The original Rx+ series of devices used timing loops for the erase/programming time based on a user supplied osc freq. These were done in ranges. Something like 0 to 3 MHz delayed "n" machines cycles, 3 to 6 MHz delayed "m" machine cycles, etc - these are not the exact ranges - I just don't want to pour though the code to look them up - but you get the idea. When the Rx2 and 66x devices were made, they had a self-timed mode for erase/programming that depended on feedback from the memory. This is the fastest way (quick erase). To keep the ISP and IAP code compatible, we left the original method of timing loops in the IAP (and hence ISP) code and provided an option that allowed you to use the quick erase by specifying fosc = 0. It sounds like an out-of-range number, like FFh, will default to the longest erase time. I only wrote the ISP portion, I didn't write the IAP so I would have to go look at the IAP.