SMARTDrive and 32-Bit Disk Access

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Article ID: 83325
                             INTRODUCTION
                             ============

Microsoft Windows operating system version 3.1 has a number of
enhancements over previous versions that improve both its usability
and its performance. Foremost among these enhancements are Windows
3.1's improved features for accessing and utilizing the hard drive of
your PC.


                       SMARTDRIVE 4.0 DISK CACHE
                       =========================

Microsoft's first release of the Windows graphical environment
included a disk caching program called SMARTDrive 3.03. SMARTDrive has
been improved with each subsequent release of Windows and has been
entirely redesigned in Windows version 3.1. SMARTDrive 4.0 in Windows
3.1 provides faster and more intelligent disk caching that takes
better advantage of the revolutionary way that Windows and Windows-
based applications use extended memory.

SMARTDrive 4.0 gives Windows 3.1:

    - Improved performance by speeding up hard disk and RAM access.
    - Improved stability in 386 enhanced mode by double buffering data
      when needed.

The remainder of this section compares the performance of SMARTDrive
4.0 to SMARTDrive 3.x and illustrates how SMARTDrive 4.0 works at a
more technical level.


WHAT IS SMARTDRIVE?
===================

SMARTDrive is a disk caching program that intercepts system calls to
the hard disk to control read/write access to the disk. SMARTDrive
intercepts any calls to the hard disk and loads the needed data into a
cache it establishes in RAM. (Unless specified otherwise, SMARTDrive
automatically determines a reasonable cache size based on the amount
of free extended memory available when it initially loads; the maximum
amount it will take is around 2 MB.) Subsequent calls to the hard disk
are intercepted by SMARTDrive, which scans the cache for the requested
data. If the data is already present in the cache, SMARTDrive can
access it directly in RAM. If the data is not in the cache, SMARTDrive
accesses the hard disk and loads the necessary data into the cache.
The least recently used data residing in RAM is cached back to the
hard disk to make room for the new data. By loading blocks of data
from the hard disk into RAM, SMARTDrive helps decrease the number of
calls to the hard disk. This can dramatically speed up applications
that repeatedly access the hard disk for data because accessing the
hard disk is considerably slower than accessing RAM. Essentially,
SMARTDrive attempts to maintain information in RAM that an application
needs at any given time from the hard disk.


SMARTDRIVE 3.X
==============

To understand why SMARTDrive needed to be redesigned, lets look at
how SMARTDrive 3.x works and examine some of the shortcomings of its
design. First, SMARTDrive 3.x is a read-only track cache, which means
that it caches on a track basis and for read operations only. This
means that the internal data structures of the SMARTDrive program are
tied to the logical geometry of the disk (that is, what MS-DOS sees).
In the diagram below, you can see that SMARTDrive 3.x caches at the
ROM BIOS INT13 level and uses the BIOS-specified disk geometry to
decide the size of its caching element (that is, track size). INT13 is
the most common method for interacting with hard disks (and has
evolved into the industry standard). SMARTDrive under Windows 3.0 can
be loaded into either extended memory or expanded memory.


          Application
       (Read/Write INT21)
              |
- - - - - - - - - - - - - - - - -
              |
           MS-DOS
       Call HD Driver
              |
          HD Driver
           INT 13
              |
              --------->  SMARTDrive 3.x
                           Chains INT13
              ---------<
              |
          ROM BIOS
              |

         Hard Drive


SMARTDRIVE 4.0
==============

SMARTDrive 4.0 is designed as a block-oriented disk cache. It hooks
into the system at the MS-DOS device driver level rather than at the
ROM BIOS INT13 level (as with SMARTDrive 3.x). Each block device
driver on the MS-DOS device driver chain is "front-ended" by a
SMARTDrive 4.0 module that provides the actual caching. This new
design yields the following benefits:

    - It is independent of the INT13 interface.
    - It is independent of disk geometry.

         Application
     (Read/Write INT21)
              |
- - - - - - - - - - - - - - -
              |
           MS-DOS
        Call HD Driver
              |
              --------->  SMARTDrive 4.0
                           HOOKS Driver
              ---------<
              |
           HD Driver
           INT 13
              |
          ROM BIOS
              |

         Hard Drive


Independent of INT13 Interface
------------------------------

Many device drivers do not use the INT13 (BIOS) interface (for
example, Bernoulli drives, some hard cards, and many SCSI and WORM
drives). This means that SMARTDrive 4.0 can cache these devices,
whereas SMARTDrive 3.x cannot. SMARTDrive 4.0 can cache any disk
controller that uses an MS-DOS block device driver.

