The difficulty that was encountered initially was that disks defined with more than 1024 cylinders could be formatted and used by MSDOS software, but would overwrite cylinder 0 if attempts were made to write to Cylinder 1024. Since lower numbered cylinders are typically allocated first, these disks would be put into service, work for months or years, then -- as they approached being full, the data would self destruct. This problem resolved itself in a year or so when disks with capacities greater than 504 MB started to appear. Those required CHS translation (with or without Logical Block Addressing -- LBA). A side affect of CHS translation was that users quit attempting to configure disks as having more than 1024 cylinders since CHS translation required 1024 or fewer cylinders. Heads or sectors per track were simply adjusted upwards to compensate for the reduced cylinders.
A second, more lasting, problem came about because to this day, the BIOS uses basic INT13 based logic to boot almost all hard drives including SCSI drives. That means that even with CHS, LBA and Extended INT13 allowing very large hard drives to be used, the partition booted by the BIOS must have its boot record on cylinders 0-1023. This is almost never an issue with single OS disks. It is usually overcome on multiple OS boot disks by using boot manager software or by using relatively small partitions to boot from. Logical mounting of the rest of the storage is deferred until the OS is booted.
Return To Index Copyright 1994-2002 by Donald Kenney.