Contents of the INTLEAVE.DOC file
* THE GUIDE TO HARD DISK SECTOR INTERLEAVING *
* by *
* Steven Gibson *
* GIBSON RESEARCH CORP. *
* Box 6024, Dept C *
* Irvine, CA, 92716 *
* (714) 854-1520 *
In these "sophisticated" days of computers, where AUTOEXEC.BAT
and CONFIG.SYS make casual conversation, it's rare to find a
topic as interesting and critical but still mis-understood, as
hard disk sector interleaving.
Our researches into this area have discovered that MOST IBM AND
COMPATIBLE personal computers are performance-crippled by mis-
interleaved hard disk drives.
Unfortunately, many disk controller companies, in competing with
each other, have set their disk interleave defaults too tightly
for many computers. Such "specsmanship" directly hurts the
innocent computer user (you) by dramatically limiting his hard
disk data transfer rate. This GUIDE carefully explains the
situation and shows how to use the two included programs to
determine whether your own IBM PC or compatible's hard disk
drives have their sector interleave set correctly.
Responding to this problem, Gibson Research Corp., publisher of
the popular display screen enhancement utility FlickerFree, has
recently developed an inexpensive software SOLUTION which first
determines your system's optimum hard disk interleave factor
then RESETS IT while leaving all your hard disk data in place!
UNDERSTANDING HARD DISK SECTOR INTERLEAVING
It's a rare person who would not wish for additional performance
from his personal computer's hard disk drive. While much
attention is given to the drive's Average Seek Time, which is a
measure of the time required to move the read/write head from
one track to another, there is another vital detail which
determines overall hard disk performance and which is subject to
the user's control.
We will see that the too often neglected SECTOR INTERLEAVING
factor of a hard disk has a dramatic impact on data transfer
As most people know, the information stored on a floppy or hard
disk is arranged in a series of concentric circular paths called
tracks. The disk drive's read/write head is specifically
positioned over any desired track with an operation called a
SEEK. Thus an obvious limit on the speed with which a drive can
find or place information would be the so called track-to-track
and average seek times.
A single track of a standard IBM compatible PC contains
approximately nine thousand bytes of data. But since we usually
deal with data in much smaller chunks, each track is divided
into smaller sections called sectors. Think of a spinning pizza
which has been cut into seventeen identical, and numbered,
slices. (Drives with RLL encoding pack 50% more data onto every
track resulting in more than thirteen thousand bytes per track
divided into 25 or 26 sectors.)
Now suppose that we need to read the information contained in
sector 1 of our current track. We patiently wait for sector 1
to rotate under our read/write head, reading its data at that
time. After absorbing this freshly read information, we realize
that we also need to read the next sector, number 2. However,
by the time sector 1 has been moved into our computer and we've
decided to read sector 2, the beginning of sector 2 has already
started passing under the read/write head. So we have no choice
but to wait for the disk to rotate all the way around once more
to deliver sector 2. If we wished to read a nine thousand byte
file composed of all seventeen disk sectors on this track,
seventeen complete rotations of the disk, one for each sector,
would be required!
It wasn't long before a bright engineer realized that the entire
problem could be easily resolved by spreading the sequentially
numbered sectors out around the disk: Instead of placing sector
2 immediately after sector 1, sector 2 could be placed a few
sectors later! In this way, after reading sector 1, sector 2
would be just about ready for reading by the time we were ready
for it. Such an elegant solution!
If, for example, sequentially (logically) numbered sectors were
staggered out every three physical sectors, then each rotation
of the disk could read every third sector. Therefore only three
revolutions of the disk would be required to read an entire
track. Quite an improvement over 17 revolutions! This sector
staggering is known as SECTOR INTERLEAVING or SECTOR MAPPING.
The physical spacing between logically consecutive sectors is
known as the INTERLEAVE FACTOR. This example used an interleave
factor of three, shown as "3:1" and pronounced: "3 to 1".
The new higher-density RLL controllers need to be correctly
interleaved too. With 26 sectors per track a non-interleaved or
mis-interleaved disk would require 26 revolutions for an entire
Now here's the real rub: In the current environment of mix and
match highly modular personal computing, responsibility for
determining and setting your hard disk drive's optimal sector
interleave factor has "fallen through the cracks" as it were.
You've never worried about it have you? If you're inclined to
believe that someone else has, (like your local dealer perhaps)
you might be in for a real surprise. Experiments with a wide
variety of computers, drives, controllers, clock speeds, and
interleave factors have shown that the hard disks of MOST
PERSONAL COMPUTER SYSTEMS ARE NOT PROPERLY INTERLEAVED TODAY!