Independent of Disk Geometry
----------------------------

Some disk managers and disk controllers use a disk geometry mapping
scheme that causes the logical geometry to be different from the
physical geometry (for example, many PS/2 systems, Ontrack's Disk
Manager, and several disk controllers). The INT13-based disk caches
are sensitive to this and often have problems. For example, some disk
management software actually changes the ROM BIOS-specified disk
geometry, which confuses INT13-based disk caches (including SMARTDrive
3.x).

There usually is an interface to determine the true geometry, but this
requires detection of the specific disk manager driver and generally
complicates the disk cache software. Often, logical tracks actually
cross physical track boundaries, causing the track caches to incur
performance penalties (intertrack seeks and rotational latencies, for
example). To get around the ROM BIOS 1024-cylinder limitation, disk
managers and controllers sometimes fold multiple tracks into one
logical track. This results not only in problems with the physical
track boundaries as noted above, buts forces track caches to have a
very large track buffer as well. In some cases, this buffer can be as
large as 31.5K and must reside in conventional memory. The design of
SMARTDrive 4.0 eliminates this geometry mismatch problem.


OTHER SMARTDRIVE 4.0 FEATURES
=============================

Write-Behind Cache
------------------

SMARTDrive 4.0 is a write-behind cache, also known as lazy write. A
write-behind cache adds significant performance improvements wherever
files are being written to disk. For example, an application writes
data to what it thinks is the hard disk. SMARTDrive takes this data
and places it in the cache, rather than physically writing it to the
disk. This write-behind data stays in the cache until one of the
following occurs:

1. The cache fills up. The oldest block in the cache is freed up, and
   if it is write-behind data, physically written to the disk.

2. The system goes idle. SMARTDrive writes the oldest write-behind
   data block to the physical disk. As long as the system is idle,
   SMARTDrive continues to write to the disk until all the write-
   behind data is written. This write-behind data also comes from
   Windows-based and MS-DOS-based applications that are idle.

3. SMARTDrive detects that you have pressed CTRL+ALT+DEL. When you
   restart by pressing CTRL+ALT+DEL, SMARTDrive writes all write-
   behind data to the physical disk. This is a synchronous operation--
   SMARTDrive does not give up control until all data is written.

4. A block is older than five seconds. If a block is older than five
   seconds, it is written to the physical disk.

Shrink Algorithm
----------------

Similar to SMARTDrive 3.x, SMARTDrive 4.0 implements a shrink
algorithm that frees memory for Windows (that is, the cache set up by
SMARTDrive is removed from memory). The difference is that SMARTDrive
4.0 watches for the Windows startup broadcast while SMARTDrive 3.x
provides an IOCTL interface. The net effect is identical, but the
SMARTDrive 4.0 code is much simpler. Upon exiting Windows, the process
is reversed and the memory is reacquired by SMARTDrive 4.0.

Automatic Load High
-------------------

When an upper memory block (UMB) provider (such as EMM386.EXE) is
loaded, SMARTDrive 4.0 (SMARTDRV.EXE) automatically attempts to load
itself high. Unlike the Windows 3.0 SMARTDrive, SMARTDrive 4.0 can
only be loaded into extended memory.


DOUBLE BUFFERING AND BUS MASTERING
==================================

The second major benefit of SMARTDrive 4.0 is the protection it
provides through double buffering. Double buffering is a response to
bus mastering, which is supported by certain (usually older) disk
controllers. Bus mastering refers to a situation in which the hard
disk controller takes over the bus to directly transfer data to or
from system RAM, bypassing the CPU entirely. A problem occurs when
Windows 3.x virtual machines and/or popular memory managers (such as
EMM386.EXE and QEMM.SYS) are running in virtual 8086 mode. The read or
write address that is passed to MS-DOS from the bus master controller
is often not the same as the actual physical memory address. This can
cause data to be read from the wrong location, or worse, can cause
data to be written to the wrong RAM address, resulting in erratic
system behavior and potential file corruption.

