This barrier, often just called the "8 GB barrier", is one of the most
important in the hard disk world. Now that hard disk capacities have moved into the tens
of gigabytes and beyond, it gets most of the attention that the old 504 MiB / 528 MB barrier used to get in the
mid-to-late 1990s. Many people run into this particular barrier as they attempt to upgrade
systems originally purchased in the late 1990s with hard disks of 1 GB to 8 GB or so in
size.
Like most of the others, this barrier is also based on a BIOS limitation. It is a
tougher nut to crack than most of the smaller-valued barriers, however. The reason for
this is that with this particular barrier, we have actually come up against one of the
traditional limits of how hard disks are used in the PC: the
Int13h interface. That standard allocates 10 bits for the cylinder number (and thus a
maximum of 1,024 cylinders), 8 bits for the head number (maximum of 256) and 6 bits for
the sector number (maximum of 63, since the number 0 is not used). Multiplying these
together, and assuming the standard of 512 bytes per sector, you get a maximum of
8,455,716,864 bytes. This is the largest hard disk size that can be addressed using the
standard Int13h interface.
Unlike the old 504 MiB barrier, there is no translation that can get around this
because it isn't the result of a combination of limitations like the 504 MiB
barrier is. It is in fact the limit of how hard disks can be represented using the BIOS
Int 13h routines used by DOS and applications to access the hard disk. To get around this
barrier, we must change the way hard disks are accessed entirely. This means leaving
Int13h behind and using Int13h extensions.
Note: Int13h extensions require support from both the BIOS and
the operating system. Some older operating systems do not support Int13h extensions, and
there are no plans to provide it for them. In particular, all versions of straight
non-Windows DOS (6.22 and earlier), and Windows NT version 3.5 will not support Int13h
extensions and cannot use hard disks over 8.4 GB in size.
Note: Some systems have a
smaller Int13h capacity limit due to the use of modified translation to avoid presenting
geometry with 240 heads to the operating system. See
here for details.
Next: The Windows 95 Limit (29.8 GiB / 32.0 GB) Barrier
