| Overcoming
the 8.4-Gbyte Barrier
As PC
storage requirements grow, new bottlenecks appear. Once managed,
corrected bottlenecks have traditionally unleashed growth for PC storage
capacities. The two primary issues, BIOS and operating
system limitations, create inhibitors to new capacity
points, unless they are anticipated and dealt with.
Resolving
the BIOS Issue
Basic input/output
system (BIOS)
is a set of programs permanently stored in a read-only memory
chip (ROM)
that provides the most basic control and management of a computer’s
hardware. When a computer is switched on, the BIOS conducts a series of
complex tests of the installed devices. During run-time, the BIOS
provides the operating system and application programs with access to
those devices.
BIOS
limitations previously occurred at 528-Mbyte and 2.1-Gbyte capacities.
The next BIOS frontier is the 8.4-Gbyte barrier. This barrier occurs
because of the BIOS’s addressing limitation. Unlike previous BIOS
limitations, the solution is more complex, but like previous
constraints, upgrading to the latest version BIOS will solve this
problem.
The
8.4-Gbyte constraint is based on an obscure method of causing the PC to
perform a disc drive operation called an interrupt
(INT). When the BIOS wants to get data onto or off of the hard disc, it
must send a software interrupt. The key storage interrupt is INT 13h.
Older versions of BIOS do not support this interrupt on disc drives
larger than 8.4 Gbytes. Like the rest of PC architecture, this interrupt
has been enhanced to accommodate the larger capacities required in
today’s systems.
There
are three solutions for adding INT 13h Extensions to existing systems:
upgrade system BIOS, add an intelligent host adapter card with new BIOS
on the board, or use Seagate’s DiscWizard
software. The new BIOS is a hardware solution that will allow the system
to recognize greater than 8.4-Gbyte capacity as a native function, and
the DiscWizard software bundle, utilizing DiscWizard Starter Edition, will create a
new layer of software that will translate to accommodate greater than
8.4-Gbyte capacity.
Many
Motherboard manufacturers provide BIOS upgrades; contact them or visit
their web sites for information. Intel offers flash upgrades to many of
their current BIOSs. Upgradable BIOSs include the TX, HX, VX, and LX
family of chipsets. These BIOS upgrades can be found on the Intel web
site (www.intel.com). (Tip: Pressing
the Pause key during POST will sometimes pause the screen long enough to
read
the system BIOS dates and numbers.)
Resolving
the Operating System Issue
Completely
separate from the BIOS issue is the Microsoft FAT12-16 file system
limitation that forces you to split large drives into multiple 2.1-Gbyte
or smaller partitions. Windows 95a can provide large drive support, but
it is still limited to the 2.1-Gbyte partition size.
DOS/Win3.x
If you
are using DOS/Win3.x (FAT12-16), an additional device driver
(such as ONTRACKD.sys) is required to break the 8.4-Gbyte barrier. This
driver is available by contacting Seagate
Technical Support While this driver will break the 8.4-Gbyte
barrier, partitions will still be limited to 2.1 Gbytes.
Windows 95/98
For
Windows 95 and 98 the solution is extended file system support
provided with FAT32. Because upgrades to Win95 containing FAT32 (OSR2)
are currently only available through OEMs, the best current operating
system solution to resolving this issue is to upgrade to Windows 98.
Taking this into account, Seagate's DiscWizard
software is fully compatible with the new FAT32 file systems provided in
Windows 98.
Windows NT 3.51 and 4.0
Windows
NT 3.51 and 4.0 require a bus
mastering driver to support Ultra DMA.
Versions of this driver as of 5-June-1998 do not support the full
capacity of drives larger than 8.4 Gbytes. Please check with Microsoft
or the chipset maker for more information.
Following
is some information regarding Microsoft operating systems and Ultra DMA
support:
For more
detailed information please contact www.microsoft.com.
System
BIOS Parameters for Drives Greater than 8.4 Gbytes
If
system BIOS can break the 8.4-Gbyte barrier, how system BIOS uses and
reports drive parameters becomes a bit
misleading and confusing.
Currently,
16,383 cylinders is the absolute upper limit for system BIOSs and
DOS-type operating systems. When combined with the normal 16 heads and
63 sectors per track
(16,383 x 16 x 63 x 512 bytes per sector
= 8,455,200,768 bytes), this produces a limit of about 8.4 Gbytes for
capacity reporting. The normal way of computing drive capacity
(Cylinders x Heads x Sectors x 512 bytes/sector) can no longer be used
for drives larger than 8.4 Gbytes. For drives larger than 8.4
Gbytes, we must use the total number of addressable sectors on the drive
times 512 bytes per sector. For example, the ST39140A label
that shows 17,803,440 addressable sectors also shows 16,383C, 16H, 63S.
Therefore, 17,803,440 total sectors x 512 bytes per sector =
9,115,361,280 bytes.
The
standard C-H-S parameters are displayed on the Seagate drive label and
in the Seagate Desk Reference,
along with total addressable sectors for these large drives. If
"User" is used in system BIOS, even with LBA
enabled, and the posted C-H-S parameters are entered, the drive capacity
will be limited to 8.4 Gbytes. With system BIOSs that support large
drives, you must use "Auto" with LBA enabled to get the full
capacity of the drive. (Tip: Some system BIOSs have a separate HDD
detect utility that normally uses the standard C-H-S method and will not
function correctly with large drives). Using System BIOS
"Auto" on those BIOSs that support large drives will use the
total addressable sectors on the drive to compute drive capacity. The
total capacity of the drive may or may not be displayed correctly in
system BIOS. The true capacity as seen by the system BIOS and the
operating system can only be displayed using FDISK or CHKDSK.
Although
this discussion is primarily for ATA
devices, the BIOS on some Adaptec (www.adaptec.com)
SCSI
Adapters (such as AHA2940U2W-LVD) provides Extended INT 13h support and
will successfully break the 8.4-Gbyte barrier for recognized capacity
when using an operating system that supports Extended INT 13. UNIX and
other non-DOS operating systems do not necessarily suffer from the
8.4-Gbyte limitation.
How
Operating Systems Report Drive Capacity
Windows
NT
From Windows Explorer, right click on a drive letter, then click on
Properties. This shows capacities in bytes, Mbytes, and Gbytes.
Windows
95/98
From Windows Explorer, right click on a drive letter, then click on
Properties. This shows bytes, Mbytes, and Gbytes.
DOS Prompt – CHKDSK shows bytes
DOS Prompt – FDISK shows Mbytes
DOS/Windows
3.x
CHKDSK shows bytes
FDISK shows Mbytes
Converting
from Gbytes to Mbytes to Kbytes to bytes
1 Kbyte = 1024 bytes (210)
1 Mbyte = 1,048,576 bytes (220)
1 Gbyte = 1,073,741,824 bytes (230)
Examples
8.6 Gbytes = 8.6 x 1,073,741,824 = 9,234,179,686 bytes
2047 Mbytes = 2047 x 1,048,576 = 2,146,435,072 bytes
1024 Kbytes = 1024 x 1024 = 1,048,576 bytes |
