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How It Works -- CHS Translation
Plus BIOS Types, LBA and Other Good Stuff
Version 4a
by Hale Landis (landis@sugs.tware.com)
THE "HOW IT WORKS" SERIES
This is one of several How It Works documents. The series
currently includes the following:
* How It Works -- CHS Translation
* How It Works -- Master Boot Record
* How It Works -- DOS Floppy Boot Sector
* How It Works -- OS2 Boot Sector
* How It Works -- Partition Tables
Introduction (READ THIS!)
-------------------------
This is very technical. Please read carefully. There is lots of
information here that can sound confusing the first time you read
it.
Why is an understanding of how a BIOS works so important? The
basic reason is that the information returned by INT 13H AH=08H
is used by FDISK, it is used in the partition table entries
within a partition record (like the Master Boot Record) that are
created by FDISK, and it is used by the small boot program that
FDISK places into the Master Boot Record. The information
returned by INT 13H AH=08H is in cylinder/head/sector (CHS)
format -- it is not in LBA format. The boot processing done by
your computer's BIOS (INT 19H and INT 13H) is all CHS based.
Read this so that you are not confused by all the false
information going around that says "LBA solves the >528MB
problem".
Read this so that you understand the possible data integrity
problem that a WD EIDE type BIOS creates. Any BIOS that has a
"LBA mode" in the BIOS setup could be a WD EIDE BIOS. Be very
careful and NEVER chage the "LBA mode" setting after you have
partitioned and installed your software.
History
-------
Changes between this version and the preceeding version are
marked by "!" at left margin of the first line of a changed
or new paragraph.
Version 4 -- BIOS Types 8 and 10 updated.
Version 3 -- New BIOS types found and added to this list. More
detailed information is listed for each BIOS type. A section
describing CHS translation was added.
Version 2 -- A rewrite of version 1 adding BIOS types not
included in version 1.
Version 1 -- First attempt to classify the BIOS types and
describe what each does or does not do.
Definitions
-----------
* 528MB - The maximun drive capacity that is supported by 1024
cylinders, 16 heads and 63 sectors (1024x16x63x512). This
is the limit for CHS addressing in the original IBM PC/XT
and IBM PC/AT INT 13H BIOS.
* 8GB - The maximum drive capacity that can be supported by 1024
cylinders, 256 heads and 63 sectors (1024x256x63x512). This
is the limit for the BIOS INT 13H AH=0xH calls.
* ATA - AT Attachment -- The real name of what is widely known
as IDE.
* CE Cylinder - Customer Engineering cylinder. This is the
last cylinder in P-CHS mode. IBM has always reserved this
cylinder for use of disk diagnostic programs. Many BIOS do
not account for it correctly. It is of questionable value
these days and probably should be considered obsolete.
However, since there is no industry wide agreement, beware.
There is no CE Cylinder reserved in the L-CHS address. Also
beware of diagnostic programs that don't realize they are
operating in L-CHS mode and think that the last L-CHS cylinder
is the CE Cylinder.
* CHS - Cylinder/Head/Sector. This is the traditional way to
address sectors on a disk. There are at least two types
of CHS addressing: the CHS that is used at the INT 13H
interface and the CHS that is used at the ATA device
interface. In the MFM/RLL/ESDI and early ATA days the CHS
used at the INT 13H interface was the same as the CHS used at
the device interface.
Today we have CHS translating BIOS types that can use one CHS
at the INT 13H interface and a different CHS at the device
interface. These two types of CHS will be called the logical
CHS or L-CHS and the physical CHS or P-CHS in this document.
L-CHS is the CHS used at the INT 13H interface and P-CHS is
the CHS used at the device interface.
The L-CHS used at the INT 13 interface allows up to 256 heads,
up to 1024 cylinders and up to 63 sectors. This allows
support of up to 8GB drives. This scheme started with either
ESDI or SCSI adapters many years ago.
The P-CHS used at the device interface allows up to 16 heads
up to 65535 cylinders, and up to 63 sectors. This allows
access to 2^28 sectors (136GB) on an ATA device. When a P-CHS
is used at the INT 13H interface it is limited to 1024
cylinders, 16 heads and 63 sectors. This is where the old
528MB limit originated.
ATA devices may also support LBA at the device interface. LBA
allows access to approximately 2^28 sectors (137GB) on an ATA
device.
