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Normal / Standard CHS Mode
The normal or default mode used by "regular" hard disks that are below 504 MiB in size is called variously Normal or CHS mode, where "CHS" stands for "cylinder, head, sector", the three parameters used in hard disk geometry specifications. In this mode, there is no translation done at the BIOS level, and the logical geometry presented by the disk is used by the BIOS directly. Remember that this is still logical geometry. The actual physical geometry is known only to the disk controller.
Each hard disk using CHS mode is limited to 1,024 cylinders, 16 heads and 63 sectors, or 504 binary megabytes. This is also the only mode available on older BIOSes, from before about 1994. When hard disks above 504 MiB in size are used with one of these older BIOSes, the infamous 504 MiB barrier is hit.