SMARTDrive May Be Required to Use EMM386.SYS or EMM386.EXE (83141)

The information in this article applies to:
    Microsoft MS-DOS operating system 4.0
    Microsoft MS-DOS operating system 4.01
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22
    Microsoft Windows 3.0
    Microsoft Windows 3.0a
    Microsoft Windows 3.1
    Microsoft Windows 3.11

This article was previously published under Q83141


Some hard drive controllers use direct memory addressing (DMA) that can interfere with EMM386.SYS, EMM386.EXE, or running Windows in 386 enhanced mode. Symptoms of this interference include:
    System stops responding (hangs) when loading programs that use expanded memory provided by EMM386.EXE.
    System hangs when attempting to load programs with DEVICEHIGH= or LOADHIGH (LH) into upper memory blocks (UMBs) provided by EMM386.EXE.
    Data corruption on the hard disk drive when running Windows in 386 enhanced mode.
    CHKDSK or MS-DOS 6.2 ScanDisk reports data corruption (such as truncated or invalid directories).
In this case, SMARTDrive should be loaded in conventional memory before loading any device drivers that use expanded memory provided by EMM386.EXE or before loading any drivers into UMBs using DEVICEHIGH= statements. For example:
If you are using Microsoft Windows 3.1, use:
   device=c:\windows\smartdrv.exe /double_buffer


MS-DOS versions 4.x and Windows 3.0 include EMM386.SYS, an expanded memory emulator. MS-DOS versions 5.x and 6.0 and Windows 3.1 include EMM386.EXE, which also can be used to create UMBs along with providing expanded memory. In all versions, EMM386.EXE uses the virtual 8086 mode of the 80386 and 80486 processors to make extended memory appear to be in the upper memory area. This allows EMM386.EXE to simulate expanded memory or create UMBs out of extended memory; in actuality, there isn't any RAM where the UMBs or page frame appears to be.

These disk controllers attempt to directly transfer information from disk to memory, without involving the CPU. By bypassing the CPU to transfer information from disk to memory, the controller tries to access nonexistent RAM. This usually causes the system to hang, but it can also cause data corruption on the disk.

A protocol called Virtual DMA Services (VDS) has been developed to work around this problem. Most hard disk drives being produced now are VDS-compliant. A VDS-compliant hard disk drive knows when the machine is in virtual 8086 mode and communicates with the virtual mode monitor (in this case, Windows or EMM386.EXE) to ensure that information is put in the correct place.

SMARTDrive also works around this problem by hooking all disk transfers into its own buffer. The DMA works in this case because SMARTDrive is loaded in conventional memory (hence the requirement that SMARTDrive be loaded low). Then, SMARTDrive transfers the information into the buffer where it really needs to be placed. This is called "double buffering."

Using SMARTDRV.SYS (Versions 3.x)

SMARTDRV.SYS versions 3.x, included with Windows 3.0 and MS-DOS versions 4.x and 5.x, perform both double buffering and disk caching. When loaded, SMARTDRV.SYS performs double buffering if it detects that it is required. Double buffering can also be forced by loading SMARTDrive with the /B+ option.

WARNING: Some drives have more than 1024 cylinders or use sector sizes of other than 512 bytes per sector. If this is the case, SMARTDRV.SYS should not be used. SMARTDRV.SYS versions 3.06 (included in Windows 3.0a) and 3.13 (included in MS-DOS 5.0) refuse to load if a third-party partitioning utility (commonly used on such systems) is detected. SMARTDRV.SYS can be forced to load despite this check by using the /P switch. For example, to forcibly load SMARTDRV.SYS, add the following command to the CONFIG.SYS file:

device=<path>\smartdrv.sys /p

To forcibly load SMARTDRV.SYS with forced double buffering, add the following command to the CONFIG.SYS file:

device=<path>\smartdrv.sys /b+ /p

NOTE: Use the /P option only if the disk drive has fewer than 1024 cylinders and the ROM BIOS supports the disk drive size; otherwise, the /P option can lead to data corruption. For information on using disks with more than 1024 cylinders with MS-DOS, query on the following words in the Microsoft Knowledge Base:

1024 and cylinders and workarounds

Using SMARTDRV.EXE (Version 4.0)

Windows 3.1 and MS-DOS 6.0 include SMARTDrive version 4.0. SMARTDrive 4.0 is loaded differently depending on whether double buffering, disk caching, or both is desired. To load SMARTDrive for double buffering, use the following line in the CONFIG.SYS file:
   device=c:\windows\smartdrv.exe /double_buffer+
The double buffer portion must be loaded in conventional memory; it displays an error and exits if loaded into a UMB. The disk cache (usually loaded from the AUTOEXEC.BAT file) can still be loaded into a UMB and tries to do so automatically if MS-DOS is providing UMBs.


For information on using SMARTDRV.EXE with third-party partitioning software, query on the following words in the Microsoft Knowledge Base:

SMARTDRV.EXE and partitioning

For information on using SMARTDRV.EXE with MS-DOS 6, refer to SMARTDrive Help.

Modification Type: Major Last Reviewed: 5/10/2003
Keywords: kbinfo KB83141