When paging is enabled on an 80386 or faster processor, it is possible
for the physical address (the actual location of the memory) to be
different from the virtual address (the address that programs are
using to access memory). Paging is enabled by all device drivers that
can provide load high capability and by Windows in 386 enhanced
mode. For example, say SMARTDrive wants to write to an area of memory
that has a virtual address in conventional memory, but a physical
address in extended memory. When the hard drive controller takes over
the bus and transfers data from the UMB to the disk, the controller
does not use the CPU, so there is no address translation, resulting in
the wrong data being written to disk. To avoid this problem, double
buffering copies the data into a buffer in conventional memory and
then passes the information to the hard disk controller. So, the
virtual address always equals the physical address when using devices
that only recognize conventional memory.

Installing Double Buffering
---------------------------

To install the SMARTDrive double-buffering feature, add the following
line to your CONFIG.SYS file:

   device=c:\windows\smartdrv.exe /double_buffer

Including this line does not install the cache, only the double-
buffering driver. The cache must be installed from the AUTOEXEC.BAT
file or MS-DOS command line.

Most disk controllers do not need double buffering. These include all
MFM, RLL, and IDE controllers as well as many ESDI and SCSI devices.
The Setup program in Windows 3.1 will install the double buffer in the
following cases:

1. If the computer has an 80386 or faster processor

2. If 32-Bit Disk Access was not installed in the SYSTEM.INI file (see
   next section)

You should install the double buffer in these cases in the interest of
added safety. Another new feature of SMARTDrive 4.0 that helps you
determine if double buffering is necessary is the smartdrv command.
Once your system is running with SMARTDrive loaded, type "smartdrv"
(without the quotation marks) at the MS-DOS prompt and press ENTER.
Information similar to the following is displayed:

     Microsoft SMARTDrive Disk Cache version 4.0
     Copyright 1991, 1992 Microsoft Corp.
     Cache size: 1,048,576 bytes
     Cache size while running Windows: 1,048,576 bytes

                   Disk Caching Status


     drive    read cache    write cache    buffering
     ---------------------------------------------------
     A:       yes           no             no
     B:       yes           no             no
     C:       yes           yes            yes
     D:       yes           yes            ---

     For help, type "Smartdrv /?".

Note the column labeled "buffering." For each drive that is being
cached, one of three values is displayed--"yes," indicating that
double buffering is needed; "no," indicating that double buffering is
not needed; or "---," indicating that SMARTDrive has not yet
determined the necessity of double buffering. If the buffering column
displays a "no" for each drive, the double buffer driver is
unnecessary, and you can safely remove it from your CONFIG.SYS file.

Setting Up SMARTDrive 4.0
-------------------------

The Setup program in Microsoft Windows 3.1 automatically installs
SMARTDrive (SMARTDRV.EXE) in the first line of your AUTOEXEC.BAT file.
In general, you should let the Setup program configure SMARTDrive's
cache size for you. However, please note it is critical that
SMARTDRV.EXE load after any block devices that reside on your system,
such as:

    - Hard disk partitioning software (for example, DMDRVR.BIN and
      SSTOR.SYS)
    - Hard disk drivers (for example, SCSI drives, hard cards, and
      Bernoulli drives)
    - Data encryption software (for example, Stacker and DoubleDisk)

The configuration of SMARTDRV.EXE is controlled by command-line
parameters. Most configuration options can be modified without
restarting; however, without the Windows Resource Kit utility
SMARTDrive Monitor (SMARTMON.EXE), the configuration cannot be altered
while running Windows. Also note that SMARTDrive:

    - Needs a minimum cache size of 16K.
    - Creates a cache size of 16K by default if you supply an invalid
      cache size.
    - Gives you benefits up to a cache size of 2 MB, whereupon the
      benefits typically plateau.


                          32-BIT DISK ACCESS
                          ==================

Another new feature of Windows 3.1 is its use of 32-Bit Disk Access.
32-Bit Disk Access is not a program but rather a term used to refer to
a system of Windows components working together. The main purpose of
these components is to allow Windows to interact directly with the
hard disk, bypassing the BIOS, which is normally used for
communicating with the hard disk. Under 386 enhanced mode Windows,
where 32-Bit Disk Access is supported, you get the following benefits:

    - Faster hard disk access
    - Ability to run more MS-DOS-based applications
    - Faster overall system response, even when running MS-DOS-based
      applications in the background
    - More powerful off-the-shelf disk utilities to choose from, since
      independent software vendors (ISVs) are writing programs that
      take advantage of this new set of services


WHAT IS 32-BIT DISK ACCESS?
===========================

    - A SYSTEM OF WINDOWS COMPONENTS. Some of these components are
      called 32-Bit Disk Access devices, but they are useless without
      the rest of the components to support them. Each of these
      components is discussed below.

