Computations with register machines

As an example, the following program for a 3-register machine starting with initial condition {n, 0, 0} will compute {Round[√n], 0, 0}:

{d[1, 4], i[1], d[1, 15], i[2], d[1, 6], d[1, 11], i[1], d[2, 7], d[3, 7], d[1, 15], d[3, 4], i[3], d[2, 12], d[3, 4]}