A SCSI host adapter can convert a L-CHS directly to an LBA
used in the SCSI read/write commands. On a PC today, SCSI is
also limited to 8GB when CHS addressing is used at the INT 13H
interface.
* EDPT - Enhanced fixed Disk Parameter Table -- This table
returns additional information for BIOS drive numbers 80H and
81H. The EDPT for BIOS drive 80H is pointed to by INT 41H.
The EDPT for BIOS drive 81H is pointed to by INT 46H. The
EDPT is a fixed disk parameter table with an AxH signature
byte. This table format returns two sets of CHS information.
One set is the L-CHS and is probably the same as returned by
INT 13H AH=08H. The other set is the P-CHS used at the drive
interface. This type of table allows drives with >1024
cylinders or drives >528MB to be supported. The translated
CHS will have <=1024 cylinders and (probably) >16 heads. The
CHS used at the drive interface will have >1024 cylinders and
<=16 heads. It is unclear how the IBM defined CE cylinder is
accounted for in such a table. Compaq probably gets the
credit for the original definition of this type of table.
* FDPT - Fixed Disk Parameter Table - This table returns
additional information for BIOS drive numbers 80H and 81H.
The FDPT for BIOS drive 80H is pointed to by INT 41H. The
FDPT for BIOS drive 81H is pointed to by INT 46H. A FDPT does
not have a AxH signature byte. This table format returns a
single set of CHS information. The L-CHS information returned
by this table is probably the same as the P-CHS and is also
probably the same as the L-CHS returned by INT 13H AH=08H.
However, not all BIOS properly account for the IBM defined CE
cylinder and this can cause a one or two cylinder difference
between the number of cylinders returned in the AH=08H data
and the FDPT data. IBM gets the credit for the original
definition of this type of table.
* LBA - Logical Block Address. Another way of addressing
sectors that uses a simple numbering scheme starting with zero
as the address of the first sector on a device. The ATA
standard requires that cylinder 0, head 0, sector 1 address
the same sector as addressed by LBA 0. LBA addressing can be
used at the ATA interface if the ATA device supports it. LBA
addressing is also used at the INT 13H interface by the AH=4xH
read/write calls.
* L-CHS -- Logical CHS. The CHS used at the INT 13H interface by
the AH=0xH calls. See CHS above.
* MBR - Master Boot Record (also known as a partition table) -
The sector located at cylinder 0 head 0 sector 1 (or LBA 0).
This sector is created by an "FDISK" utility program. The MBR
may be the only partition table sector or the MBR can be the
first of multiple partition table sectors that form a linked
list. A partition table entry can describe the starting and
ending sector addresses of a partition (also known as a
logical volume or a logical drive) in both L-CHS and LBA form.
Partition table entries use the L-CHS returned by INT 13H
AH=08H. Older FDISK programs may not compute valid LBA
values.
* OS - Operating System.
* P-CHS -- Physical CHS. The CHS used at the ATA device
interface. This CHS is also used at the INT 13H interface by
older BIOS's that do not support >1024 cylinders or >528MB.
See CHS above.
Background and Assumptions
--------------------------
First, please note that this is written with the OS implementor
in mind and that I am talking about the possible BIOS types as
seen by an OS during its hardware configuration search.
It is very important that you not be confused by all the
misinformation going around these days. All OS's that want to be
co-resident with another OS (and that is all of the PC based OS's
that I know of) MUST use INT 13H to determine the capacity of a
hard disk. And that capacity information MUST be determined in
L-CHS mode. Why is this? Because: 1) FDISK and the partition
tables are really L-CHS based, and 2) MS/PC DOS uses INT 13H
AH=02H and AH=03H to read and write the disk and these BIOS calls
are L-CHS based. The boot processing done by the BIOS is all
L-CHS based. During the boot processing, all of the disk read
accesses are done in L-CHS mode via INT 13H and this includes
loading the first of the OS's kernel code or boot manager's code.
Second, because there can be multiple BIOS types in any one
system, each drive may be under the control of a different type
of BIOS. For example, drive 80H (the first hard drive) could be
controlled by the original system BIOS, drive 81H (the second
drive) could be controlled by a option ROM BIOS and drive 82H
(the third drive) could be controlled by a software driver.
