                ATA/ATA-1/ATA-2/IDE/EIDE/etc FAQ

                    Part 1 of ? -- The Basics

                      Version 0b -- 7 Feb 95

             by Hale Landis -- landis@sugs.tware.com

Note:  Major changes from the previous version are marked by a
   "!" at the left margin on the first line of the changed
   paragraph.

First the "legal" stuff...

   1) This FAQ is not intended to replace any other FAQ on this
   subject but is an attempt to provide historical and technical
   information about the ATA interface.

   2) This FAQ is not an endorsement of any vendor's product(s).

   3) This FAQ is not a recommendation to purchase any vendor's
   product(s).

   4) Every effort is made to insure that all of the information
   presented here is not copyrighted, not proprietary and
   unrestricted.

   4) When opinions are stated they are clearly identified,
   including the person's name and email address.  Such opinions
   are offered as long as they contribute to the understanding of
   the subject being discussed.  No "flames" allowed.

This is the first version of this FAQ.  It will take some time to
get all the significant information into it so it will be rapidly
growing and changing during the next several weeks or months.  I
don't even know how many parts there will be yet!  Versions will
be numbered with simple integer numbers (no 1.1, 1.2, etc)
starting at 0.

If you have a question that is not answered here or if you have
unrestricted material that you would like to contribute, please
email it to landis@sugs.tware.com.  DO NOT send material that is
copyrighted, proprietary or otherwise restricted in any way -- I
can't use such material in this document.

I don't have FTP access to anything at this time so I leave it to
others to put this at the appropriate FAQ FTP sites.

About myself:  Until recently I worked for Seagate where I was
one of several people that attended the ATA, ATA-2, PCMCIA and
SFF meetings for Seagate.  I was also the manager of a software
development group that created much of the engineering test
software for ATA hard disk drives.  I am now a consultant and I
still attend the ATA-2 meetings.

Table of Contents
-----------------

Part 1 - The Basics

   Glossary
   Basic Questions

Part 2 - BIOS and Drivers

   TBD


Glossary
--------

Read and understand these terms.  You will be lost and confused
if you don't!  Many of these are describe in much greater detail
in other parts of this FAQ.

ATA or AT Attachment

   ATA is the proper and correct name for what most people call
   IDE.  In this document, ATA refers to all forms of ATA (ATA-1,
   ATA-2, etc, IDE, EIDE, etc).  The ATA interface uses a single
   40-conductor cable in most desktop systems.

ATA-1

   ATA-1 is the common name of the original ATA (IDE)
   specification.  ATA-1 is not an official standard yet.  Final
   approval is pending.

ATA-2 or ATA Extensions

   ATA-2 is the common name of the new ATA specification.  ATA-2
   is still in early draft form and has not been submitted for
   approval as an official standard.

ATA-3

!  ATA-3 is the common name of a future version of the ATA
   specification.  The ATA-3 working group has held several
   meeting but the only things adopted so far are a DMA
   version of the Identify command, a description of
   "device 1 only configurations" and a set of "security"
   commands.

!  There is much discussion going on concerning merging ATA-3
   with ATAPI.  This will require some kind of "command overlap"
   capability.  The details of this are consumming much of the
   meeting time.

ATAPI or ATA Packet Interface

   ATAPI is a proposed new interface specification.  Initially it
   will probably be used for CD-ROM and tape devices.  It uses
   the ATA hardware interface at the physical level but uses a
   subset of the SCSI command set at the logical level.  The
   ATAPI specification work is currently being done in the SFF
   committee.

!  The ATAPI folks have delayed forwarding their CD-ROM
   specification from SFF to X3T10 so the X3T10 ATAPI working
   group has nothing to work on yet and have held no meetings.

! Block Mode

!  Block mode is the name given to the use of the ATA Read
   Multiple and Write Multiple commands.  These commands generate
   a single interrupt to the host system for each block of
   sectors transfered.  The traditional Read Sectors and Write
   Sectors commands generate an interrupt to the host for each
   sector transfered.

