Cylinders versus tracks...

On 22 Dec 2006 at 10:46, Dave Dunfield wrote: Much worse. In fact, when talking about floppy addressing, one needs to take into account the following: a. The physical position of the sector/side/track on the disk. b. The address in the header for each sector on the disk c. What the driver calls them d. What the operating system calls them For example, if the OS/driver combination uses a transparent sparing method, there is a translation between (c) and (d) that maps out bad sectors, so the disk appears to be a continuous span of unflawed sectors. Another example is the insistence of CP/M on 128 byte sectors--there's a blocker/deblocker that maps 128 byte sectors to 256 byte or larger sectors. For (b) to (c), the driver generally performs a translation from some sort of linear consecutive addressing to whatever the floppy sector addresses are. Usually, this is a form of sector "skewing" , but it needn't be--I've got at least one OS sample that places sector 0 on track 20 on a 40 track disk (SSDD, so tracks=cylinders in this case). The sequence then proceeds from track 20 outward to track 0, then inward from track 21 to track 39. Similarly, on double-sided diskettes, addressing can proceed from sector, then side, then cylinder or sector, cylinder then side. Some formats go from physical cylinder 0 to cylinder n on one side, then from cylinder n in reverse down to cylinder 0 on the other side. Others start at cylinder 0 again. Some formats start on the bottom surface; others on the top. For (a) to (b), just about anything in the address fields will generally work, unless a controller insists on a particular convention. A skew may be built in, so that driver software doesn't have to deal with it. Controllers that can do multisector will benefit from sector addresses that are consecutive by 1, but there's no inherent demand that it will be so. All of which makes talking about cylinder, track and sector on floppies extremely confusing. I try to disambiguate a bit by using the term "physical" to refer to the absolute location of sides and tracks, but use the header ID value to refer to sector numbering. Thus, if the first sector on a track is addressed as 129, I'll talk about sector 129, not sector 0. Cheers, Chuck