INT 13h / extended INT 13h
The BIOS interrupt 13h provides functions for direct access of the disk.
These functions are implemented by the BIOS, i.e. these functions can
be used without the need of any disk drivers. This is important especially
for booting, since at this moment no operating system and no hard disk drivers
are loaded. Typically the INT 13h is used for reading the boot sector from the
disk. The INT 13h uses the CHS notation with a width of 24 bit in order to
address a sector. Therefore the INT 13h can access at most 8 GB of a disk.
If the disk is larger, then in the best case the first 8 GB are accessible
by the INT 13h.
Extended INT 13h:
The extended INT 13h is the extension of the INT 13h. This extension has been
specified at the beginning of the Nineties by IBM and Microsoft. One
difference (among others) of the extended INT 13h is that it uses LBA numbers
with a width of 64 bit for addressing disk sectors. Therefore the extended
INT 13h supports disks up to a size of 8 billion TB. This limit exceeds
today's typical disk capacities (8 GB) by 12 orders of magnitude. Hence it
should be possible to use the extended INT 13h for "some" more years.
Basically the configuration program Boot-US uses "only" 32 bit
for the LBA numbers.
Please note, however, that also the partition table uses LBA numbers with a
width of 32 bit. Even these 32 bit still allow to address all sectors up to a
disk size of 2048 GB (2 TB).