sectors from 1 to 255. This means the reported number of heads never exceeds 16 (0-15), the number of sectors can be 255 (1-255; though 63 is often the largest used) and the number of cylinders can be as large as 65,536 (0-65535), limiting disk size to 128 GiB (?137.4 GB), assuming 512 byte sectors. These values can be accessed by issuing the ATA command "Identify Device" (ECh) to the drive.

However, the IBM BIOS implementation defined in the INT 13h