    - A REPLACEMENT FOR THE HARD DISK BIOS. 32-Bit Disk Access serves
      as a device driver that interacts with the hard disk controller.
      32-Bit Disk Access watches for INT13H calls and handles them if
      they are for the disk it represents; otherwise, it passes the
      calls on to the BIOS in the usual way, as if 32-Bit Disk Access
      were not available.

    - AN EXACT MATCH FOR THE HARD DISK CONTROLLER (NOT THE SPECIFIC
      HARD DISK). There are some standards for hardware communication
      that work on many different controllers. The most widely adopted
      standard is that introduced by Western Digital in its 1003
      controllers. This is the standard supported by Windows 3.1 and
      more than 90 percent of hard disk controllers, with the
      exception of ESDI and SCSI controllers.

    - AN OPEN STANDARD FOR CERTAIN CONTROLLERS. 32-Bit Disk Access is
      an open standard that is being promoted to all manufacturers of
      hard disk controllers. This means that every manufacturer has
      the opportunity to write a device driver that supports 32-Bit
      Disk Access functionality for its hardware. Many hardware
      manufacturers are writing 32-Bit Disk Access device drivers that
      will be available at the time Windows 3.1 ships or shortly
      thereafter.

    - AN OPTION, NOT A SYSTEM REQUIREMENT. Without the 32-Bit Disk
      Access device driver installed, you lose the hardware-access
      benefits provided by 32-Bit Disk Access, but your system is
      otherwise unaffected. Windows 3.1 without 32-Bit Disk Access has
      the same disk access capabilities of Windows 3.0.


DISK ACCESS UNDER MS-DOS
========================

The 32-Bit Disk Access feature bypasses the system BIOS to more
directly control hard disk access. It acts, in a sense, as the hard
disk controller. Before discussing how disk access has changed under
Windows 3.1, let's look at how MS-DOS handles calls to the hard disk.

In the MS-DOS-based PC, when an application wants to read from the
disk, it tracks the following sequence:

                                         H/W
         INT21H          INT13H          I/O
        ------>           ----->        ----->
 -----           ------           ----          ----------     -----
| APP |         |MS-DOS|         |BIOS|        |Controller| ->|Drive|
 -----           ------           ----          ----------     -----


1. The application makes an INT21H call to MS-DOS.

2. MS-DOS locates the requested part of the file on the hard disk.

3. MS-DOS issues an INT13H call to talk to the hard disk BIOS. The
   BIOS contains a program to control each device in the system.

4. The BIOS driver talks directly to the hard disk controller. Each
   hard disk controller requires its own BIOS driver, but these BIOS
   drivers are usually not specific to the hard disk itself, but
   rather to the controller board (or adapter) that connects the hard
   disk to the rest of the system.

This system works well, because MS-DOS is able to identify the files
and directories and keep track of their locations, regardless of the
type of hard disk MS-DOS is accessing. But, on the other hand, the
system BIOS is wholly linked to the hard disk, which provides disk
independence for the applications but does not allow the BIOS to
differentiate between a call from MS-DOS or a program such as
SMARTDrive.

But remember that not all disk device drivers reside in ROM. If you
purchase an off-the-shelf disk drive and install it, your computer's
ROM BIOS may not know how to communicate with it. If this is the case,
you must install a device driver that is specific to your new hard
disk. This additional device driver is usually installed in your
CONFIG.SYS file when you install the software that comes with your
hard disk. With some devices, such as a CD-ROM drive, a device driver
that provides the BIOS interface and INT13H interface to the CD-ROM
drive appears in your AUTOEXEC.BAT file. This type of device driver
traps INT13H calls to the hard disk and makes sure the calls are
understood by the hard disk. Usually, the device driver handles any
attempts to access the hard disk; the calls never get passed on to the
real ROM BIOS. If the call is not for the device that the device
driver represents, then the INT13H call is passed on to the BIOS,
where it is handled in the usual way. 32-Bit Disk Access represents a
way for Windows to provide a BIOS that better suits customers' device
support needs.


