User Guide

since this was done in different ways, there appeared several BIOS

functioning modes:

• Normal. In this mode cylinder number takes 12 bits, and head number

takes 6 bits, allowing for 4096 cylinders and 64 heads.

• Large and LBA. In these modes cylinder number takes 10 bits and head

number takes 8 bits, allowing for 1024 cylinders and 256 heads.

Few operating systems support the Normal mode. In the LBA mode errors in

operating systems in calculating the hard disk parameters result in limitation

of 255 heads, limiting the size to ~8032 megabytes.

BIOS extension that allowed addressing sectors via absolute numbers instead

of cylinder, head, and sector numbers was a considerable advance. Absolute

number takes 64 bits, thus allowing addressing hard disks of enormous

capacity.

So the troubles with limitations arise in the following cases:

• If an application (or an operating system) does not support BIOS

extension, it will be able to see not more than 1024 * number_of_heads

* sectors_per_track sectors or 4096 * number_of_heads *

sectors_per_track sectors if the application supports the Normal mode,

usually these are 8064 megabytes.

• If the program has an error in calculating hard disk parameters (all

MS-DOS version have this error), and BIOS reports 256 heads on the

hard disk.

• If BIOS does not support extension (usually these are BIOSes that were

released before 1994).

In some BIOSes even the extension does not allow to address more than

8064 megabytes.

BIOS problems can be solved by installing some program like EZ-Drive, DM6

DDO, MaxBlast, that uploads its own interrupt 13h code that is free of above-

mentioned disadvantages. Acronis OS Selector whenever possible works

through BIOS extension and is fully compatible with EZ-Drive type programs.