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MS-DOS compatibility mode

Posted on 1997-10-30
Last Modified: 2013-12-16
I was working perfectly with my PC and Windows95 when I decided to change my CPU. So I replaced the Cyrix 166+ that I had with an Intel 200wMMX. After the installation when I powered  the PC everything seems to be allright (I mean the new CPU was correctly identified and no wrong messages) but when the Windows95 appeared everything was frozen. I reboot the PC and the windows started normally. When I opened the control panel-system I saw that there was a problem with the Primary IDE controller (single fifo) and the message "Compatibility mode paging reduces overall system performance. All HDDs using MS-DOS compatibility mode file system". After that I saw that Drive C: (Western Digital AC33100) was accessable but Drive D: ( slave Western Digital AC25400) was not. What can I do?
Question by:zozefina

Expert Comment

ID: 1751356
Did you install another harddisk driver?  Like Bus mastering drivers for your harddisk?  If so. most probably is that your harddisk drivers are not compatible with your harddisk.  You'll have to install the normal drivers.


Expert Comment

ID: 1751357
What happens is you put your "old" CPU back in?
Try removing all system ressources in the list of peripherals. BIOS, parralel and COM ports, and all those little things. Then detect new hardware after having rebooted.
Does that help?


Expert Comment

ID: 1751358
Check and see if your bios hard disk is configured properly (look out for your hard disk access type). It might be erased due to over long power loss.

Is your win95 version osr2? Previous versions do not do well with large partitions, try upgrading.
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Expert Comment

ID: 1751359
Sorry, correct me if I'm wrong.....

I thought your primary hd controller should show "Dual fifo"?

LVL 14

Accepted Solution

smeebud earned 100 total points
ID: 1751360
Two things to try:
GP Fault or Fatal Exception Error on Intel MMX CPU

Last reviewed: July 18, 1997
Article ID: Q156492
The information in this article applies to:

* Microsoft Windows 95


When you are running a program or using a device driver that calls
MMX- specific instructions in the retail release of Windows 95 on a
system with an Intel processor supporting the MMX instruction set,
the following symptoms may occur:

* A general protection (GP) fault error message
* A fatal exception error message on a blue screen

This problem does not occur with Windows 95 OEM Service Release 2
or later.


The retail release of Windows 95 does not properly support some MMX
instructions if they are called from a device driver or from
program code running in protection level 0 (Ring 0, also known as
privileged or Kernel mode). If these instructions are issued while
the processor is operating in Ring 0, a GP fault or other serious
error may occur.


Microsoft has confirmed this to be a problem in Microsoft Windows
95. An update to address this problem is now available, but is not
fully regression tested and should be applied only to computers
experiencing this specific problem. Unless you are severely
impacted by this specific problem, Microsoft does not recommend
implementing this update at this time. Contact Microsoft Technical
Support for additional information about the availability of this
Troubleshooting MS-DOS Compatibility Mode on Hard

Article ID: Q130179

Microsoft Windows 95


The Performance tab in System properties shows that one or more
of the hard disks in your computer is using MS-DOS Compatibility
mode. MS-DOS compatibility mode may be in use for either the
file system or for virtual memory. You may receive the following

Compatibility Mode Paging reduces overall system performance


MS-DOS Compatibility mode may be in use for any of the
following reasons:

* An "unsafe" device driver, memory-resident program, or virus
hooked the * INT21h or INT13h chain before Windows 95
loaded. * The hard disk controller in your computer was not
detected by Windows 95. * The hard disk controller was removed
from the current configuration in Device * Manager. * There is a
resource conflict between the hard disk controller and another *
hardware device. * The Windows 95 protected-mode driver is
missing or damaged. * The Windows 95 32-bit protected-mode
disk drivers detected an unsupportable * configuration or
incompatible hardware.


To correct the problem, follow these steps:

1. Use the Performance tab in System properties to identify which
drive is 2. using MS-DOS Compatibility mode and why.

NOTE: Floppy disk drives and CD-ROM drives operating in
MS-DOS Compatibility mode cause the Performance tab to
display the message "Some drives are using MS-DOS
compatibility" for the file system, but this article applies only to
troubleshooting hard disks operating in MS-DOS

Compatibility mode.

a. If the driver name listed as causing MS-DOS Compatibility
is MBRINT13.SYS, your computer may be infected with a
boot-sector virus, or you are running real-mode geometry
translation software (for an IDE hard disk with more than 1024
cylinders) that is not compatible with Windows 95
protected-mode disk drivers.

