one number from the other and doing the comparison of the result with
zero:
A > B? ====> (A-B) > 0? (See? it is the same comparison)
This way the comparison would be true if bit Z=0 and bit N=0 (the
result of the substraction is not zero (as it has to be greater than
zero) and it is not negative)
Another example which only needs one comparison code bit:
A < B? ====> (A-B) < 0?
To be True, the comparison ought to check only the N bit to be 1, as
this indicates that the result is negative (that is, less than zero).
And the same goes with A >= B which requires N=0
and with A <= B which requires (N=1 OR (Z=1 AND N=0)) which is the same
as, in simplified form, (N=1 OR Z=1)
A=B and A!=B imply only checking the Z bit (after the substraction A-B)
but as you can see there are cases in which it is wiser to have all the
condition code bits at hand, as we are not messing with a picky program
that checks first if there has been an Overflow (quite a natural thing
to do) or the carry bit. You must have in mind that if you have just
one bit then the programs will have to repeat the operations that are
checked as many times as bits in the complete comparison codes are
needed (that is two times for a A<B) and also many branches inside this
code... just try to assemble by hand any simple program which has to
make integer comparisons and you will see how performance can be
affected. I hope this helped. Regards,
Victor Lopez
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