longmult := proc(a::integer,b::integer)
    local A,B,m,n,i,j;
    # Note, return a*b; works in Maple for any sized integer
    A := convert(a,base,10);
    B := convert(b,base,10);
    m := numelems(A);
    n := numelems(B);
    add( add( A[i]*B[j]*10^(j-1), j=1..n )*10^(i-1), i=1..m );
end;

> longmult( 2^64, 2^64 );
                    340282366920938463463374607431768211456