"Original" IBM PC

IIRC, most 8" drives were 48tpi, and had 77 cylinders. What I don't know is the radius of the outside cylinder (which would be track 0 on side 0 IIRC). It's got to be A little under 4", of course. I don;t have a spare 8" disk to rip apart and measure the diameter of the magnetic disk itself, nor do I know how far in track 0 is fro mthe edge of the disk. Once we know that, obvious places pf the directory are on track 0, track 1 (allowing track 0 to be used for a bootstrap) and track 38 (in the middle). Which would be 0, 1/48" and 38/48" in from the position of track 0. [Yes I did once -- but only once -- make the mistake of forgetting to divide the change in diameter by 2 when turning a metal part. In other words I thought 'Oh, I need to take 50 thou off the diameter' and then moved my cutting too in my 50 thou (in several passes of course) on the lathe. Result : the part was undersized, I'd reduced the radius by 50 thou, and thus the diameter by 100 thou. I am otld some Americal lathes have the cross-slide calibrated so that the cutting tool moves half the distance it claims, the side is calibrated for the change in diameter. British lathes do what they say, the slide is calibrated for the actual distance the tool moves. Which is actually more convenient, particualrly if you also do milling or drilling in the the lathe.] -tony