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+<!-- $Id: esdi.sgml,v 1.2 1995-10-07 04:31:20 jfieber Exp $ -->
+<!-- The FreeBSD Documentation Project -->
+
+<!--
+ <title>An introduction to ESDI hard disks and their use with FreeBSD</title>
+ <author>(c) 1995, Wilko Bulte, <tt/wilko@yedi.iaf.nl/
+ <date>Tue Sep 12 20:48:44 MET DST 1995</date>
+
+ Copyright 1995, Wilko C. Bulte, Arnhem, The Netherlands
+
+ <abstract>
+ This document describes the use of ESDI disks in combination
+ with the FreeBSD operating system. Contrary to popular
+ belief, this is possible and people are using ESDI based
+ systems succesfully! This document tries to explain you
+ how to do this.
+
+ If you find something missing, plain wrong or have useful
+ comments on how to improve
+ the document please send mail to <tt/wilko@yedi.iaf.nl/
+ </abstract>
+-->
+
+ <sect><heading>ESDI hard disks and FreeBSD<label id="esdi"></heading>
+
+ <p><em>Copyright &copy; 1995, &a.wilko;.<newline>24 September 1995.</em>
+
+ ESDI is an acronym that means Enhanced Small Device Interface.
+ It is loosely based on the good old ST506/412 interface originally
+ devised by Seagate Technology, the makers of the first affordable
+ 5.25" winchester disk.
+
+ The acronym says Enhanced, and rightly so. In the first place
+ the speed of the interface is higher, 10 or 15 Mbits/second
+ instead of the 5 Mbits/second of ST412 interfaced drives.
+ Secondly some higher level commands are added, making the ESDI
+ interface somewhat 'smarter' to the operating system driver
+ writers. It is by no means as smart as SCSI by the way. ESDI
+ is standardised by ANSI.
+
+ Capacities of the drives are boosted by putting more sectors
+ on each track. Typical is 35 sectors per track, high capacity
+ drives I've seen were up to 54 sectors/track.
+
+ Although ESDI has been largely obsoleted by IDE and SCSI interfaces,
+ the availability of free or cheap surplus drives makes them
+ ideal for low (or now) budget systems.
+
+ <sect1><heading>Concepts of ESDI</heading>
+ <p>
+ <sect2><heading>Physical connections</heading>
+ <p>
+ The ESDI interface uses two cables connected to each drive.
+ One cable is a 34 pin flatcable edge connector that carries
+ the command and status signals from the controller to the
+ drive and viceversa. The command cable is daisy chained
+ between all the drives. So, it forms a bus onto which all
+ drives are connected.
+
+ The second cable is a a 20 pin flatcable edge connector that
+ carries the data to and from the drive. This cable is radially
+ connected, so each drive has it's own direct connection to the
+ controller.
+
+ To the best of my knowledge PC ESDI controllers are limited
+ to using a maximum of 2 drives per controller. This is
+ compatibility feature(?) left over from the WD1003 standard
+ that reserves only a single bit for device addressing.
+
+ <sect2><heading>Device addressing</heading>
+ <p>
+ On each command cable a maximum of 7 devices and 1 controller
+ can be present. To enable the controller to uniquely
+ identify which drive it addresses, each ESDI device is equipped
+ with jumpers or switches to select the devices address.
+
+ On PC type controllers the first drive is set to address 0,
+ the second disk to address 1. <it>Always make sure</it> you
+ set each disk to an unique address! So, on a PC with it's
+ two drives/controller maximum the first drive is drive 0, the
+ second is drive 1.
+
+ <sect2><heading>Termination</heading>
+ <p>
+ The daisy chained command cable (the 34 pin cable remember?)
+ needs to be terminated at the last drive on the chain.
+ For this purpose ESDI drives come with a termination resistor
+ network that can be removed or disabled by a jumper when it
+ is not used.
+
+ So, one and <it>only</it> one drive, the one at
+ the fartest end of the command
+ cable has it's terminator installed/enabled. The controller
+ automatically terminates the other end of the cable.
+ Please note that this implies that the controller must be
+ at one end of the cable and <it>not</it> in the middle.
+
+ <sect1><heading>Using ESDI disks with FreeBSD</heading>
+ <p>
+ Why is ESDI such a pain to get working in the first place?
+
+ People who tried ESDI disks with FreeBSD are known to have
+ developed a profound sense of frustration. A combination of
+ factors works against you to produce effects that are
+ hard to understand when you have never seen them before.