So many computers are so badly interleaved that it's quite
likely that you could increase your own hard disk's performance,
by FOUR TO SEVEN TIMES just by optimally setting your disk's
The interleave factor can be either too "loose" or too "tight".
The result of operating with an interleave factor which is too
loose is lower performance than a particular drive-controller-
computer combination could achieve with tighter interleaving.
For example the original IBM PC/XT is interleaved at 6:1 but can
readily achieve 5:1 in a standard 4.77 Mhz machine and 4:1 in an
8 Mhz PC. This means that disks on accelerated PC's can read
and write at 150% of their current rate!
The consequence of operating with an interleave factor which is
too tight is more disastrous, since missing that next sector
induces the significant delay of another entire disk revolution!
You might be saying to yourself "so what's an extra disk
revolution between friends?", but consider this: If your disk
is properly interleaved, say with a factor of 3, the entire nine
thousand byte track will be read or written in just 3 disk
revolutions. However, if the particular controller in your
particular computer cannot achieve that interleave of 3, then 17
or 18 entire revolutions will be required to read the same
3 revolutions versus 18 is a performance difference of 600%!
Since many files and most programs are well over nine thousand
bytes long, and since DOS DOES REQUEST an entire track transfer
in such cases, these 17-revolution track reads add up quickly.
You would certainly feel the difference between waiting 3
seconds for a program to start up, versus 18 seconds!
The damning thing about proper hard disk sector interleave is
its total dependency upon EVERY factor of your system. For
example, the AT&T 6300 requires 20 REVOLUTIONS for a full track
transfer when reading a disk which is interleaved at the most
common factor of 3:1! Even it's faster brother, the 6300 Plus,
requires 14 revs! We have found that the AT&T's hard disk data
transfer rate can be made FOUR TIMES FASTER when its interleave
is optimized for it! If you're using a 6300 or 6300+, your disk
transfer rate is one quarter of what it could be!
Another example: Western Digital's RLL hard disk controller
formats drives at a default interleave of 3. Believe it or not,
this interleave is too tight for good old standard 4.77 Mhz XT!
Consequently 28 entire disk revolutions are required to read a
SINGLE track! If the interleave is set to 4 then only 4 revs.
are required to read the same data, for a 700% throughput boost!
So by now you're dying to know what's happening with the hard
disk sector interleave of your own computers! What interleave
factors are being used, are they correct, and what you can do
about it if not!
Included with this INTLEAVE.DOC file are two small programs:
SPINTEST.COM and SPINTIME.COM
The main program, SPINTEST.COM determines how many disk
revolutions your IBM compatible personal computer requires to
read an entire 17 sector track from its hard disk. (SPINTEST is
also fully compatible with the 25 or 26 sector tracks used with
the high-density RLL controllers.)
Running SPINTEST on the many computers at Gibson Research, and
in many local computer dealerships, we discovered something
quite surprising: Most computers being sold today are horribly
interleaved. We turned up many machines which required more
than 17 entire revolutions just to read or write one full track!
These same machines flew along at 4:1 but not at 3:1.
For example, Gibson Research has a vintage Leading Edge Model-D
with a standard Western Digital hard disk controller. This
machine always seemed to run more quickly than a newer generic
PC clone equipped with a different Western Digital controller.
Both hard disks received their original low-level (interleaving)
format at the Western Digital default interleave of 3. After
all, those dealers must have thought, Western Digital knows
what's best. Right? NOT NECESSARILY!
Since the controller AND computer together determine the optimal
interleave, it is IMPOSSIBLE for any controller manufacturer to
set a single optimal interleave for everyone. In fact, even the
controller's revision level can be a determiner! One of our two
WD controllers ALWAYS MISSES sectors when the disk is inter-
leaved at 3, while the other controller makes 3:1 ... in the
To test this, one of our hard disks was given a low-level format
at an interleave of four. Afterward, the SPINTEST timing
diagnostic program and a stop watch confirmed our suspicions:
An MS-DOS file copy operation on the hard disk which had
required 139 seconds when the disk was interleaved at 3:1 now
took only 39 seconds at an interleave of 4:1!
Interestingly, that "slower" controller which can't make the
interleave of 3 on a standard 4.77 Mhz XT does just fine on a
faster 8 Mhz clone computer. So as you can see, the COMPUTER
makes a difference too! Any way you look at it, the issue of
sector interleaving involves the complex interactions among the
disk drive, the controller make and model (even the revision
level), the processor, and system clock rate.
OVERVIEW OF THE INTERLEAVE DIAGNOSTIC PROGRAMS
The issues of hard disk sector interleaving have gone unnoticed
for so long because there has never been a clear way to see
what's really going on deep inside a hard disk. After seeing
the importance of this issue, we decided to change this. The
two programs SPINTEST and SPINTIME determine, for any standard
MFM or RLL controller, exactly how many revolutions the disk
requires to transfer one entire track of data. The programs
were hand written in 100% machine language (as is everything
Steve Gibson writes and Gibson Research publishes) to give them
the required measuring resolution speed.
