Perimeter of the Ellipse (HP-15C)

[Gerson W. Barbosa]

Thank you very much, C.Ret, for the HP-15C version!
The HP-15C was my first calculator and I am supposed to know how to program it, yet I had no idea on how to start.
The results almost exactly match my latest table, except for 1, 2 or 3 ULPs. There was only one result with a 5-ULP difference, but it turned out to be still close to the actual result, but on the other side.
Only about 8 seconds on my HP-15C, purchased brand-new in ‘83 or ‘84. As a comparison, one AGM iteration takes 2.5 seconds, but then again my program might not be nearly as optimized as it could.