32-BIT DISK ACCESS SPEEDS UP DISK ACCESS
========================================

On many machines, the resident hard disk BIOS is slow and inefficient.
Since 32-Bit Disk Access is specific to devices, it is often more
efficient, leading to very impressive performance increases. 32-Bit
Disk Access also allows Windows 3.1 to handle BIOS requests in
protected mode, rather than in real mode, which may also speed up disk
access. The following diagram charts the flow of calls under Windows
in standard mode:

                                         H/W
         INT21H          INT13H          I/O
        ------>           ----->        ----->
 -----           ------           ----          ----------     -----
| APP |         |MS-DOS|         |BIOS|        |Controller| ->|Drive|
 -----           ------           ----          ----------     -----


Under 386 enhanced mode Windows, the flow of calls is more
complicated:


         -------------------------------------------------
        |         Enhanced Mode Virtual Devices           |
         -------------------------------------------------
Protected   / \          /    \ 
  Mode     /   \        /      \ 
----------/-----\------/--------\-----------------------------------
Virtual  /       \    /          \ 
 Mode   /         \  /            \ 
   -----   INT21H  ------   INT13H ----    H/W   ----------     -----
  | APP |         |MS-DOS|        |BIOS|   I/O  |Controller|-->|Drive|
   -----           ------          ----  ----->  ----------     -----

Windows 3.1 and Windows applications run in protected mode. Windows
has to switch back into real mode (or virtual 8086 mode) to run older
code (for MS-DOS and the BIOS calls, for example). Switching modes is
very time consuming. For example, when a non-Windows application
running under 386 enhanced mode tries to read from a file, the
following occurs:

   Windows starts out in virtual mode running the application. Windows
   makes an INT21H call to read from the file. 386 enhanced mode traps
   this interrupt and switches to protected mode, where a number of
   virtual device drivers look at the call and try to qualify it. If
   none of the device drivers intercepts the call, the call is
   accepted and is subsequently handed off to MS-DOS.

   Windows then switches back to virtual mode to let MS-DOS handle the
   call. MS-DOS reads a particular location on the disk and then
   generates an INT13H call to talk to the BIOS. Windows in 386
   enhanced mode then traps the call again and switches to protected
   mode to perform certain tasks, hands the call back to the real
   BIOS, and switches back to virtual mode.

   The BIOS handles the call and accesses the hard disk controller to
   set the physical read in motion. Once the BIOS reads the call,
   Windows goes back to protected mode to perform more tasks and then
   returns to virtual mode to let MS-DOS see the returned call.
   Windows returns the INT21H call, switches to protected mode to
   perform yet more tasks, and then finally switches back to virtual
   mode to communicate with the application that issued the call to
   begin with.

All this mode switching takes time but is necessary due to the nature
of the 80386 processor, MS-DOS, and the PC BIOS. With a 32-Bit Disk
Access device installed, Windows can start trapping INT13H calls and
handling them entirely in protected mode. With 32-Bit Disk Access, the
call diagram now looks like this:

   ----------------------------     H/W     ----------     -----
  |Enhanced Mode   |32-Bit Disk|    I/O    |Controller| ->|Drive|
  |Virtual Devices |     Access|  ----->    ----------     -----
   -----------------------------
Protected    /   \           / 
  Mode      /     \         / 
-----------/-------\-  ----/---------------------------------------
Virtual   /         \     / 
 Mode    /           \   / 
      -----   INT21H  ------  INT13H   ----
     | APP |         |MS-DOS|         |BIOS|
      -----           ------           ----


With 32-Bit Disk Access, Windows no longer has to go through two mode
transitions--one mode to get to the BIOS and the other mode to get
back. Only two mode switches per INT13H call, rather than four, are
now being performed.


32-BIT DISK ACCESS SUPPORT FOR NON-WINDOWS APPLICATIONS
=======================================================

Sometimes Windows shows an adequate amount of free memory and large
Windows-based applications run just fine, yet an "Out of Memory"
message is generated when a non-Windows application is started. Also,
in spite of the available memory, Windows-based applications may slow
down and constantly access the hard disk after a non-Windows
application is started.

These discrepancies occur because of the difference between virtual
memory (which is often large) and physical memory (RAM, which is
usually limited). On a given machine, virtual memory might be four
times the size of RAM. But, while virtual memory works well for
Windows-based applications, it doesn't always meet all the system's
memory needs for MS-DOS-based applications.

