A forward problem for the Dirac system is to find u = ((u1(x, t))(u2(x,t))) obeying iu(t) + ((0) (1) (-1 0))u(x) + ((p) (q) (q) (-p))u = 0 for x> 0, t> 0; u( x, 0) = ((0) (0)) for x >= 0, and u(1)(0, t) = f( t) for t > 0, with the real p = p(x), q = q( x). An input-output map R: u(1)( 0, .) double right arrow u2( 0, .) is of the convolution form Rf = if + r * f, where r = r( t) is a response function. By hyperbolicity of the system, for any T > 0, function r I-0 0, given r I-0