+
+ This has also led to the popular legend ESDI and FreeBSD
+ is a plain NO-GO.
+ The following sections try to list all the pitfalls and
+ solutions.
+
+ <sect2><heading>ESDI speed variants</heading>
+ <p>
+ As briefly mentioned before, ESDI comes in two speed flavours.
+ The older drives and controllers use a 10 Mbits/second
+ data transfer rate. Newer stuff uses 15 Mbits/second.
+
+ It is not hard to imagine that 15 Mbits/second drive cause
+ problems on controllers laid out for 10 Mbits/second.
+ As always, consult your controller <it>and</it> drive
+ documentation to see if things match.
+
+ <sect2><heading>Stay on track</heading>
+ <p>
+ Mainstream ESDI drives use 34 to 36 sectors per track.
+ Most (older) controllers cannot handle more than this
+ number of sectors.
+ Newer, higher capacity, drives use higher numbers of sectors
+ per track. For instance, I own a 670 Mb drive that has
+ 54 sectors per track.
+
+ In my case, the controller could not handle this number
+ of sectors. It proved to work well except that it only
+ used 35 sectors on each track. This meant losing a
+ lot of diskspace.
+
+ Once again, check the documentation of your hardware for
+ more info. Going out-of-spec like in the example might
+ or might not work. Give it a try or get another more
+ capable controller.
+
+ <sect2><heading>Hard or soft sectoring</heading>
+ <p>
+ Most ESDI drives allow hard or soft sectoring to be
+ selected using a jumper. Hard sectoring means that the
+ drive will produce a sector pulse on the start of each
+ new sector. The controller uses this pulse to tell when
+ it should start to write or read.
+
+ Hard sectoring allows a selection of sector size (normally
+ 256, 512 or 1024 bytes per formatted sector). FreeBSD uses
+ 512 byte sectors. The number of sectors per track also varies
+ while still using the same number of bytes per formatted sector.
+ The number of <em>unformatted</em> bytes per sector varies,
+ dependent on your controller it needs more or less overhead
+ bytes to work correctly. Pushing more sectors on a track
+ of course gives you more usable space, but might give
+ problems if your controller needs more bytes than the
+ drive offers.
+
+ In case of soft sectoring, the controller itself determines
+ where to start/stop reading or writing. For ESDI
+ hard sectoring is the default (at least on everything
+ I came across). I never felt the urge to try soft sectoring.
+
+ In general, experiment with sector settings before you install
+ FreeBSD because you need to re-run the low-level format
+ after each change.
+
+ <sect2><heading>Low level formatting</heading>
+ <p>
+ ESDI drives need to be low level formatted before they
+ are usable. A reformat is needed whenever you figgle
+ with the number of sectors/track jumpers or the
+ physical orientation of the drive (horizontal, vertical).
+ So, first think, then format.
+ The format time must not be underestimated, for big
+ disks it can take hours.
+
+ After a low level format, a surface scan is done to
+ find and flag bad sectors. Most disks have a
+ manufacturer bad block list listed on a piece of paper
+ or adhesive sticker. In addition, on most disks the
+ list is also written onto the disk.
+ Please use the manufacturer's list. It is much easier
+ to remap a defect now than after FreeBSD is installed.
+
+ Stay away from low-level formatters that mark all
+ sectors of a track as bad as soon as they find one
+ bad sector. Not only does this waste space, it also
+ and more importantly causes you grief with bad144
+ (see the section on bad144).
+
+ <sect2><heading>Translations</heading>
+ <p>
+ Translations, although not exclusively a ESDI-only problem,
+ might give you real trouble.
+ Translations come in multiple flavours. Most of them
+ have in common that they attempt to work around the
+ limitations posed upon disk geometries by the original
+ IBM PC/AT design (thanks IBM!).
+
+ First of all there is the (in)famous 1024 cylinder limit.
+ For a system to be able to boot, the stuff (whatever
+ operating system) must be in the first 1024 cylinders
+ of a disk. Only 10 bits are available to encode the
+ cylinder number. For the number of sectors the limit
+ is 64 (0-63).
+ When you combine the 1024 cylinder limit with the 16 head
+ limit (also a design feature) you max out at fairly limited
+ disk sizes.
+
+ To work around this problem, the manufacturers of ESDI
+ PC controllers added a BIOS prom extension on their boards.