Because each non-Windows application runs in its own virtual machine,
each application takes up 640K or more of virtual memory. When the
virtual machine is actually running, all of the program code needs to
occupy RAM. Parts of the non-Windows application cannot be paged out
while keeping other parts in RAM, as is the case with Windows-based
applications. The virtual machine in which the non-Windows application
runs must be treated as a single block. This is true even if the
application is currently inactive in the background.

With Windows-based applications, most of the program remains resident
on the hard disk, while only the parts that are actually being used
are loaded in RAM. Problems occur when Windows tries to do this with
non-Windows-based applications.

If a part of the application that is not currently being used--a data
buffer, for example--is paged out, it can always be read back in
whenever the application needs to access it. However, if the
application makes a call to MS-DOS to read some data from the disk
into a buffer, MS-DOS starts handling the call by making a call to
INT13H to talk to the BIOS, which in turn talks to the hard disk
controller, resulting in the transfer of data into the buffer. So when
the application needs to transfer the page into the buffer, which is
currently out on disk, the application tries to call MS-DOS to read
the page.

Because MS-DOS is already occupied, the application can't call MS-DOS;
and MS-DOS is not reentrant, so the second call from the application
fails. Nor can the application have the page read by calling the BIOS,
because the BIOS is also in the middle of a call, so it can't be
reentered either. This situation results in a type of deadlock: the
application can't let the current call finish until the page is put in
the buffer, but the page cannot be placed in the buffer until the
current call finishes.

The only way to ensure that this deadlock situation does not occur is
to make sure that Windows never has to page anything in while the
application is in the middle of an MS-DOS or BIOS call. Because
Windows cannot predict ahead of time what the application will try to
access, the only way to be 100-percent safe from deadlocking is to
make sure everything the application might try to access is already in
RAM (that is, never paged out to disk). In other words, all 640K-plus
of the virtual machine in which the non-Windows application is running
has to remain resident in RAM, thereby using up a lot of physical
memory. Running several non-Windows-based applications at once uses
large amounts of RAM, so you may soon end up with only a small amount
of free RAM left for everything else you're trying to do, such as run
Windows-based applications. When you check the Program Manager About
dialog box (from the Help menu), it shows that you have plenty of
memory--virtual memory--and you do. But if you try to start another
non-Windows application, which requires yet another 640K of RAM, the
application won't start. So, Windows starts running sluggishly and/or
may generate the "Out of Memory" message.

So how does 32-Bit Disk Access help? Windows 3.0 offered a contiguous
paging file (which is created with Swapfile) for any pages using the
INT13H BIOS calls. Windows 3.1 handles the same task by making calls
to the 32-Bit Disk Access system. Unlike MS-DOS or the BIOS, the 32-
Bit Disk Access system can queue up multiple requests and is
completely reentrant. Windows no longer needs to rely on MS-DOS or the
BIOS; so regardless of what the application is doing, Windows can
always page calls in from the hard disk. Free from a potential
deadlock situation, Windows 3.1 can now make the virtual machines that
are running completely pageable. Non-Windows-based applications no
longer need to take up so much RAM, since they can now use virtual
memory the way Windows-based applications do. When Windows needs some
RAM space, it can now page out pieces of these virtual machines.

With Windows 3.1 and 32-Bit Disk Access, you can run multiple non-
Windows-based applications and Windows-based applications without
generating an "Out of Memory" message. When the About box tells you
that you have 14 MB of memory, you can actually use all 14 MB for
running your applications, whether they are Windows or non-Windows.

Improved Performance for Non-Windows Applications Under Windows
---------------------------------------------------------------

In the previous example, we saw how 32-Bit Disk Access in Windows can
page non-Windows-based applications to free up enough RAM for those
applications when they need to use it. The 32-Bit Disk Access feature
also improves Windows performance all around, making the system run
much more quickly.

Switching between non-Windows-based applications by pressing ALT+TAB
is faster with 32-Bit Disk Access. Without 32-Bit Disk Access, a non-
Windows application can only be swapped out to disk if it is
configured to not run in the background. This means when you press
ALT+TAB to switch to such an application, Windows has to read the
entire program into RAM, causing the task switching to take a long
time. When Windows is paging using 32-Bit Disk Access, Windows can
load in just the small amount of information that the application is
actually using, so task switching becomes almost instant. And, since
only a small part of the application needs to be in RAM at one time,
Windows may not have to access the hard disk at all.