Also, be aware that each drive could be a different type, for
example, drive 80H could be an MFM drive, drive 81H could be an
ATA drive, drive 82H could be a SCSI drive.
Third, not all OS's understand or use BIOS drive numbers greater
than 81H. Even if there is INT 13H support for drives 82H or
greater, the OS may not use that support.
Fourth, the BIOS INT 13H configuration calls are:
* AH=08H, Get Drive Parameters -- This call is restricted to
drives up to 528MB without CHS translation and to drives up to
8GB with CHS translation. For older BIOS with no support for
>1024 cylinders or >528MB, this call returns the same CHS as
is used at the ATA interface (the P-CHS). For newer BIOS's
that do support >1024 cylinders or >528MB, this call returns a
translated CHS (the L-CHS). The CHS returned by this call is
used by FDISK to build partition records.
* AH=41H, Get BIOS Extensions Support -- This call is used to
determine if the IBM/Microsoft Extensions or if the Phoenix
Enhanced INT 13H calls are supported for the BIOS drive
number.
* AH=48H, Extended Get Drive Parameters -- This call is used to
determine the CHS geometries, LBA information and other data
about the BIOS drive number.
* the FDPT or EDPT -- While not actually a call, but instead a
data area, the FDPT or EDPT can return additional information
about a drive.
* other tables -- The IBM/Microsoft extensions provide a pointer
to a drive parameter table via INT 13H AH=48H. The Phoenix
enhancement provides a pointer to a drive parameter table
extension via INT 13H AH=48H. These tables are NOT the same
as the FDPT or EDPT.
Note: The INT 13H AH=4xH calls duplicate the older AH=0xH calls
but use a different parameter passing structure. This new
structure allows support of drives with up to 2^64 sectors
(really BIG drives). While at the INT 13H interface the AH=4xH
calls are LBA based, these calls do NOT require that the drive
support LBA addressing.
CHS Translation Algorithms
--------------------------
NOTE: Before you send me email about this, read this entire
section. Thanks!
As you read this, don't forget that all of the boot processing
done by the system BIOS via INT 19H and INT 13H use only the INT
13H AH=0xH calls and that all of this processing is done in CHS
mode.
First, lets review all the different ways a BIOS can be called
to perform read/write operations and the conversions that a BIOS
must support.
! * An old BIOS (like BIOS type 1 below) does no CHS translation
and does not use LBA. It only supports the AH=0xH calls:
INT 13H (L-CHS == P-CHS) ATA
AH=0xH --------------------------------> device
(L-CHS) (P-CHS)
* A newer BIOS may support CHS translation and it may support
LBA at the ATA interface:
INT 13H L-CHS ATA
AH=0xH --+--> to --+----------------> device
(L-CHS) | P-CHS | (P-CHS)
| |
| | P-CHS
| +--> to --+
| LBA |
| |
| L-CHS | ATA
+--> to -----------------+---> device
LBA (LBA)
* A really new BIOS may also support the AH=4xH in addtion to
the older AH\0xH calls. This BIOS must support all possible
combinations of CHS and LBA at both the INT 13H and ATA
interfaces:
INT 13H ATA
AH=4xH --+-----------------------------> device
(LBA) | (LBA)
|
| LBA
+--> to ---------------+
P-CHS |
|
INT 13H L-CHS | ATA
AH=0xH --+--> to --+------------+---> device
(L-CHS) | P-CHS | (P-CHS)
| |
| | P-CHS
| +--> to --+
| LBA |
| |
| L-CHS | ATA
+--> to -----------------+---> device
LBA (LBA)
You would think there is only one L-CHS to P-CHS translation
algorithm, only one L-CHS to LBA translation algorithm and only
one P-CHS to LBA translation algorithm. But this is not so.
Why? Because there is no document that standardizes such an
algorithm. You can not rely on all BIOS's and OS's to do these
translations the same way.
The following explains what is widely accepted as the
"correct" algorithms.
An ATA disk must implement both CHS and LBA addressing and
must at any given time support only one P-CHS at the device
interface. And, the drive must maintain a strick relationship
between the sector addressing in CHS mode and LBA mode. Quoting
the ATA-2 document:
LBA = ( (cylinder * heads_per_cylinder + heads )
* sectors_per_track ) + sector - 1
where heads_per_cylinder and sectors_per_track are the current
translation mode values.
