<!-- $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, <firstname.lastname@example.org/
<date>Tue Sep 12 20:48:44 MET DST 1995</date>
Copyright 1995, Wilko C. Bulte, Arnhem, The Netherlands
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 <email@example.com/
<sect><heading>ESDI hard disks and FreeBSD<label id="esdi"></heading>
<p><em>Copyright © 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>
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
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.
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.
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>
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
<sect2><heading>ESDI speed variants</heading>
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>
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
<sect2><heading>Hard or soft sectoring</heading>
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
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>
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).
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
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
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.
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>
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
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.
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
# 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
<sect2><heading>Tuning your ESDI kernel setup</heading>
<sect1><heading>Particulars on ESDI hardware</heading>
<sect2><heading>Adaptec 2320 controllers</heading>
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>
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
<sect2><heading>Ultrastor U14F controllers</heading>
According to multiple reports from the net, Ultrastor ESDI
boards work OK with FreeBSD. I lack any further info on
<sect1><heading>Tracking down problems</heading>
<sect1><heading>Further reading<label id="esdi:further-reading"></>
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:
<item>Enhanced Small Device Interface (ESDI) [X3.170-1990/X3.170a-1991]
[X3T10/792D Rev 11]
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
For info on Western Digital controllers see <htmlurl
Andrew Gordon for sending me an Adaptec 2320 controller and ESDI disk