+ This BIOS extension handles disk I/O for booting (and for
+ some operating systems <it>all</it> disk I/O) by using
+ translation. For instance, a big drive might be presented
+ to the system as having 32 heads and 64 sectors/track.
+ The result is that the number of cylinders is reduced to
+ something below 1024 and is therefore usable by the system
+ without problems.
+ It is noteworthy to know that FreeBSD after it's kernel has
+ started no longer uses the BIOS. More on this later.
+
+ A second reason for translations is the fact that most
+ older system BIOSes could only handle drives with 17 sectors
+ per track (the old ST412 standard). Newer system BIOSes
+ usually have a user-defined drive type (in most cases this is
+ drive type 47).
+
+ <em>Whatever you do to translations after reading this document,
+ keep in mind that if you have multiple operating systems on the
+ same disk, all must use the same translation</em>
+
+ While on the subject of translations, I've seen one controller
+ type (but there are probably more like this) offer the option
+ to logically split a drive in multiple partitions as a BIOS
+ option. I had select 1 drive == 1 partition because this
+ controller wrote this info onto the disk. On powerup it
+ read the info and presented itself to the system based on
+ the info from the disk.
+
+ <sect2><heading>Spare sectoring</heading>
+ <p>
+ Most ESDI controllers offer the possibility to remap bad sectors.
+ During/after the low-level format of the disk bad sectors are
+ marked as such, and a replacement sector is put in place
+ (logically of course) of the bad one.
+
+ In most cases the remapping is done by using N-1 sectors on
+ each track for actual datastorage, and sector N itself is
+ the spare sector. N is the total number of sectors physically
+ available on the track.
+ The idea behind this is that the operating system sees
+ a 'perfect' disk without bad sectors. In the case of
+ FreeBSD this concept is not usable.
+
+ The problem is that the translation from <it>bad</it> to <it>good</it>
+ is performed by the BIOS of the ESDI controller. FreeBSD,
+ being a true 32 bit operating system, does not use the BIOS
+ after it has been booted. Instead, it has device drivers that
+ talk directly to the hardware.
+
+ <em>So: don't use spare sectoring, bad block remapping or
+ whatever it may be called by the controller manufacturer when you
+ want to use the disk for FreeBSD.</em>
+
+ <sect2><heading>Bad block handling</heading>
+ <p>
+ The preceding section leaves us with a problem. The controller's
+ bad block handling is not usable and still FreeBSD's filesystems
+ assume perfect media without any flaws.
+ To solve this problem, FreeBSD use the <it>bad144</it> tool.
+ Bad144 (named after a Digital Equipment standard for bad block
+ handling) scans a FreeBSD slice for bad blocks. Having found
+ these bad blocks, it writes a table with the offending block
+ numbers to the end of the FreeBSD slice.
+
+ When the disk is in operation, the diskaccesses are checked
+ against the table read from the disk. Whenever a blocknumber
+ is requested that is in the bad144 list, a replacement block
+ (also from the end of the FreeBSD slice) is used.
+ In this way, the bad144 replacement scheme presents 'perfect'
+ media to the FreeBSD filesystems.
+
+ There are a number of potential pitfalls associated with
+ the use of bad144.
+ First of all, the slice cannot have more than 126 bad sectors.
+ If your drive has a high number of bad sectors, you might need
+ to divide it into multiple FreeBSD slices each containing less
+ than 126 bad sectors. Stay away from low-level format programs
+ that mark <em>every</em> sector of a track as bad when
+ they find a flaw on the track. As you can imagine, the
+ 126 limit is quickly reached when the low-level format is done
+ this way.
+
+ Second, if the slice contains the root filesystem, the slice
+ should be within the 1024 cylinder BIOS limit. During the
+ boot process the bad144 list is read using the BIOS and this
+ only succeeds when the list is within the 1024 cylinder limit.
+ <em>Note</em> that the restriction is not that only the root
+ <em>filesystem</em> must be within the 1024 cylinder limit, but
+ rather the entire <em>slice</em> that contains the root filesystem.
+
+
+ <sect2><heading>Kernel configuration</heading>
+ <p>
+ ESDI disks are handled by the same <it>wd</it>driver as
+ IDE and ST412 MFM disks. The <it>wd</it> driver should work
+ for all WD1003 compatible interfaces.
+
+ Most hardware is jumperable for one of two different I/O
+ address ranges and IRQ lines. This allows you to have
+ two wd type controllers in one system.
