Store it as two long long
values. Here's a sketch of a solution:
#include <stdio.h>
typedef unsigned long long ull;
void multiply( ull a, ull b, ull * lo, ull * hi )
{
ull ah = a >> 32;
ull bh = b >> 32;
ull al = a & 0xFFFFFFFF;
ull bl = b & 0xFFFFFFFF;
ull mid = ah * bl + al * bh;
ull albl = al * bl;
ull imm = mid + ( albl >> 32 );
*lo = ( mid << 32 ) + albl;
*hi = ah * bh + ( imm >> 32 );
}
int main()
{
ull n1 = 10000000000LL;
ull n2 = 10000000000LL;
ull lo, hi;
multiply( n1, n2, &lo, &hi );
printf( "result in hex is %llx%016llx\n", hi, lo );
return 0;
}
Outputs:
result in hex is 56bc75e2d63100000