This algorithm can also be used by a BIOS or an OS to convert
a L-CHS to an LBA as we'll see below.
This algorithm can be reversed such that an LBA can be
converted to a CHS:
cylinder = LBA / (heads_per_cylinder * sectors_per_track)
temp = LBA % (heads_per_cylinder * sectors_per_track)
head = temp / sectors_per_track
sector = temp % sectors_per_track + 1
While most OS's compute disk addresses in an LBA scheme, an OS
like DOS must convert that LBA to a CHS in order to call INT 13H.
Technically an INT 13H should follow this process when
converting an L-CHS to a P-CHS:
1) convert the L-CHS to an LBA,
2) convert the LBA to a P-CHS,
If an LBA is required at the ATA interface, then this third
step is needed:
3) convert the P-CHS to an LBA.
All of these conversions are done by normal arithmetic.
However, while this is the technically correct way to do
things, certain short cuts can be taken. It is possible to
convert an L-CHS directly to a P-CHS using bit a bit shifting
algorithm. This combines steps 1 and 2. And, if the ATA device
being used supports LBA, steps 2 and 3 are not needed. The LBA
value produced in step 1 is the same as the LBA value produced in
step 3.
AN EXAMPLE
Lets look at an example. Lets say that the L-CHS is 1000
cylinders 10 heads and 50 sectors, the P-CHS is 2000 cylinders, 5
heads and 50 sectors. Lets say we want to access the sector at
L-CHS 2,4,3.
* step 1 converts the L-CHS to an LBA,
lba = 1202 = ( ( 2 * 10 + 4 ) * 50 ) + 3 - 1
* step 2 converts the LBA to the P-CHS,
cylinder = 4 = ( 1202 / ( 5 * 50 )
temp = 202 = ( 1202 % ( 5 * 50 ) )
head = 4 = ( 202 / 50 )
sector = 3 = ( 202 % 50 ) + 1
* step 3 converts the P-CHS to an LBA,
lba = 1202 = ( ( 4 * 5 + 4 ) * 50 ) + 3 - 1
Most BIOS (or OS) software is not going to do all of this to
convert an address. Most will use some other algorithm. There
are many such algorithms.
BIT SHIFTING INSTEAD
If the L-CHS is produced from the P-CHS by 1) dividing the
P-CHS cylinders by N, and 2) multiplying the P-CHS heads by N,
where N is 2, 4, 8, ..., then this bit shifting algorithm can be
used and N becomes a bit shift value. This is the most common
way to make the P-CHS geometry of a >528MB drive fit the INT 13H
L-CHS rules. Plus this algorithm maintains the same sector
ordering as the more complex algorithm above. Note the
following:
Lcylinder = L-CHS cylinder being accessed
Lhead = L-CHS head being accessed
Lsector = L-CHS sector being accessed
Pcylinder = the P-CHS cylinder being accessed
Phead = the P-CHS head being accessed
Psector = P-CHS sector being accessed
NPH = is the number of heads in the P-CHS
N = 2, 4, 8, ..., the bit shift value
The algorithm, which can be implemented using bit shifting
instead of multiply and divide operations is:
Pcylinder = ( Lcylinder * N ) + ( Lhead / NPH );
Phead = ( Lhead % NPH );
Psector = Lsector;
A BIT SHIFTING EXAMPLE
Lets apply this to our example above (L-CHS = 1000,10,50 and
P-CHS = 2000, 5, 50) and access the same sector at at L-CHS
2,4,3.
Pcylinder = 4 = ( 2 * 2 ) + ( 4 / 5 )
Phead = 4 = ( 4 % 5 )
Psector = 3 = 3
As you can see, this produces the same P-CHS as the more
complex method above.
SO WHAT IS THE PROBLEM?
The basic problem is that there is no requirement that a CHS
translating BIOS followed these rules. There are many other
algorithms that can be implemented to perform a similar function.
Today, there are at least two popular implementions: the Phoenix
implementation (described above) and the non-Phoenix
implementations.
SO WHY IS THIS A PROBLEM IF IT IS HIDDEN INSIDE THE BIOS?
Because a protected mode OS that does not want to use INT 13H
must implement the same CHS translation algorithm. If it
doesn't, your data gets scrambled.
WHY USE CHS AT ALL?
In the perfect world of tomorrow, maybe only LBA will be used.
