root/drivers/block/README.ide

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README.ide -- Information regarding ide.c and ide-cd.c
================================================================================
Supported by:  mlord@bnr.ca           -- disks, interfaces, probing
               snyder@fnald0.fnal.gov -- cdroms, ATAPI, audio

(see description later on below for handling BIG IDE drives with >1024 cyls).

Major features of ide.c & ide-cd.c:

        - support for up to two IDE interfaces on one or two IRQs
        - support for any mix of up to four disk and/or cdrom drives
        - support for reading IDE ATAPI cdrom drives (NEC, MITSUMI, VERSA)
        - support for audio functions on FX400,NEC-260 cdrom drives (others?)
        - auto-detection of interfaces, drives, IRQs, and disk geometries
        - support for BIOSs which report "more than 16 heads" on disk drives
        - uses LBA (slightly faster) on disk drives which support it
        - support for lots of fancy (E)IDE drive functions with hdparm utility
        - optional (compile time) support for 32-bit VLB data transfers
        - support for IDE multiple (block) mode (same as hd.c)
        - support for interrupt unmasking during I/O (better than hd.c)
        - auto detection/use of multiple (block) mode settings from BIOS
        - improved handshaking and error detection/recovery
        - can co-exist with hd.c to control only the secondary interface

Under construction:

        - support for interface speed selection on jumperless interfaces
        - improved detection of non-standard IDE ATAPI cdrom drives

To access devices on the second interface, device entries must first be
created in /dev for them.  To create such entries, simply run the included
shell script:   MAKEDEV.ide1

ide.c automatically probes for the primary and secondary interfaces,
for the drives/geometries attached to those interfaces, and for the
IRQ numbers being used by the interfaces (normally IRQ14 & IRQ15).

The primary and secondary interfaces may share a single IRQ if necessary,
at a slight performance penalty, whether on separate cards or a single VLB card.

Drives are normally found by auto-probing and/or examining the CMOS/BIOS data.
For really weird situations, the apparent (fdisk) geometry can also be specified
on the kernel "command line" using LILO.  The format of such lines is:   

        hdx=cyls,heads,sects,wpcom,irq
or      hdx=cdrom

where hdx can be any of {hda,hdb,hdc,hdd}, or simply hd, for the "next" drive
in sequence.  Only the first three parameters are required (cyls,heads,sects),
and wpcom is ignored for IDE drives.

        Example:  hdc=1050,32,64 hdd=cdrom

If an irq number is given, it will apply to both drives on the same interface,
either {hda,hdb} or {hdc,hdd}.  The results of successful auto-probing may
override the physical geometry/irq specified, though the "original" geometry
is retained as the "logical" geometry for partitioning purposes (fdisk).

Courtesy of Scott Snyder, the driver now supports ATAPI cdrom drives
such as the NEC-260 and the new MITSUMI triple/quad speed drives.
Such drives will be identified at boot time, as hda,hdb,hdc or hdd,
just like a harddisk.

If for some reason your cdrom drive is *not* found at boot time, you can force
the probe to look harder by supplying a kernel command line parameter
via LILO, such as:  hdc=cdrom

For example, a GW2000 system might have a harddrive on the primary
interface (/dev/hda) and an IDE cdrom drive on the secondary interface
(/dev/hdc).  To mount a CD in the cdrom drive, one would use something like:

        ln -sf /dev/hdc /dev/cdrom
        mkdir /cd
        mount /dev/cdrom /cd -t iso9660 -o ro,block=2048

Please pass on any feedback on the cdrom stuff to the author & maintainer,
Scott Snyder (snyder@fnald0.fnal.gov).

Note that present, the kernel is unable to execute demand-paged binaries
directly off of the cdrom (due to the different block size).

The hdparm.c program for controlling various IDE features is now packaged
separately.  Look for it on popular linux FTP sites.

mlord@bnr.ca
snyder@fnald0.fnal.gov
================================================================================

How To Use *Big* IDE drives with Linux/DOS
------------------------------------------
All IDE drives larger than 504MB ("528Meg") use a "physical" geometry which
has more than 1024 cylinders.  This presents two problems to most systems:

        1. The INT13 interface to the BIOS only allows 10-bits for cylinder
        addresses, giving a limit of 1024cyls for programs which use it.

