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Cylinder/Head/Sector. To set up an IDE hard drive on an old computer that doesn't auto-detect, these are the numbers you need to know.

Incedentally, Cylinder refers to the number of tracks each platter has, Head is the number of heads, and therefore twice the number of platters, and Sector is the number of physical sectors each track is divided into.

Ranges for C/H/S (Cylinders/Heads/Sectors) adressing of a PC's IDE/ATA hard drive - useful if (for example) you emulate PCs/compatibles with something like bochs/plex86 (freemware):

The maximum parameter values accepted by the original int-13h BIOS routines for accessing the hard drive are 1024 for cylinders*, 256 for heads, and 63 for sectors. At 512 bytes per sector, this gives a maximum addressable disk capacity of [1024 * 256 * 63 * 512 =] 8,455,716,864 bytes (8.4 GB/7.875 gibibytes).

Certain actual (ATA) drives at the time were using translation mechanisms targetted at 16383 cylinders, 16 heads, and 63 sectors per track - which (note 1024*256/16 = 16384) also achieves an 8.4 gigabyte capacity. Subsequently, there is a combined limit at 528MB (504 MiB) in BIOSes that predate Logical Block Address (LBA) translation, since 1024/16/63 = 528,482,304 bytes.

Modern drives (larger than 8.4GB) cannot be accessed by the regular (read "legacy", if you'd rather) BIOS routines, so the drives specify the capacity 16383/16/63 to the BIOS, thereby communicating to the OS that it needs to use the "extended int 13h" routines to determine the true physical drive size.

(*Presumably the reason certain LILO versions complain about certain sizes of hard drive/partition arrangements)

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