But today we are faced with the following problems:
* Some drives >528MB don't implement LBA.
* Some drives are optimized for CHS and may have lower
performance when given commands in LBA mode. Don't forget
that LBA is something new for the ATA disk designers who have
worked very hard for many years to optimize CHS address
handling. And not all drive designs require the use of LBA
internally.
* The L-CHS to LBA conversion is more complex and slower than
the bit shifting L-CHS to P-CHS conversion.
* DOS, FDISK and the MBR are still CHS based -- they use the
CHS returned by INT 13H AH=08H. Any OS that can be installed
on the same disk with DOS must understand CHS addressing.
* The BIOS boot processing and loading of the first OS kernel
code is done in CHS mode -- the CHS returned by INT 13H AH=08H
is used.
* Microsoft has said that their OS's will not use any disk
capacity that can not also be accessed by INT 13H AH=0xH.
These are difficult problems to overcome in today's industry
environment. The result: chaos.
DANGER TO YOUR DATA!
See the description of BIOS Type 7 below to understand why a
WD EIDE BIOS is so dangerous to your data.
The BIOS Types
--------------
I assume the following:
a) All BIOS INT 13H support has been installed by the time the OS
starts its boot processing. I'm don't plan to cover what
could happen to INT 13H once the OS starts loading its own
device drivers.
b) Drives supported by INT 13H are numbered sequentially starting
with drive number 80H (80H-FFH are hard drives, 00-7FH are
floppy drives).
And remember, any time a P-CHS exists it may or may not account
for the CE Cylinder properly.
I have identified the following types of BIOS INT 13H support as
seen by an OS during its boot time hardware configuration
determination:
BIOS Type 1
Origin: Original IBM PC/XT.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. There is no CHS translation. INT 13H AH=08H
returns the P-CHS. The FDPT should contain the same P-CHS.
Description: Supports up to 528MB from a table of drive
descriptions in BIOS ROM. No support for >1024 cylinders or
drives >528MB or LBA.
Support issues: For >1024 cylinders or >528MB support, either
an option ROM with an INT 13H replacement (see BIOS types 4-7)
-or- a software driver (see BIOS type 8) must be added to the
system.
BIOS Type 2
Origin: Unknown, but first appeared on systems having BIOS
drive type table entries defining >1024 cylinders. Rumored to
have originated at the request of Novell or SCO.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. INT 13H AH=08H should return a L-CHS with the
cylinder value limited to 1024. Beware, many BIOS perform
a logical AND on the cylinder value. A correct BIOS will
limit the cylinder value as follows:
cylinder = cylinder > 1024 ? 1024 : cylinder;
An incorrect BIOS will limit the cylinder value as follows
(this implementation turns a 540MB drive into a 12MB drive!):
cylinder = cylinder & 0x03ff;
The FDPT will return a P-CHS that has the full cylinder
value.
Description: For BIOS drive numbers 80H and 81H, this BIOS
type supports >1024 cylinders or >528MB without using a
translated CHS in the FDPT. INT 13H AH=08H truncates
cylinders to 1024 (beware of buggy implementations). The FDPT
can show >1024 cylinders thereby allowing an OS to support
drives >528MB. May convert the L-CHS or P-CHS directly to an
LBA if the ATA device supports LBA.
Support issues: Actual support of >1024 cylinders is OS
specific -- some OS's may be able to place OS specific
partitions spanning or beyond cylinder 1024. Usually all OS
boot code must be within first 1024 cylinders. The FDISK
program of an OS that supports such partitions uses an OS
specific partition table entry format to identify these
partitions. There does not appear to be a standard (de facto
or otherwise) for this unusual partition table entry.
Apparently one method is to place -1 into the CHS fields and
use the LBA fields to describe the location of the partition.
This DOES NOT require the drive to support LBA addressing.
Using an LBA in the partition table entry is just a trick to
get around the CHS limits in the partition table entry. It is
unclear if such a partition table entry will be ignored by an
OS that does not understand what it is. For an OS that does
not support such partitions, either an option ROM with an INT
13H replacement (see BIOS types 4-7) -or- a software driver
(see BIOS type 8) must be added to the system.
Note: OS/2 can place HPFS partitions and Linux can place
Linux partitions beyond or spanning cylinder 1024. (Anyone
know of other systems that can do the same?)
