Decimal to Binary conversion

#include <iostream>
using namespace std;
int main()
{
    unsigned int a,t,mask=0;
    mask=~mask;
    cout<<"Insert an integer number <= "<<mask<<endl;
    cin>>a;
    mask=mask<<(sizeof(mask)*8-1);
    while(mask!=0)
    {
        t=a&mask;
        if(t==0)
            cout<<'0';
        else
            cout<<'1';
        mask=mask>>1;
    }
    cout<<endl;
    system("PAUSE");
    return 0;
}