        2. The physical geometry fields of the disk partition table only
        allows 10-bits for cylinder addresses, giving a similar limit of 1024
        cyls for operating systems that do not use the "sector count" fields
        instead of the physical Cyl/Head/Sect (CHS) geometry fields.

Neither of these limitations affects Linux itself, as it (1) does not use the
BIOS for disk access, and it (2) is clever enough to use the "sector count"
fields of the partition table instead of the physical CHS geometry fields.

        a) Most folks use LILO to load linux.  LILO uses the INT13 interface
        to the BIOS to load the kernel at boot time.  Therefore, LILO can only
        load linux if the files it needs (usually just the kernel images) are
        located below the magic 1024 cylinder "boundary".

        b) Many folks also like to have bootable DOS partitions on their
        drive(s).  DOS also uses the INT13 interface to the BIOS, not only
        for booting, but also for operation after booting.  Therefore, DOS
        can normally only access partitions which are contained entirely below
        the magic 1024 cylinder "boundary".

There are at least seven commonly used schemes for kludging DOS to work
around this "limitation".  In the long term, the problem is being solved
by introduction of an alternative BIOS interface that does not have the
same limitations as the INT13 interface.  New versions of DOS are expected
to detect and use this interface in systems whose BIOS provides it.

But in the present day, alternative solutions are necessary.

The most popular solution in newer systems is to have the BIOS shift bits
between the cylinder and head number fields.  This is activated by entering
a translated logical geometry into the BIOS/CMOS setup for the drive.
Thus, if the drive has a geometry of 2100/16/63 (CHS), then the BIOS could
present a "logical" geometry of 525/64/63 by "shifting" two bits from the
cylinder number into the head number field for purposes of the partition table, CMOS setup, and INT13 interfaces.  Linux kernels 1.1.39 and higher detect and
"handle" this translation automatically, making this a rather painless solution
for the 1024 cyls problem.  If for some reason Linux gets confused (unlikely),
then use the kernel command line parameters to pass the *logical* geometry,
as in:  hda=525,64,63

If the BIOS does not support this form of drive translation, then several
options remain, listed below in order of increasing popularity:

        - rewrite LILO to bypass the BIOS and talk directly to the drive
        - boot from a floppy disk instead of the hard drive (takes 10 seconds)
        - use a partition below the 1024 cyl boundary to hold the linux
        boot files (kernel images and /boot directory), and place the rest
        of linux anywhere else on the drive.  These files can reside in a DOS
        partition, or in a tailor-made linux boot partition.

If you cannot use drive translation, *and* your BIOS also restricts you to
entering no more than 1024 cylinders in the geometry field in the CMOS setup,
then you'll have to set it to 1024.  As of v3.5 of this driver, Linux will
automatically determine the *real* number of cylinders for fdisk to use,
allowing easy access to the full disk capacity.

Regardless of what you do, all DOS partitions *must* be contained entirely
within the first 1024 logical cylinders.  For a 1Gig WD disk drive, here's
a good "half and half" partitioning scheme to start with:

        geometry = 2100/16/63
        /dev/hda1 from cyl    1 to  992         dos
        /dev/hda2 from cyl  993 to 1023         swap
        /dev/hda3 from cyl 1024 to 2100         linux

After installing slackware (or whatever), boot from floppy and mount the dos
partition under /dos, and then move the /boot directory to /dos/boot,
and move the /vmlinuz kernel file into /dos/boot.  Then create a symlink
to put it back where the filesystem standard wants it:  ln -s /dos/boot /boot
Finally, edit /etc/lilo.conf and change the reference to /vmlinuz to point
at /boot/vmlinuz and then re-run lilo.  Have Fun.  :)

If you "don't do DOS", then partition as you please, but remember to create
a small partition to hold the /boot directory (and vmlinuz) as described above
such that it all lies within the first 1024 cylinders.

Western Digital now ships a "DiskManager 6.03" diskette with all of their big
hard drives.  Burn it!  That idiotic piece of garbage isn't even universally
compatible with DOS, let alone other operating systems like Linux.  Eventually
some kind person will kludge Linux to work with it, but at present the two
are completely incompatible.  If you have this version of DiskManager on your
hard disk already, it can be exterminated at the expense of all data on the
drive (back it up elsewhere), by using the "DM" command from the diskette
as follows:   DM /Y-

mlord@bnr.ca

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