BIOS Type 3
Origin: Unknown, but first appeared on systems having BIOS
drive type table entires defining >1024 cylinders. Rumored to
have originated at the request of Novell or SCO.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H. INT 13H AH=08H can return an L-CHS with more
than 1024 cylinders.
Description: This BIOS is like type 2 above but it allows up
to 4096 cylinders (12 cylinder bits). It does this in the INT
13H AH=0xH calls by placing two most significant cylinder bits
(bits 11 and 10) into the upper two bits of the head number
(bits 7 and 6).
Support issues: Identification of such a BIOS is difficult.
As long as the drive(s) supported by this type of BIOS have
<1024 cylinders this BIOS looks like a type 2 BIOS because INT
13H AH=08H should return zero data in bits 7 and 6 of the head
information. If INT 13H AH=08H returns non zero data in bits
7 and 6 of the head information, perhaps it can be assumed
that this is a type 3 BIOS. For more normal support of >1024
cylinders or >528MB, either an option ROM with an INT 13H
replacement (see BIOS types 4-7) -or- a software driver (see
BIOS type 8) must be added to the system.
Note: Apparently this BIOS type is no longer produced by any
BIOS vendor.
BIOS Type 4
Origin: Compaq. Probably first appeared in systems with ESDI
drives having >1024 cylinders.
BIOS call support: INT 13H AH=0xH and EDPT for BIOS drives
80H and 81H. If the drive has <1024 cylinders, INT 13H AH=08H
returns the P-CHS and a FDPT is built. If the drive has >1024
cylinders, INT 13H AH=08H returns an L-CHS and an EDPT is
built.
Description: Looks like a type 2 BIOS when an FDPT is built.
Uses CHS translation when an EDPT is used. May convert the
L-CHS directly to an LBA if the ATA device supports LBA.
Support issues: This BIOS type may support up to four drives
with a EDPT (or FDPT) for BIOS drive numbers 82H and 83H
located in memory following the EDPT (or FDPT) for drive 80H.
Different CHS translation algorithms may be used by the BIOS
and an OS.
BIOS Type 5
Origin: The IBM/Microsoft BIOS Extensions document. For many
years this document was marked "confidential" so it did not
get wide spread distribution.
BIOS call support: INT 13H AH=0xH, AH=4xH and EDPT for BIOS
drives 80H and 81H. INT 13H AH=08H returns an L-CHS. INT 13H
AH=41H and AH=48H should be used to get P-CHS configuration.
The FDPT/EDPT should not be used. In some implementations the
FDPT/EDPT may not exist.
Description: A BIOS that supports very large drives (>1024
cylinders, >528MB, actually >8GB), and supports the INT 13H
AH=4xH read/write functions. The AH=4xH calls use LBA
addressing and support drives with up to 2^64 sectors. These
calls do NOT require that a drive support LBA at the drive
interface. INT 13H AH=48H describes the L-CHS used at the INT
13 interface and the P-CHS or LBA used at the drive interface.
This BIOS supports the INT 13 AH=0xH calls the same as a BIOS
type 4.
Support issues: While the INT 13H AH=4xH calls are well
defined, they are not implemented in many systems shipping
today. Currently undefined is how such a BIOS should respond
to INT 13H AH=08H calls for a drive that is >8GB. Different
CHS translation algorithms may be used by the BIOS and an OS.
Note: Support of LBA at the drive interface may be automatic
or may be under user control via a BIOS setup option. Use of
LBA at the drive interface does not change the operation of
the INT 13 interface.
BIOS Type 6
Origin: The Phoenix Enhanced Disk Drive Specification.
BIOS call support: INT 13H AH=0xH, AH=4xH and EDPT for BIOS
drives 80H and 81H. INT 13H AH=08H returns an L-CHS. INT 13H
AH=41H and AH=48H should be used to get P-CHS configuration.
INT 13H AH=48H returns the address of the Phoenix defined
"FDPT Extension" table.
Description: A BIOS that supports very large drives (>1024
cylinders, >528MB, actually >8GB), and supports the INT 13H
AH=4xH read/write functions. The AH=4xH calls use LBA
addressing and support drives with up to 2^64 sectors. These
calls do NOT require that a drive support LBA at the drive
interface. INT 13H AH=48H describes the L-CHS used at the INT
13 interface and the P-CHS or LBA used at the drive interface.