THE COMPONENTS OF THE 32-BIT DISK ACCESS SYSTEM
===============================================

The 32-Bit Disk Access system is composed of the following 386
enhanced mode virtual devices:

    - WDCTRL, which is the 32-Bit Disk Access device that talks to
      standard Western Digital 1003 or ST506 hard disk controllers
      (about 90 percent of the installed base). WDCTRL is only
      installed if Setup detects a compatible hard disk controller.

    - BlockDev, which coordinates calling block I/O services between
      devices and the 32-Bit Disk Access devices that provide those
      services for specific hard disk controllers. BlockDev is always
      installed.

    - PageFile, which handles the virtual memory paging file and calls
      through BlockDev if any 32-Bit Disk Access devices are
      available. PageFile is always installed.

    - INT13, which traps and emulates INT13H BIOS calls by calling
      BlockDev. INT13 is only installed if at least one 32-Bit
      Disk Access device is present.

Because WDCTRL is the only 32-Bit Disk Access device included in the
Windows box, the standard Windows components only support 32-Bit Disk
Access for Western Digital-standard controllers. When you run Setup,
it automatically detects these controllers and adds the following
lines to the [386Enh] section of your SYSTEM.INI file:

     device=*int13
     device=*wdctrl

If you want to disable 32-Bit Disk Access, start the Control Panel and
choose the 386 Enhanced icon, or comment the lines out of your system
by placing a semicolon (;) at the beginning of both lines. You do not
need to delete any files from your hard disk, as the 32-Bit Disk
Access devices are contained within the file WIN386.EXE.


32-BIT DISK ACCESS AND HARD DISK PROTECTION
===========================================

Microsoft identified machines where the use of WDCTRL can stop the
system. Usually, this is caused by hard disk controllers that appear
to be Western Digital 1003 compatible but are really not. WDCTRL has
been put though rigorous testing to make sure it is as safe as
possible, and is designed with a built-in safety measure--every time
it starts, it performs elaborate tests to make sure it can communicate
in the same language as the hard disk controller before it attempts to
read and write data. WDCTRL starts testing by taking a peripheral look
at some data on the hard disk for tell-tale traces of a Western
Digital 1003-compatible controller. If the controller passes that
test, WDCTRL then starts calling up larger pieces of data to try to
elicit the correct response. Finally, after much redundant checking,
WDCTRL tries actually reading data from the disk. If it is able to
read data, WDCTRL then tries to write data and read it back. Only
after the read/write test is passed does WDCTRL continue and start
Windows, reassured that it will not cause any catastrophic errors on
the hard disk.

However, there are still a few exceptions. There are drives that
appear to be WD1003 compatible but are not, and which cannot be
detected by WDCTRL. In addition, there can be problems even with
compatible controllers on some portable computers, specifically those
that power down the hard disk to conserve power without telling the
running software. If the disk is powered down and then back up while
it is being tracked by WDCTRL, the tracking information will be
incorrect, and serious damage can result. For these reasons, 32-Bit
Disk Access is not turned on by default. When you install Windows, 32-
Bit Disk Access is disabled until you make the decision to enable it
through Control Panel. Only original equipment manufacturers (OEMs)
who preinstall Windows 3.1 on a 100-percent-compatible system can turn
32-Bit Disk Access on before the user receives the system without
jeopardizing the user's hard disk.

When something does go wrong, WDCTRL may do different things,
depending on where it is in its verification code. The initial tests
have reported WDCTRL as extremely safe. The controllers that WDCTRL
fails with are considered incompatible with 32-Bit Disk Access, so
WDCTRL simply does not load when it senses these unsupported
controllers. This procedure is invisible to the user; he or she just
won't receive the benefits of 32-Bit Disk Access. If WDCTRL fails a
test after it is installed, Windows displays an error message that
warns that something is wrong, most likely that the hard disk might
have been damaged, and advises to restart the machine (so that WDCTRL
does not load, and 32-Bit Disk Access is disabled).


THIRD-PARTY 32-BIT DISK ACCESS DEVICES
======================================

Windows 3.1 ships with only one 32-Bit Disk Access device--WDCTRL.
However, Microsoft is working with most of the major hardware and
software vendors to help them design 32-Bit Disk Access devices for
their products. Concerns regarding distribution and quality of testing
for these third-party devices are currently being addressed.
			
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Article ID: 83325 - Last Review: June 17, 2014 - Revision: 2.0
Keywords: 
kbinfo KB83325
Retired KB Content Disclaimer
This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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