For information about real-mode geometry translation software
that is compatible with Windows 95 protected-mode disk drivers,
please see the following article in the Microsoft Knowledge Base:

ARTICLE-ID: Q126855 TITLE : Windows 95 Support for Large
IDE Hard Disks

Disk Manager 6.03 is supported in protected mode on hard disks
on the primary IDE channel and when DriveSpace disk
compression is not installed. For drives on the secondary IDE
channel, Disk Manager 7.0 or later is required. When using the
DriveSpace compression software that is included with Microsoft
Windows 95 and Microsoft Plus!, Disk Manager 7.04 or later
must be used. For more information, please see the following
article in the Microsoft Knowledge Base:

ARTICLE ID: Q126855 TITLE : Windows 95 Support for Large
IDE Hard Disks

For information about detecting and removing boot-sector viruses,
please see the following articles in the Microsoft Knowledge Base:

ARTICLE-ID: Q82923 TITLE : Methods to Detect a
Boot-Sector Virus

ARTICLE-ID: Q129972 TITLE : Description of Computer

ARTICLE-ID: Q49500 TITLE : List of Anti-Virus Software

b. If a driver that is listed in the CONFIG.SYS file is named,
contact the driver's manufacturer to determine whether there is a
version of the driver that allows protected-mode access in
Windows 95.

If no driver is listed on the Performance tab, continue with Step 2.

3. Check to make sure that the hard disk controller is listed in
Device 4. Manager. If it is not listed, install it with the Add New
Hardware Wizard. 5. If the Wizard does not detect the controller,
run the Wizard again but do 6. not let the Wizard detect the
hardware in your computer. Instead, select 7. the controller from
the hardware list. If the controller is not listed, contact 8. the
manufacturer of the hard disk controller to determine whether there
is 9. a Windows 95 protected-mode disk driver or a Windows 3.1
32-bit disk 10. (FastDisk) driver available.

NOTE: If the hard disk controller is listed in Device Manager but
has a red X over it, it has been removed from the current hardware
profile. Click Properties for the controller in Device Manager and
then click the check box corresponding to the current hardware
profile under Device Usage. 11. If the hard disk controller is listed
in Device Manager but has a yellow exclamation 12. point over it,
there is an IRQ, I/O, DMA, or RAM address conflict with another
13. device, the protected-mode driver is missing or damaged, or
the "Disable all 32-bit 14. protected-mode disk drivers" check box
is selected in File System properties.

a. Check to make sure that the "Disable all 32-bit protected-mode
drivers" check box has not been selected on the Troubleshooting
tab in File System properties. To access this tab, double-click
System in Control Panel, click the Performance tab, and then click
File System.

b. Resolve any resource (IRQ, I/O, DMA, or RAM address)
with other devices. Consult the controller's documentation for
information about resource usage and changing resource usage.

c. Check to make sure that the protected-mode driver is in the

Windows\SYSTEM\IOSUBSYS directory and is loading
properly. To determine which driver is providing 32-bit disk
access, click Properties for the controller in Device Manager and
click the Driver tab to see which driver files are associated with the

NOTE: If you are using an IDE, EIDE, or ESDI hard disk
controller, the Driver tab may not be present when you click
Properties for the controller in Device Manager. Unless you are
using a third-party driver, Esdi_506.pdr is the protected-mode
driver that is used to provide 32-bit disk access for these

Restart Windows 95 and press F8 at the "Starting Windows 95"
message. Select a Logged (/BOOTLOG.TXT) start. Examine the
just-created BOOTLOG.TXT file to determine if the driver listed
above is loading properly.

If the BOOTLOG.TXT file shows an "Init Failure" or "Load
Failure" message for the driver listed above, proceed with step D.
If the BOOTLOG.TXT file shows an
"INITCOMPLETESUCCESS" message for the drive listed
above, examine the IOS.LOG file.