CAM (Common Access Method) Committee

   The Common Access Method committee, now disbanded, worked on
   two specifications: the CAM SCSI and CAM ATA specifications.
   Both specifications were forwarded to the X3T9 committee for
   further work years ago.

CHS or Cylinder/Head/Sector

   CHS is the old and traditional way to address data sectors on
   a hard disk.  This style of addressing relates a sector's
   address to the position of the read/write heads.  In today's
   ATA devices, all sector addresses used by the host are logical
   and have nothing to do with the actual physical position of
   the sector on the media or the actual position of the
   read/write heads.

Command Block
Control Block

   These are names given to the I/O register interface used by
   ATA devices.  It refers to a set of I/O registers, or I/O
   ports and I/O port addresses used to program the device.
   These names replace the older term Task File.

DMA or Direct Memory Access

   DMA is a method of data transfer between two devices that does
   not use the system's main processor as part of the data path.
   DMA requires lots of hardware:  a DMA arbitration unit, a DMA
   data transfer unit and host bus signals that enable the DMA
   controller to assume control of the host system's bus.  When
   the DMA controller has control of the host system's bus, it
   moves data between the two devices by generating the
   appropriate bus read/write cycles.  For the ATA READ DMA
   command this means generating an I/O read cycle and then a
   memory write cycle for each 16-bit word being transferred.
   For the ATA WRITE DMA command, a memory read cycle is followed
   by an I/O write cycle for each 16-bit word transferred.

EIDE or Enhanced IDE

   EIDE is a marketing program started by Western Digital to
   promote certain ATA-2 features including ATAPI.  WD has
   encouraged other product vendors to mark their products as
   "EIDE compatible" or "EIDE capable".

ESDI

   See MFM.

Fast ATA

   Fast ATA is a Seagate marketing program used to promote
   certain ATA-2 features in newer ATA devices.  Seagate has
   encouraged other product vendors to mark their products as
   "Fast ATA compatible" or "Fast ATA capable".

Host or Host System

   The computer system that the ATA device is attached to.

HBA or Host Bus Adapter or Host Adapter

   The hardware that converts host bus signals to/from ATA
   interface signals.  An ATA-1 host adapter is generally a very
   simple piece of hardware.  An ATA-2 host adapter can be simple
   or complex.

IDE

   IDE can mean any number of things:  Imbedded Device (or Drive)
   Electronics (yes, you can spell embedded with an "i"),
   Intelligent Device (or Drive) Electronics, etc.  The term IDE
   is the trademark of someone (Western Digital does not claim
   IDE as theirs but they do claim EIDE).  Many hard disk vendors
   do not use IDE to describe their products to avoid any
   trademark conflicts.

LBA or Logical Block Addressing

   LBA is a newer (for ATA it is newer) way to address data
   sectors on a hard disc.  This style of addressing uses a
   28-bit binary number to address a sector.  LBA numbers start
   at zero.  In today's ATA devices, all sector addresses used by
   the host are logical and have nothing to do with the actual
   physical location of the sector on the media.

Local Bus

   Usually this refers to the processor's local bus in a high
   performance computer system.  Usually the processor, the
   external processor instruction/data cache, the main memory
   controller and the bridge controller for the next low speed
   system bus, for example, a PCI bus, are located on the local
   bus.  Lower speed local buses may have connectors that allow
   the attachment of other devices.  For example, the VL-Bus is a
   local bus that can allow attachment of video, SCSI or ATA
   controllers.  It is very difficult to attach other devices to
   high speed (say faster than 100MHz) local buses due to
   electrical restrictions that come into play at those higher
   speeds.

Master

   ATA device 0.  Device 0, the master, is the "master" of
   nothing.  See Slave.

Megabyte or MB

   Megabyte or MB is 1,000,000 bytes or 10^6 bytes.  IT IS NOT
   1,048,576 bytes or 2^20 bytes, repeat NOT!

MFM

   In this document MFM refers to any of the older hard disk
   controller interfaces, MFM, RLL and ESDI.  It is used to
   describe any hard disk controller that uses the Task File
   interface on the host side and the ST506/ST412 interface
   on the drive side.