This BIOS supports the INT 13 AH=0xH calls the same as a BIOS
type 4. The INT 13H AH=48H call returns additional information
such as host adapter addresses, DMA support, LBA support, etc,
in the Phoenix defined "FDPT Extension" table.
Phoenix says this this BIOS need not support the INT 13H
AH=4xH read/write calls but this BIOS is really an
extension/enhancement of the original IBM/MS BIOS so most
implementations will probably support the full set of INT 13H
AH=4xH calls.
Support issues: Currently undefined is how such a BIOS should
respond to INT 13H AH=08H calls for a drive that is >8GB.
Different CHS translation algorithms may be used by the BIOS
and an OS.
Note: Support of LBA at the drive interface may be automatic
or may be under user control via a BIOS setup option. Use of
LBA at the drive interface does not change the operation of
the INT 13 interface.
BIOS Type 7
Origin: Described in the Western Digital Enhanced IDE
Implementation Guide.
BIOS call support: INT 13H AH=0xH and FDPT or EDPT for BIOS
drives 80H and 81H. An EDPT with a L-CHS of 16 heads and 63
sectors is built when "LBA mode" is enabled. An FDPT is built
when "LBA mode" is disabled.
Description: Supports >1024 cylinders or >528MB using a EDPT
with a translated CHS *** BUT ONLY IF *** the user requests
"LBA mode" in the BIOS setup *** AND *** the drive supports
LBA. As long as "LBA mode" is enabled, CHS translation is
enabled using a L-CHS with <=1024 cylinders, 16, 32, 64, ...,
heads and 63 sectors. Disk read/write commands are issued in
LBA mode at the ATA interface but other commands are issued in
P-CHS mode. Because the L-CHS is determined by table lookup
based on total drive capacity and not by a multiply/divide of
the P-CHS cylinder and head values, it may not be possible to
use the simple (and faster) bit shifting L-CHS to P-CHS
algorithms.
When "LBA mode" is disabled, this BIOS looks like a BIOS type
2 with an FDPT. The L-CHS used is taken either from the BIOS
drive type table or from the device's Identify Device data.
This L-CHS can be very different from the L-CHS returned when
"LBA mode" is enabled.
This BIOS may support FDPT/EDPT for up to four drives in the
same manner as described in BIOS type 4.
The basic problem with this BIOS is that the CHS returned by
INT 13H AH=08H changes because of a change in the "LBA mode"
setting in the BIOS setup. This should not happen. This use
or non-use of LBA at the ATA interface should have no effect
on the CHS returned by INT 13H AH=08H. This is the only BIOS
type know to have this problem.
Support issues: If the user changes the "LBA mode" setting in
BIOS setup, INT 13H AH=08H and the FDPT/EDPT change
which may cause *** DATA CORRUPTION ***. The user should be
warned to not change the "LBA mode" setting in BIOS setup once
the drive has been partitioned and software installed.
Different CHS translation algorithms may be used by the BIOS
and an OS.
BIOS Type 8
Origin: Unknown. Perhaps Ontrack's Disk Manager was the
first of these software drivers. Another example of such a
driver is Micro House's EZ Drive.
BIOS call support: Unknown (anyone care to help out here?).
Mostly likely only INT 13H AH=0xH are support. Probably a
FDPT or EDPT exists for drives 80H and 81H.
! Description: A software driver that "hides" in the MBR such
that it is loaded into system memory before any OS boot
processing starts. These drivers can have up to three parts:
a part that hides in the MBR, a part that hides in the
remaining sectors of cylinder 0, head 0, and an OS device
driver. The part in the MBR loads the second part of the
driver from cylinder 0 head 0. The second part provides a
replacement for INT 13H that enables CHS translation for at
least the boot drive. Usually the boot drive is defined in
CMOS setup as a type 1 or 2 (5MB or 10MB drive). Once the
second part of the driver is loaded, this definition is
changed to describe the true capacity of the drive and INT 13H
is replaced by the driver's version of INT 13H that does CHS
translation. In some cases the third part, an OS specific
device driver, must be loaded to enable CHS translation for
devices other than the boot device.
