From: Sergio Masci (sergio@titan.demon.co.uk)
Date: Fri 10 Mar 2000 - 18:36:05 IST
>
>
> This is somewhat off-topic, but I figure if anyone can answer it, you guys
> can. Thanks.
>
> ==========
>
> Ok, you smart guys, how do I get the upper 32 bits of a number when using
> a 32-bit compiler? I.e., >> 32 would work on a 64-bit system, but you
> can't do that on an x86, right?
>
> Here's an example using a "Watcom" compiler:
>
> /*------------------------------------------------------------------------*
> BIGMUL -> 32bit * 32bit = 64bit. We need the upper 32 bit.
> On a 64-bit compiler, it would be (num1 * num2) >> 32;
> *------------------------------------------------------------------------*/
> int BIGMUL(int a, int b);
> #pragma aux BIGMUL = \
> "imul edx" \
> parm [eax] [edx] \
> value [edx];
>
>
> How I rewrite this for gcc?
>
> Thanks!
>
> -Jay
>
The simplest way is to use the 'long long' data type. This provides 64 bit
ints on 32 bit machines.
e.g.
long BIGMUL(int a, int b)
{
long long xa, xb;
long c;
xa = a;
xb = b;
c = (xa * xb) >> 32
return c;
}
you could of course do it like this:
long BIGMUL(int a, int b)
{
return (long)(((long long)a * b) >> 32);
}
When I last looked, this was no where near as efficent as in line assembler
but if you're only using this occationally that wont make any difference.
Regards
Sergio
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