OS

   Operating System.

PC Card ATA
PCMCIA

   We can thank the Personal Computer Memory Card International
   Association for the PC Card specification.  The PCMCIA is a
   nonprofit industry association.  The PC Card ATA
   specification is another form of the ATA interface used by
   PCMCIA compatible ATA devices.  This interface uses the PCMCIA
   68-pin connector.  Most 68-pin ATA devices are dual mode --
   they can operate as either a PCMCIA PC Card ATA device or as a
   68-pin ATA device.

PCI

   We can thank Intel and the other members of the PCI committee
   for the PCI bus specification.  PCI is intended to be the next
   high performance computer bus.  PCI is not generally described
   as a processor local bus.

PIO or Programmed Input/Output

   PIO is a method of data transfer between two devices that uses
   the system's main processor as part of the data path.  On
   x86 systems, the REP INS and REP OUT instructions
   implement this data transfer method.  INS reads and I/O port
   and writes the data into memory.  OUTS reads data from memory
   and writes the data to an I/O port.  Each time an INS or OUTS
   instruction is executed, the memory address is updated.  The
   REP prefix causes the instructions to be repeated until a
   counter reaches zero.

RLL

   See MFM.

Slave

   ATA device 1.  Device 1, the slave, is a "slave" to nothing.
   See Master.

Task File

   This is the name given to the I/O register interface used by
   MFM controllers.  It refers to a set of I/O registers, or I/O
   ports and I/O port addresses used to program the controller.
   In ATA, this name has been replaced by the terms Command Block
   and Control Block.

SCSI

   See the SCSI FAQ.

SFF or Small Form Factor

   The SFF committee is an ad hoc committee formed by most of the
   major storage device and system vendors to set standards for
   the physical layout of hard disk and other devices.  SFF has
   published many specifications that describe the physical
   mounting and connector specifications for hard disk devices,
   including ATA devices.  During a brief period of time when the
   X3T9 committee was not doing much work on the ATA-1 interface,
   the SFF committee published several specifications that were
   not really part of the original SFF charter.  Most, if not
   all, of these nonphysical specifications have now been
   incorporated into the latest X3T9 or X3T10 ATA specifications.
   ATAPI is currently an SFF specification.

ST506 and ST412

   This is the common name for the hard disk controller to hard
   disk drive interface used by MFM, RLL and ESDI controllers and
   disk drives.  ST stands for Seagate Technology.  The ST506 and
   ST412 were the Seagate products that set the de facto
   standards for this interface many years ago.  This interface
   is composed of two cables:  a 34-conductor and a 20-conductor
   cable.

VESA and VL-Bus

   We can thank the Video Electronics Standards Association for
   the VESA Local Bus or VL-Bus specification.  The VL-Bus is one
   example of a local bus.  VESA is a nonprofit industry
   association like the PCMCIA.

WG or Working Group

   The actual work on various specifications and standards
   documents within the X3T9, X3T10 and SFF committees happens in
   working group meetings.  Most WG meetings are held monthly.

X3T9 and X3T10

   These are the names of the official standards committees that
   have worked on the ATA-1 and ATA-2 specifications.  X3T9 was
   responsible for the SCSI and ATA-1 specifications and
   standards.  X3T10 has replaced X3T9 and is now responsible for
   the current SCSI and ATA specifications and standards work.

528MB

   This term is used in this document to describe the capacity
   boundary that exists in most x86 system software.  This
   boundary limits the size of an ATA disk drive to 528MB.  For
   cylinder/head/sector style addressing of disk data sectors,
   this number is computed as follows:

   a) the number of cylinders are limited to 1024, numbered
   0-1023.

   b) the number of heads (per cylinder) are limited to 16,
   numbered 0-15,

   c) the number of sectors (per track) are limited to 63,
   numbered 1-63.

   d) a sector is 512 bytes,

   e) 528MB means the following values:

      ( 1024 * 16 * 63 ) or 1,032,192 data sectors

      or

      ( 1024 * 16 * 63 * 512 ) or 528,482,304 bytes.