! I don't know the details of how these drivers respond to INT
13H AH=08H or how they set up drive parameter tables (anyone
care to help out here?). Some of these drivers convert the
L-CHS to an LBA, then they add a small number to the LBA and
finally they convert the LBA to a P-CHS. This in effect skips
over some sectors at the front of the disk.
Support issues: Several identified -- Some OS installation
programs will remove or overlay these drivers; some of these
drivers do not perform CHS translation using the same
algorithms used by the other BIOS types; special OS device
drivers may be required in order to use these software drivers
For example, under MS Windows the standard FastDisk driver
(the 32-bit disk access driver) must be replaced by a driver
that understands the Ontrack, Micro House, etc, version of INT
13H. Different CHS translation algorithms may be used by the
driver and an OS.
! The hard disk vendors have been shipping these drivers with
their drives over 528MB during the last year and they have
been ignoring the statements of Microsoft and IBM that these
drivers would not be supported in future OS's. Now it appears
that both Microsoft and IBM are in a panic trying to figure
out how to support some of these drivers in WinNT, Win95 and
OS/2. It is unclear what the outcome of this will be at this
time.
! NOTE: THIS IS NOT A PRODUCT ENDORSEMENT! An alternate
solution for an older ISA system is one of the BIOS
replacement cards. This cards have a BIOS option ROM. AMI
makes such a card called the "Disk Extender". This card
replaces the motherboard's INT 13H BIOS with a INT 13H BIOS
that does some form of CHS translation. Another solution for
older VL-Bus systems is an ATA-2 (EIDE) type host adapter card
that provides a option ROM with an INT 13H replacement.
BIOS Type 9
Origin: SCSI host adapters.
BIOS call support: Probably INT 13H AH=0xH and FDPT for BIOS
drives 80H and 81H, perhaps INT 13H AH=4xH.
Description: Most SCSI host adapters contain an option ROM
that enables INT 13 support for the attached SCSI hard drives.
It is possible to have more than one SCSI host adapter, each
with its own option ROM. The CHS used at the INT 13H
interface is converted to the LBA that is used in the SCSI
commands. INT 13H AH=08H returns a CHS. This CHS will have
<=1024 cylinders, <=256 heads and <=63 sectors. The FDPT
probably will exist for SCSI drives with BIOS drive numbers of
80H and 81H and probably indicates the same CHS as that
returned by INT 13H AH=08H. Even though the CHS used at the
INT 13H interface looks like a translated CHS, there is no
need to use a EDPT since there is no CHS-to-CHS translation
used. Other BIOS calls (most likely host adapter specific)
must be used to determine other information about the host
adapter or the drives.
The INT 13H AH=4xH calls can be used to get beyond 8GB but
since there is little support for these calls in today's OS's,
there are probably few SCSI host adapters that support these
newer INT 13H calls.
Support issues: Some SCSI host adapters will not install
their option ROM if there are two INT 13H devices previously
installed by another INT 13H BIOS (for example, two
MFM/RLL/ESDI/ATA drives). Other SCSI adapters will install
their option ROM and use BIOS drive numbers greater than 81H.
Some older OS's don't understand or use BIOS drive numbers
greater than 81H. SCSI adapters are currently faced with the
>8GB drive problem.
BIOS Type 10
Origin: A european system vendor.
BIOS call support: INT 13H AH=0xH and FDPT for BIOS drives
80H and 81H.
Description: This BIOS supports drives >528MB but it does not
support CHS translation. It supports only ATA drives with LBA
capability. INT 13H AH=08H returns an L-CHS. The L-CHS is
converted directly to an LBA. The BIOS sets the ATA drive to
a P-CHS of 16 heads and 63 sectors using the Initialize Drive
Parameters command but it does not use this P-CHS at the ATA
interface.
! Support issues: OS/2 will probably work with this BIOS as
long as the drive's power on default P-CHS mode uses 16 heads
and 63 sectors. Because there is no EDPT, OS/2 uses the ATA
Identify Device power on default P-CHS, described in
Identify Device words 1, 3 and 6 as the current P-CHS for the
drive. However, this may not represent the correct P-CHS. A
newer drive will have the its current P-CHS information in
Identify Device words 53-58 but for some reason OS/2 does not
use this information.
/end of part 2 of 2/
--
\\===============\\=======================\\
\\ Hale Landis \\ 303-548-0567 \\
// Niwot, CO USA // landis@sugs.tware.com //
//===============//=======================//
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