Windows 95 creates an IOS.LOG file in the Windows directory if
any drives are using MS-DOS Compatibility mode. The first few
lines of the IOS.LOG file may contain information describing why
the protected-mode disk driver failed to load. Please have this
information available if you contact Microsoft Product Support
Services about this problem.

d. Make sure the protected-mode driver is not damaged.

For all ESDI and IDE drives, Windows 95 uses ESDI_506.PDR
in the IOSUBSYS directory to provide 32-bit disk access. For
SCSI controllers, Windows 95 uses SCSIPORT.PDR and a
"mini-port" (.MPD) driver to provide 32-bit disk access.

Manually extract the appropriate .PDR or .MPD files from the
Windows 95 disks or CD-ROM, or run Setup and choose the
Verify option.

15. Check to see if the Mh32bit.386 driver is being loaded in the
System.ini file. 16. Check for a line that reads
"device=mh32bit.386." This driver is installed by 17. MicroHouse
EZ-Drive software, and is not compatible with the Windows 95
18. protected-mode disk drivers. This driver is not removed by
Windows 95 Setup. 19. Contact the hard disk controller's
manufacturer for information about Windows 20. 95 compatibility.
You may be able to get protected-mode, 32-bit disk access 21. in
Windows 95 by using one of the following methods:

- Disable any enhanced features (such as caching, fast or turbo
mode, reduced data transfer rates, and so on) on the controller
(SCSI, IDE, or ESDI) or system BIOS (IDE only).

- obtain a protected-mode Windows 95 disk driver, or Windows
3.1 FastDisk driver for the controller.


A real-mode driver is "safe" if its functionality does not exceed the
functionality of the corresponding Windows 95 protected-mode
driver. If a real-mode driver is safe, the protected-mode driver can
take over all I/O operations for the corresponding device.
Otherwise, Windows 95 routes all I/O operations through the
real-mode driver.

An example of an unsafe driver is a real-mode IDE/ESDI driver
that uses dynamic encryption for security reasons. Since Windows
95 does not provide encryption, Windows 95 does not allow the
protected-mode IDE/ESDI driver to take over the real-mode
driver. Any real-mode driver with functionality on the following list
is considered unsafe:

* Data compression that is not compatible with DoubleSpace *
Data encryption * Disk mirroring * Bad sector mapping * Fault
tolerance (for example, maintenance of ECC correction on a
separate disk) * Vendor-specific IOCTLs * Microsoft-defined
IOCTLs with vendor-extended features

The safe driver list (the IOS.INI file) is a Windows 95-maintained
list of safe drivers. Each entry in the list identifies a driver or TSR
that Windows 95 can take over with the corresponding
protected-mode driver. The safe driver list includes the name of
the driver or TSR. This name should be the same as the name in

Windows 95 does not store the version number of the driver or
TSR in the list, so it is the responsibility of the vendor to change the
name of the driver if a future version of the driver is enhanced in a
manner that makes the driver unsafe.

By default, the following drivers are considered safe:

* MS-DOS 5.0-compatible real-mode block device drivers *
INT 13 monitors (hooks INT 13 for monitoring INT 13 I/O but
does not access * the hardware directly or modify the I/O buffer)
* INT 13 hooker (hooks INT 13 for altering INT 13 I/O but does
not access the * hardware directly) * INT 13 driver (provides
INT 13 functionality and directly accesses the hardware) * ASPI
Manager (implements ASPI for MS-DOS specification) * CAM
Manager (implements MS-DOS CAM specification)

NOTE: If the real-mode driver you are using has better
performance or provides some functions that are not be present in
the Windows 95 protected-mode driver, the driver's vendor
should remove the driver from the safe driver list. The system will
use real mode to access the drive. If the real-mode driver you are
using can be safely taken over by protected-mode drivers, the
driver's vendor can add that driver to the safe driver list.

Disk Manager is manufactured by OnTrack Computer Systems, a
vendor independent of Microsoft; we make no warranty, implied
or otherwise, regarding this product's performance or reliability.

EZ-Drive is manufactured by Micro House, a vendor independent
of Microsoft; we make no warranty, implied or otherwise,
regarding this product's performance or reliability.

Let me know how it goes.


Expert Comment

ID: 1751361
I had the same problem.  I finally solved it by reinstalling Win95.  It is a relatively easy thing to do.

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