68-pin ATA

   This refers to a variation of the ATA interface that uses the
   PCMCIA 68-pin physical interface but does not use the PCMCIA
   electrical or logical interface.  Most 68-pin ATA devices are
   dual mode -- they can operate as either a PCMCIA PC Card ATA
   device or as a 68-pin ATA device.  This interface was
   developed within the SFF committee and is now included in
   ATA-2.

Basic Questions
---------------

### Where did ATA come from?

   What we now call the ATA-1 interface was developed for Compaq
   many years ago by Imprimus (then part of CDC, now part of
   Seagate) and Western Digital.  The first ATA-1 hard disk
   drives were made by Imprimus but it was Conner that made the
   interface so popular.

### How is ATA different from MFM?

   From the host software standpoint, ATA is very much like the
   Task File interface used by MFM controllers.  A properly
   written host software driver should not notice any difference
   between the MFM Task File interface and the ATA Command Block
   interface while doing basic commands such as Read/Write
   Sectors.

   At the hardware level, ATA uses a single cable between a host
   bus adapter and the ATA device, where the MFM controller
   interface uses two cables between the controller and the
   drive.

   In the MFM environment, the controller is one piece of
   hardware and the drive another piece of hardware.  Most likely
   these two pieces of hardware are from different vendors.  The
   MFM controller is dependent on the design of both the host bus
   and on the drive.

   In the ATA environment, the host adapter is the one piece of
   hardware that is dependent on the host system bus design.  The
   ATA interface is (mostly) system independent.  All of the
   hard disk controller and drive logic is contained in the ATA
   device hardware.  This gives the hard disk designer complete
   control over both the controller and drive functions.

### Why is ATA so popular?

   Two basic things make ATA so popular today:  cost and hard
   disk drive technology.  An ATA-1 host adapter is cheap,
   usually much less than $25US and it uses only one cable.  On
   the technology side, current hard disk features, such as,
   defect handling, error recovery, zone recording, cache
   management and power management require that the controller be
   fully integrated with the read/write channel, the servo system
   and spindle hardware of the disk drive.

### What are the basics of the ATA interface?

   The ATA interface is a very simple interface based on an ISA
   bus I/O device architecture.  The interface consists of two
   sets of I/O registers, mostly 8-bit, for passing command and
   status information.  The registers are like a set of mail
   boxes with a door on front and back connected such that both
   doors can not be open at the same time.  The front door is
   open when the Busy bit in the Status register is zero and the
   host can read and write the registers.  The back door is open
   when the Busy bit in the Status register is one and the ATA
   device can read or write the registers.

   The physical interface contains just enough signals for a 16
   bit data bus, five register address bits, and a few control
   signals like read register, write register and reset.

   ATA devices look like traditional hard disk
   drives even though some are not really a hard disc with
   rotating platters.  User data is recorded in 512 byte sectors.
   Each sector has a sector address.  There are two ways to
   express sector addresses:  by cylinder/head/sector (CHS) or by
   logical block address (LBA).  CHS is standard, LBA is optional.

### What is EIDE or Fast ATA?

   Both are marketing programs used to promote various ATA-2
   features, mostly the faster data transfer rates defined by
   ATA-2.

   ---

   WD defines EIDE as:

   * Support for drives larger than 528MB.

   * Support for two connectors to allow up to four drives.

   * Support for CD-ROM and tape peripherals.

   * Support for 11.1/16.6 Mbytes/second, I/O Channel Ready PIO
     data transfers.

   * Support for 13.3/16.6 Mbytes/second, DMA data transfers.

   ---

???Seagate defines Fast ATA as:

   * Support for PIO mode 3 (11.1 MB/sec) and DMA mode 1(13.3
     MB/sec).

   * Support for Multi-sector [Read/Write Multiple] transfers.

   * Support for >528 MB.

   * Support for Identify Drive Extensions & Set Transfer Mode
     Extensions.

   * Backward compatibility with ATA-1.

   ---

   What does all of this mean to us?