THE SPINTEST PROGRAM
SPINTEST determines exactly how many revolutions your hard disk
currently requires to perform a full track transfer. DOS
transfers a full track whenever programs or files larger than 9K
are read or written ... which is most of the time. SPINTEST
does NOT directly read your drive's interleave, but the drives
interleaving performance can be easily inferred from SPINTEST's
full track transfer revolution count.
SPINTEST averages the time required for each of over two hundred
full track reads in order to accurately determine the revolution
count per read, then the number of revolutions required to read
just one track and the controller's full-track data transfer
rate are computed. SPINTEST only reads data from your drive so
data is NEVER altered.
Over two hundred track reads were used because of the
inconsistent disk transfer behavior of AT&T's 6300 machines.
These exhibit a maddening inconsistency in their ability to
transfer disk data. Sometimes they get the next sector, and
often not. This means that a much looser than normal interleave
generates MUCH BETTER overall disk throughput for AT&Ts. For
this reason, and to guarantee correct results on any machines
which may behave similarly, SPINTEST performs many track reads
and averages the results.
THE SPINTIME PROGRAM
Since SPINTEST measures track read time, it must assume a given
ratio between elapsed time and disk rotation rate. Some weirdo
clone computers have a clock which runs faster than normal when
in their "turbo" (8 Mhz) mode, and some lap computers have tiny
hard disks spinning at weird speeds. If these do not sound like
your situation SPINTEST will deliver correct readings and you'll
not need SPINTIME's confirming measurements.
But if EITHER of these cases might be you (if you have a weirdo
clone computer or hard disk sporting laptop) SPINTIME will tell
you for sure. SPINTEST is only usable when SPINTIME gives
standard readings. The single (not surprising) exception to
this is for the AT&T 6300 and 6300 Plus machines. Due the
overall problems they experience with disk transfers, SPINTIME
may show a lower-than-3600 RPM reading. For 6300's this is
SPINTIME is interesting regardless since it determines EXACTLY
how fast your hard disk is really spinning!
To run SPINTEST, simply type its name at the DOS prompt.
IMPORTANT!: Read the section IMPORTANT SPINTEST AND SPINTIME
NOTES below, before you begin!
Since SPINTEST takes the average of over 200 track reads, the
time required to run this test will vary between approximately
14 seconds for a fast 2:1 interleaved machine to 98 seconds for
an incorrectly interleaved 26-sector RLL encoded hard disk. So
simply type: SPINTEST at the DOS prompt and wait a minute or
SPINTIME has two purposes: To determine the speed of your
system's clock (if you suspect that it might be weird) , and to
verify that your system's hard disk is spinning at close to the
standard 3600 RPM standard. Your system's clock is checked by
measuring SPINTIME's exact execution time. It should require
EXACTLY 60 SECONDS to run ... no matter what. During that time,
it is busy watching your hard disk spin, counting every revolu-
tion. After EXACTLY 60 seconds, it displays your drive's exact
RPM (within its measuring resolution.) IF SPINTIME's total
execution time is NOT EXACTLY 60 seconds, OR the displayed RPM
is not close to 3600, SPINTEST's reported revolution count won't
mean anything either.
SPINTIME's real value is to assure you that SPINTEST is
delivering worthwhile answers.
IMPORTANT SPINTEST AND SPINTIME NOTES!
Both programs assume and require DOS 2.0 or higher. They
operate on your system's FIRST hard disk only (drive C). The
hard disk must be bootable, have its controller plugged into a
slot, not requiring special CONFIG.SYS device drivers for
SPINTEST and SPINTIME will never disturb your hard disk data,
but if your disk is some kind of odd-ball they may not function
The tests will be more accurate if your normal collection of
memory resident programs (if any) are not in memory at the time.
Many resident programs "steal" so much time from the computer
that a delicate timing analysis could be upset. It would be
wise to boot your machine from a plain DOS diskette when running
these tests for maximum reliability.
Also, turn off (or better yet remove) any disk caching software
you might be using. Disk caching intercepts the disk reading
performed by the programs and renders their measurements invalid
(and rather humorous). By the way, disk caching program per-
formance is significantly improved through proper disk
WHAT IT ALL MEANS
So now you're wondering: What do the numbers mean? How do they
compare with industry norms and everyone else's? Should I be
dancing on roof tops or wringing my dealer's neck?
