🚀 go-pugleaf

sullivan@indra.com (Steve Sullivan) wrote:

>Is it possible to have a Pentium assembler routine set up so
>that fixed-point overflow in C or C++ is caught?  For example,
>would either of the following approaches work?

>Approach 1:
>   /* here carryflag() is an asm routine to return the carry flag */
>   int sum, i, j;
>   long lprod, li, lj;
>   sum = i + j;
>   if ( carryflag()) printf("sum overflow occured\n");
>   lprod = li * lj;
>   if ( carryflag()) printf("prod overflow occured\n");

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The carry flag is not used to detect overflow in signed arithmetic -
e.g:

(-1) + (-1)

will set the carry flag but the result can be represented correctly as
-2 in type int.  The carry flag is used in multi-precision arithmetic
to allow handling of large numbers (outside the range of a single
register).

For signed arithmetic you should look at the overflow flag.

It may be possible to stay within C or C++ by trying the reverse
operation:

sum = i + j;
if (sum - j != i) printf("sum overflow occured\n");
lprod = li * lj;
if (lj && lprod/lj != li) printf("prod overflow occured\n");

Regards,
Andy Robb.

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