   Support for the ATA-2 high speed PIO and DMA data transfer
   modes is both a hardware and software issue.

   Support for more than one hard disc controller (or ATA host
   adapter) requires the BIOS and/or the operating system to
   support more than one Task File or Command/Control Block
   register set on the host bus.

   The 528MB problem is due to the original design of the x86
   BIOS which limits cylinders to 1024 and sectors to 63.  The
   ATA interface allows up to 65,535 cylinders, 16 heads and 255
   sectors -- that's about 136GB (137GB if is LBA is used).
   Support for devices over 528MB requires the BIOS and/or
   operating system to support some form of CHS translation.
   Note that LBA alone does not solve this problem (in fact,
   LBA may make things more complex).

   Support for CD-ROM and tape will probably be done via the
   ATAPI interface which uses a different command structure than
   ATA.  That makes ATAPI another host software issue.

### What does an ATA-1 host adapter do?

   An ATA-1 host adapter is a simple piece of logic whose main
   purpose is to reduce the system bus address lines from 12 (or
   more) down to 5. It may also buffer some signals giving some
   degree of electrical isolation between the host bus (usually
   an ISA or EISA bus) and the ATA bus.  In ATA-1, the ATA
   interface is controlled directly by the host bus so that all
   timings are controlled by the host bus timing.

### What does an ATA-2 host adapter do?

   This answer is complex because it depends on how smart your
   ATA-2 host adapter is.  First, an ATA-2 host adapter supports
   the ATA-2 higher speed data transfer rates.  That requires
   that the host adapter is attached to something other than an
   ISA or EISA bus.  Second, an ATA-2 host adapter may perform
   32-bit wide transfers on the host bus.  This requires FIFO
   logic and data buffers in the host adapter.  Third, an ATA-2
   host adapter may use a different data transfer protocol on the
   host side than is used on the ATA device side.

! ### Can I put an ATA-2 device on an ATA-1 host adapter?
! ### Can I put an ATA-1 device on an ATA-2 host adapter?

   The answer to both questions is yes, as long as the electrical
   timing specifications of the device are not violated.  In
   general it is impossible for an ATA-1 host adapter to violate
   the specifications of an ATA-2 device.  It is possible for an
   ATA-2 host adapter to violate the timing specifications of an
   ATA-1 device but this is not common.  Hoever, host adapter
   hardware design errors or software driver bugs can cause such
   a problem.  The result will be corrupted data read or written
   to the ATA-1 device.

! ### I have an ATA-2 host adapter with an ATA-2 device.  I want to
! ### add an ATA-1 device to this host adapter.  Can I run the ATA-2
! ### device in its ATA-2 data transfer modes?

   Sorry, *NO* you can *NOT* run the new drive in its faster data
   transfer modes.  Be very careful, most of the ATA-2 host
   adapter vendors don't have anything in their setup
   documentation or software to prevent this sort of thing.

   When you run the new drive at a data transfer speed that is
   faster than the older drive can support, you are violating the
   electrical interface setup and hold times on the older drive.
   There is no telling what the older drive will do about this,
   but you are asking for data corruption and other nasty
   problems on your older drive.

### How many disk controllers and/or ATA host adapters and/or
### SCSI host adapters can I put in my system?

   From a hardware standpoint -- as many as you want as long as
   there are no I/O port address, memory address or interrupt
   request signal conflicts.  From a software standpoint it is a
   whole different story.

   First the simple x86 system hard disk controller
   configurations...

   a) 1 ATA with 1 or 2 drives at I/O port addresses
   1Fxh/3Fxh using interrupt request 14 (IRQ14).

   b) 1 ATA with 1 drive at I/O port addresses 1Fxh/3Fxh
   using interrupt request 14 (IRQ14) plus a SCSI with 1 drive.

   c) 1 SCSI with 1 or 2 drives.

   Other configurations are possible but are most likely not
   supported in the system or SCSI host adapter BIOS.  And if its
   not supported at the BIOS level, it is unlikely to be
   supported by an operating system, especially DOS.  The primary
   reason the above configurations are so restrictive is that the
   original IBM x86 BIOS supported only one MFM controller with a
   maximum of 2 drives.  This restriction was then coded into
   much x86 software including many early version of DOS.  The
   configurations above work because they don't break this old
   rule.

