CP/M directory entries

On 13 Apr 2012 at 19:40, Fred Cisin wrote: I'm not certain that either way is faster in practice--some BIOS versions use the recalibrate command stepping at the lowest rate, so they're actually slower than a seek. But since opening a file extent involves reading the directory entry, putting the directory in the center means that you half to seek a maximum of half the disk after an open. However, higher-numbered cylinders exhibit higher packing densities, so in theory the center tracks are less reliable than the outer ones. On the other hand, for many controllers (e.g. NEC 765) a reset involves a recalibrate, so cylinder 0 at power-on is the somewhat likely to get clobbered by a hardware glitch. I recall working on a hard disk file system where the allocation information for a cylinder was kept on the first sector of the cylinder. The master allocation bitmap recorded the cylinders that contained unallocated sectors. I don't know if it turned out to be faster. Floppies were always nasty with allocation maps getting clobbered leaving one to wonder how to put things back together again. The first allocation scheme for floppy that I ever wrote used contiguous allocation (create, write, truncate a la RT11) so losing the directory wasn't a serious issue and consecutive file access was always fast. Of course, the application had to have some sort of idea of how large a file was going to be when creating the file. Defragmenting was an obvious burden, but for many applications it was less necessary than you'd think. --Chuck