due to backward compatibility issues. Overlapping ATA CHS mapping with BIOS CHS mapping produced the lowest common denominator of 10:4:6 bits, or 1024 cylinders, 16 heads, and 63 sectors, which gave the practical limit of 1024×16×63 sectors and 528MB (504 MiB), assuming 512 byte sectors.

In order for the BIOS to overcome this limit and successfully work with larger hard drives, a CHS translation scheme had to be implemented in the BIOS disk I/O routines