   Just remember this -- most systems will always boot from
   the first drive on the first controller.  In a) that is
   ATA drive 0, in b) that is ATA drive 0, in c) that is
   SCSI drive 0.

   And now the more complex configurations...

   Once you go beyond the three configurations above all bets are
   off.  Most likely you will need operating system device
   drivers in order to access any drives beyond the first two.
   And now your real problems start especially if you like to run
   more than one operating system!

   If you do run more than one OS, then you need equivalent
   drivers for each system if you would like to access all the
   drives.  Plus it would be nice if all the drivers configured
   the drives in the same manner and supported all the possible
   partitioning schemes and partition sizes.  It would be
   especially nice if a driver would not destroy the data in a
   partition just because it did not understand the file system
   format in that partition.

   One of the things EIDE promotes is BIOS support for up to four
   ATA devices -- 2 ATA host adapters each with 1 or 2 drives.
   The first would be at I/O port addresses 1Fxh/3Fxh using
   interrupt request 14 (IRQ14) and the second at I/O port
   addresses 17xh/37xh using interrupt request 15 (IRQ15).
   Acceptance of this configuration has not been spreading like
   wild fire through the BIOS world.

   Lets look at a two complex configurations...

   a) 1 ATA with 2 drives and 1 SCSI with 1 or more drives.

   Nice configuration.  The ATA drives would be supported by the
   system BIOS and the SCSI drives may be, could be, should be,
   supported by the SCSI host adapter BIOS but probably not.  So
   in order to use the 2 SCSI drives you probably have to disable
   the BIOS on the SCSI card and then load a device driver in
   CONFIG.SYS.  And because the SCSI BIOS is disabled, you then
   need a SCSI driver for that other OS you run.

   b) 2 ATA with 1 or 2 drives on each.

   Also a nice configuration.  But because the system BIOS
   probably only supports the first controller address, you'll
   need a DOS device driver loaded in CONFIG.SYS in order to
   access the drives on the second controller.  You'll need that
   driver even if there is only one drive on the first
   controller.  You also need a similar driver to support the
   second controller in your other OS.

   Note:  I understand that OS/2 does support both MFM/ATA
   controller addresses and does allow up to four drives -- I
   have not confirmed this for myself.

! ### Are disk drives the only ATA devices?

   No.  Over the years there have been ATA tape drives, ATA
   CD-ROMS and other strange devices.  Most of these are expected
   to be added to an existing ATA host adapter as the second
   device (device 1) with an existing ATA disk drive (device 0).
   In general these require software drivers to operate with your
   OS.

   Now, we have ATAPI CD-ROM and tape devices that can be placed
   on an ATA host adapter.  And soon we should see system
   motherboard BIOS support for booting from an ATAPI CD-ROM
   device.  The general idea is that an ATAPI device can coexist
   with an ATA device on the same cable.

! ### What can be done to improve ATA device performance?

   A difficult question.  But the first step is usually to reduce
   the number of interrupts that the host sees during a read or
   write command.  ATA disk drives have three types of read/write
   commands:

   * Read Sectors / Write Sectors -- These commands are the old
   traditional data transfer commands.  These commands generate
   one interrupt to the host for each sector transfered.  These
   are PIO data in and PIO data out commands which use the host
   processor to transfer the data.

   * Read Multiple / Write Multiple -- These commands where
   defined in ATA-1 but were not used very much until recently.
   These commands generate one interrupt to the host for each
   block of sectors transfered.  The number of sector per block
   is generally 4, 8 or 16.  However, when 1 sector per block is
   used, these commands are the same as the Read/Write Sectors
   commands.  These are PIO data in and PIO data out commands
   which use the host processor to transfer the data.

   * Read DMA / Write DMA -- These commands where defined in
   ATA-1 but were not used very much until recently.  The main
   reason for not using them was that x86 system DMA transfer
   rates are much slower than PIO.  However, these command do
   generate a single interrupt at the completion of the command.

