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BIOS Types, CHS Translation, LBA and Other Good Stuff

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      BIOS Types, CHS Translation, LBA and Other Good Stuff

                            Version 4
                           2 March 95

             by Hale Landis ([email protected])

This is very technical.  Please read carefully.  There is lots of
information here that can sound confusing the first time you read
it.

Introduction (READ THIS!)
-------------------------

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
   paritions.  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.