DP Tool Club 24/CD_ASCQ_24_0995.iso/vrac/eidefaq.zip/BIOSFAQ.HTM

Text File | 1995-08-02 | 35KB | 744 lines

BIOS Types, CHS Translation, LBA and Other Good Stuff

Version 3+
11 February 95

by Hale Landis ([e-mail])

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 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 more than 1024 cylinders and 16 or
fewer 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                                   ATA
      AH=0xH  --------------------------------> device
      (P-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. I don't
know 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.

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 ATA
Identify Device command returns 16 in word 3 and 63 in word 6.

/end/ --

\\===============\\=======================\\
 \\  Hale Landis  \\      303-548-0567     \\
 // Niwot, CO USA // [e-mail] //
//===============//=======================//

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