A hard disk spins at 3600 revolutions per minute, or 60
revolutions per second. A track has 17 sectors of 512 bytes per
sector. This means that data passes under your drive's head at
a rate of 522,240 bytes per second. (This number is 768,000 for
RLL con-trollers with 25-sector tracks.) Your system will
achieve some fraction of this maximum possible rate as
determined by the number of rotations required to read or write
each track. 522,240 (or 768,000) is divided by the number of
revs per track (as SPINTEST does) to calculate your drive's data
Since your system's performance is meaningful only when compared
to other properly and improperly interleaved systems, the
following table will give you a feeling for where the industry
1:1 - There IS a controller which achieves 1:1,
unfortunately it lacks Error Correcting Code
(ECC) which is critical for reliable operation.
2:1 - Compac machines achieve 2:1. Adaptec's 2070A
RLL controller can do 2:1 with an 8 Mhz machine
even though their low-level formatter stubbornly
refuses to allow a setting of less than 3!
3:1 - Newer WD controllers and many other controllers
will handle 3:1. The Adaptec 2070A RLL con-
troller will do 3:1 in a 4.77 Mhz machine.
4:1 - Older WD controllers need 4:1, original IBM XT
controllers can do 4:1 in an 8 Mhz machine.
The AT&T 6300 Plus works best at 4:1 and WD's
RLL controller can only do 4:1.
5:1 - IBM's original XT controller can do 5:1 in a
4.77 Mhz XT but comes interleaved at 6:1. The
AT&T 6300 is best at 5:1 which increases over-
all throughput 400% over its default!
6:1 - Only the original IBM PC/XT seems to have been
way out here. These machines can be tightened
up, and if it has an inexpensive accelerator it
can probably run at 4:1 for 150% disk boost.
We have seen that WD controllers which miss at 3:1 will do
beautifully at 4:1. Several InfoWorld readers have
independently confirmed that AT&T machines (with WD controllers)
perform best at the very loose interleave of 6:1. Even the old
true blue IBM controller which runs at 6:1 can always do 5:1,
and even 4:1 in a faster than 4.77 Mhz machine.
So what system factors influence and determine the optimal
interleave setting for a given set of equipment? After a sector
of data has been read it must be moved from the controller's on-
board buffer into the computer. This is done with a process
known as DMA (Direct Memory Access.) The time required to
transfer the sector determines how soon the controller will be
ready to read the next sector. For this reason add-in
Accelerator "Turbo" Cards do not generally change a machine's
optimum interleave since the main system clock speed, which
continues to control DMA memory accesses, is not changed. The
less expensive "Speed Booster" products which alter the basic
system clock timing DO have a tremendous impact on optimal
interleave by running the DMA faster and thus moving the data in
and out faster.
GIBSON RESEARCH AND INTERLEAVING
When we began these experiments we had NO IDEA that so many
personal computers were so poorly interleaved. Without the aid
of our new special software, changing a hard disk's sector
interleave "manually" requires first backing-up all hard disk
data onto some secure medium. Then a low-level re-formatting is
performed. This unfortunately messy task was never designed for
the casual computer user since it requires unpublished knowledge
of the internal details of your controller, using the DOS DEBUG
command to poke hexadecimal values into the 8088's machine
registers and starting the low-level format. After this, the
FDISK and FORMAT commands are used to create a partition table
and lay down the high-level formatting information. After all
this, the backed up data must be copied back onto the drive.
Then, if the experimentally chosen interleave was not correct,
most of the process would have to be repeated with a different
trial interleave factor.
Gibson Research Corp., responding to the clear need for a better
solution to the task of hard disk sector interleaving optimiza-
tion has automated and streamlined this "re-interleaving"
process with a product which was designed specifically to meet
This software product quickly determines the optimal interleave
setting for any disk of any size in any system with any clock
rate. Once determined, the existing interleave of the disk can
be RESET automatically, and in just a few minutes, leaving all
your disk data intact and in place! The product also performs
several other useful hard disk utility functions which have
never before been available.
GET YOUR DISK SPINNING RIGHT!!
Even if SPINTEST turned in excellent results of 3 or 4 revs, you
should STILL check out Gibson Research's new product for the
other surprising (NEVER before offered) hard disk capabilities
it brings ... and if you received results like 10, 12, or even
17, 18, or 19 revolutions, there is NO DOUBT that your system
could be running FOUR TO FIVE TIMES FASTER ... in minutes!
To receive all the details about this product simply phone, or
mail your name and address. Ask for the INTERLEAVING INFO from
the On-Line edition of the GUIDE. ... you'll be glad you did!!
GIBSON RESEARCH CORP.
Box 6024, Dept C
Irvine, CA 92716
(If you're wondering about Gibson Research's first
product, FlickerFree, ask for some info about it too!)
* <-- The End ... Of the GUIDE --> *