   Now see the next question...

! ### What else can be done to improve ATA device performance?
! ### -or-
! ### What is PIO mode "x" ?

   An even more difficult question.  The second step is usually
   to increase the rate at which the host transfers data.

   (Ahh...  I can see the funny look on your face from here.  You
   are saying to yourself:  "the rate at which the host transfers
   data? doesn't this guy have things backwards?"  Read on...)

   The rate at which data is transferred to or from an ATA device
   is determined by only one thing:  the PIO or DMA cycle time
   the host uses.  No, the drive does not have much to do with
   this!  The only requirement is that the host not exceed the
   minimum PIO or DMA cycle times that the device supports.  For
   example, during a PIO read command when the device signals an
   interrupt to the host this means that the device is waiting
   for the host to read the next sector or block of sectors from
   the drive.  The host must execute a REP INSW instruction to do
   transfer the data.  The rate at which the host executes this
   instruction determines the PIO cycle time.  Technically, for a
   read command, the cycle time is the time from the host
   assertion of the I/O Read signal to the next time the host
   asserts the I/O Read signal.

   Be careful when looking at the table below -- the data rate is
   the data transfer rate achieved while transfering the sector
   or block or sectors.  It is an "instantanous" data rate.  The
   overall data transfer rate for a command includes many time
   consuming events such as the amount of time the host requires
   to process an interrupt.  Note that on many fast ATA drives
   today, the time it takes the host to process an interrupt is
   frequently greater than the time required to transfer the
   sector of block of sectors for that interrupt!  It is not
   uncommon for the host overhead to reduce the data rate to 1/2
   or 1/3 of the instantanous rate shown here.

   The ATA PIO modes are defined as follows:

   PIO   min cycle  data  comment
   mode    time     rate

    0      ???ns     ?MB  the rate at which a system
                          running at 4.77MHZ could
                          execute the REP INSW.

    1      ???ns     ?MB  the rate at which a system
                          running at 6MHz could
                          execute the REP INSW.

    2      240ns     8MB  the rate at which a system
                          running at 8MHz could
                          execute the REP INSW.

    3      180ns    13MB  requires an ATA-2
                          host adapter.

    4      120ns    16MB  requires an ATA-2
                          host adapter.

   The complete description of the PIO (and DMA modes is much
   more complex and will be cover in more detail later in this
   FAQ.

### Do I need BIOS or OS drivers to support more than 528MB?

   Warning:  Read the previous question before reading this one.

   Maybe, probably, yes.  The answer to this *very* complex and
   will be discussed in detail in Part 2. Here is the brief
   answer...

   A traditional x86 system BIOS supports only CHS mode
   addressing with cylinders limited to 1024, heads limited to 16
   and sectors limited to 63.  This allows addressing of drives
   up to 528MB.  These limitations come from the INT 13
   read/write calls that combine a 10-bit cylinder number with a
   6-bit sector number into a 16-bit register.

   Note that this is entirely a software problem:  the ATA
   interface supports up to 65,535 cylinders, 16 heads and 255
   sectors.

   While the head number usually requires only 4-bits, up to 6 or
   8 bits are available in the INT 13 interface.  This fact has
   allowed the SCSI folks to support big drives by increasing the
   number of heads above 16.  The SCSI host adapter BIOS converts
   this "fake" CHS address to a different CHS or an LBA when it
   issues a read/write command to the drive.  The following table
   shows some approximate drives sizes and the "fake" CHS
   parameters that you may see from a SCSI BIOS:

      cyl    head    sector   size

      1024     16        63   512MB
      1024     32        63     1GB
       512     64        63     1GB
      1024     64        63     2GB
      1024    256        63     8GB

   The last entry represents the largest possible drive that
   a traditional INT 13 BIOS can support.

