Catch Fixed Point Overflow in C or C++

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2 total messages Started by Steve Sullivan Thu, 29 Feb 1996 16:32

#11

Author: Steve Sullivan
Date: Thu, 29 Feb 1996 16:32

34 lines
1538 bytes

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");

Approach 2:
   int sum, i, j;
   long lprod, li, lj;
   setOverCheck();   /* this is an asm routine that enables
                        "interrupt on overflow" */
   try {
      sum = i + j;      /* get exception if overflow occurs */
      lprod = li * lj;  /* get exception if overflow occurs */
   }
   catch (const char * msg) { printf("overflow occured in %s\n", msg); }

Approach 2 would be much preferred, if possible, since it doesn't
require checking after each operation as in approach 1.

Where could I find such asm code, or references on how to write it?
I don't know x86 asm, but if need be I'll learn it (sigh!).
Also, would similar approaches work to detect floating point exceptions?

Many thanks for any suggestions --

Steve Sullivan

Re: Catch Fixed Point Overflow in C or C++

#12

Author: Andy J Robb
Date: Tue, 05 Mar 1996 04:05

60 lines
2312 bytes

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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