+
+ When your hardware allows non-standard strappings, you
+ can use these with FreeBSD as long as you enter the
+ correct info into the kernel config file.
+ An example from the kernel config file (they live in
+ <tt>/sys/i386/conf</tt> BTW).
+
+<tscreen><verb>
+# First WD compatible controller
+controller wdc0 at isa? port "IO_WD1" bio irq 14 vector wdintr
+disk wd0 at wdc0 drive 0
+disk wd1 at wdc0 drive 1
+
+# Second WD compatible controller
+controller wdc1 at isa? port "IO_WD2" bio irq 15 vector wdintr
+disk wd2 at wdc1 drive 0
+disk wd3 at wdc1 drive 1
+</verb></tscreen>
+
+<!--
+ <sect2><heading>Tuning your ESDI kernel setup</heading>
+ <p>
+-->
+
+ <sect1><heading>Particulars on ESDI hardware</heading>
+ <p>
+ <sect2><heading>Adaptec 2320 controllers</heading>
+ <p>
+ I succesfully installed FreeBSD onto a ESDI disk controlled by a
+ ACB-2320. No other operating system was present on the disk.
+
+ To do so I low level formatted the disk using NEFMT.EXE
+ (<it>ftp</it>able from <it>www.adaptec.com</it>) and answered NO
+ to the question whether the disk should be formatted with a
+ spare sector on each track. The BIOS on the ACD-2320 was
+ disabled. I used the 'free configurable' option in the system
+ BIOS to allow the BIOS to boot it.
+
+ Before using NEFMT.EXE I tried to format the disk using the
+ ACB-2320 BIOS builtin formatter. This proved to be a showstopper,
+ because it didn't give me an option to disable spare sectoring.
+ With spare sectoring enabled the FreeBSD installation
+ process broke down on the bad144 run.
+
+ Please check carefully which ACB-232xy variant you have. The
+ x is either 0 or 2, indicating a controller without or with
+ a floppy controller on board.
+
+ The y is more interesting. It can either be a blank,
+ a "A-8" or a "D". A blank indicates a plain 10 Mbits/second
+ controller. An "A-8" indicates a 15 Mbits/second controller
+ capable of handling 52 sectors/track.
+ A "D" means a 15 Mbits/second controller that can also
+ handle drives with > 36 sectors/track (also 52 ?).
+
+ All variations should be capable of using 1:1 interleaving. Use 1:1,
+ FreeBSD is fast enough to handle it.
+
+ <sect2><heading>Western Digital WD1007 controllers</heading>
+ <p>
+ I succesfully installed FreeBSD onto a ESDI disk controlled by a
+ WD1007 controller. To be precise, it was a WD1007-WA2. Other
+ variations of the WD1007 do exist.
+
+ To get it to work, I had to disable the sector translation and
+ the WD1007's onboard BIOS. This implied I could not use
+ the low-level formatter built into this BIOS. Instead, I grabbed
+ WDFMT.EXE from www.wdc.com Running this formatted my drive
+ just fine.
+
+ <sect2><heading>Ultrastor U14F controllers</heading>
+ <p>
+ According to multiple reports from the net, Ultrastor ESDI
+ boards work OK with FreeBSD. I lack any further info on
+ particular settings.
+
+<!--
+
+ <sect1><heading>Tracking down problems</heading>
+ <p>
+-->
+
+ <sect1><heading>Further reading<label id="esdi:further-reading"></>
+ <p>
+ If you intend to do some serious ESDI hacking, you might want to
+ have the official standard at hand:
+
+ The latest ANSI X3T10 committee document is:
+ <itemize>
+<item>Enhanced Small Device Interface (ESDI) &lsqb;X3.170-1990/X3.170a-1991&rsqb;
+ &lsqb;X3T10/792D Rev 11&rsqb;
+ </itemize>
+ On Usenet the newsgroup <htmlurl url="news:comp.periphs"
+ name="comp.periphs"> is a noteworthy place to look
+ for more info.
+
+ The World Wide Web (WWW) also proves to be a very handy info source:
+ For info on Adaptec ESDI controllers see <htmlurl
+ url="http://www.adaptec.com/">.
+ For info on Western Digital controllers see <htmlurl
+ url="http://www.wdc.com/">.
+
+ <sect1>Thanks to...
+ <p>
+ Andrew Gordon for sending me an Adaptec 2320 controller and ESDI disk
+ for testing.
+