   The system BIOS folks *must* start supporting drives over
   528MB in their BIOS by implementing some type of CHS
   translation.  To date, few systems have such BIOS.  And here
   is the bad part:  Microsoft says that the BIOS *must*
   support it in order to use it in their OS.  The algorithm is
   simple (but warning:  this is not the complete algorithm!):

      INT 13 input    action            ATA interface

      cyl number      "multiply" by n   modified cyl number
      head number     "divide" by n     modified head number
      sector number   nothing           sector number

   The value of n must be selected such that the modified head
   number is less than 16.

   LBA addressing at the hard disk drive level or at the BIOS or
   driver level is another solution.  This solution will probably
   not be popular for several more years.  It requires that the
   BIOS people implement a new INT 13 interface, called the
   Microsoft/IBM Extensions and that the OS people start using
   this new BIOS interface.  Few system BIOS support this
   alternative interface today.  Without this new interface, LBA
   support at the hard disk drive level is not required.

   So most of us have older systems without much possibility of
   getting a BIOS upgrade, so what do we do?  Well we must obtain
   one of the many driver products that are on the market that
   live in one of the disk boot sectors and "take over" the
   system BIOS INT 13 with an INT 13 that supports the
   translation.  The biggest problem with this is that the
   replacement INT 13 BIOS must live someplace in memory.  For
   DOS based systems, it can usually live at the top of the 640K
   of memory and DOS is made to think that that part of memory,
   usually around 8K bytes, does not exist.  But the protected
   mode OS's don't like this and usually wipe out the driver when
   they load their kernel.  So if you plan to run multiple OS's
   on your system, buyer beware!

   Then there is the Windows problem:  the standard FastDisk
   driver in Windows does *not* support such translation schemes
   and can not be enabled.  So make sure the driver you
   purchase also comes with a Windows FastDisk replacement.

   Buyer beware!

### Do I need BIOS or OS drivers to support the ATA-2 data
### transfer rates?

   Warning:  Read the previous two questions before reading this
   one.

   Maybe, probably, yes.  The answer to this *very* complex and
   will be discussed in detail in Part 2. Here is the brief
   answer...

   If you have a new ATA drive that supports the advanced PIO or
   DMA data transfer rates (ATA-2 PIO Mode 3 or 4, or, ATA-2 DMA
   Mode 1 or 2) then you also must have a new ATA host adapter
   that attaches to the VL-Bus or PCI bus or some other high
   speed bus (probably a 32-bit bus) in your system.  That host
   adapter has I/O registers of its own that are used to control
   its advanced features.  Controlling those advanced features
   requires software -- either in the system INT 13 BIOS or in a
   INT 13 BIOS on the host adapter card or in a driver loaded
   via the boot record or later by your OS.

   Depending on how that host adapter works you may also
   need a Windows FastDisk replacement in order to use the high
   speed data transfer modes in Windows.

   Buyer beware!

### I just purchased a new high speed host adapter for my VL-Bus
### (or PCI bus) system and a new 540MB hard disk.  How do I get
### full use out of all this new hardware?

   Did you read the previous three questions?

   You need BIOS or driver software and a Windows FastDisk
   replacement.  These *must* support both CHS translation
   (because your drive is over 528MB) and the host adapter
   hardware (to use the high speed data transfer rates).

   Some drivers on the market today use LBA addressing on the
   ATA interface to get over 528MB.  This may make your disk
   partition(s) unreadable by another OS.

   Check the hardware and software specifications of the product
   before you buy it!  Ask lots of questions -- you probably get
   lots of incorrect or misleading answers -- be prepared for
   that!  If you plan to run something other than DOS and
   Windows, especially if you plan a "dual boot" or "boot
   manager" environment, be real careful.

   Buyer beware!

   OPINION:  I know of only one product that supports all of this
   new hardware, supports over 528MB *and* supports most of the
   current OS's that are shipping including several in shipping
   in beta form.  The product is from a small two person company
   that is trying to sell the product on an OEM basis and not in
   the retail market.  - Hale Landis <landis@sugs.tware.com>

/end part 1/
-- 
\\===============\\=======================\\
 \\  Hale Landis  \\      303-548-0567     \\
 // Niwot, CO USA // landis@sugs.tware.com //
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