Microsoft KB Archive/104995

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Article ID: 104995

Article Last Modified on 5/6/2003


  • Microsoft MS-DOS 5.0 Standard Edition
  • Microsoft MS-DOS 5.0a
  • Microsoft MS-DOS 6.0 Standard Edition
  • Microsoft MS-DOS 6.2 Standard Edition
  • Microsoft MS-DOS 6.21 Standard Edition
  • Microsoft MS-DOS 6.22 Standard Edition

This article was previously published under Q104995


Your computer may not operate properly if you have a SCSI controller installed but do not have a SCSI device driver loaded in your CONFIG.SYS file. A SCSI driver can serve one or more of the following purposes:

  • Use Virtual DMA Services (VDS) to correctly compute physical addresses
  • Fix bugs in the SCSI controller ROM BIOS
  • Add features not included in the SCSI controller ROM BIOS

If your SCSI device driver is not loaded, you may have problems ranging from occasional system hangs to complete disk data loss. These symptoms are usually accelerated if you are using an upper memory block (UMB) provider (such as EMM386.EXE) or you are running Windows in 386 enhanced mode.


Any device (SCSI being the most common) that uses direct memory access (DMA) controller hardware to transfer data to and from system memory can encounter problems if it used in conjunction with an 80386 memory manager (such as EMM386.EXE, QEMM.EXE, 386MAX.EXE, and so forth).

The ROM BIOS code that supports a DMA device must be aware that under an 80386 memory manager, linear addresses used by MS-DOS programs may not correspond in the usual manner to physical addresses on the system bus.

The Virtual DMA Services is a standard supported by 80386 memory managers that allows ROM BIOS and device drivers to determine the correct "linear-to- physical" memory mapping for a particular linear address, ensuring that the correct DMA transfer address is generated for I/O requests.

SCSI controller cards can be grouped in three categories with regards to VDS compliance:

  • SCSI controller cards that have "VDS-compliant" ROM BIOS. (No additional driver software is needed.)
  • SCSI controller cards that have a non-DMA transfer mode. (These do not need to be VDS aware.)
  • SCSI controller cards that are not VDS compliant. (These require an installable MS-DOS device driver.)


If you don't have the correct driver for your SCSI controller, contact your SCSI hardware vendor. In the interim, adding the following command to your CONFIG.SYS file may help to work around the problem:


Additional query words: 6.22 5.0, 5.0a 5.00 5.00a 6.00 6.20 config.sys stop responding hang

Keywords: KB104995