Q63914: Definition of SMARTDrive Double Buffering

Article: Q63914
Product(s): Microsoft Windows 95.x Retail Product
Version(s): WINDOWS:3.0,3.0a
Operating System(s): 
Keyword(s): 
Last Modified: 08-OCT-1999

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The information in this article applies to:

- Microsoft Windows versions 3.0, 3.0a 
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SUMMARY
=======

Microsoft Windows version 3.0 does not run properly in enhanced mode on machines
equipped with Bus Master cards unless SMARTDrive is also running. The new SCSI
drive controllers are an example of such a system. If SMARTDrive is not running,
Windows 3.0 may stop responding (hang or crash) in enhanced mode on these
systems.

SMARTDrive provides double buffering when Windows is running in enhanced mode,
which resolves this problem.

MORE INFORMATION
================

Double buffering is a concept that addresses some problems with certain types of
hardware. The definition of double buffering, in the case of SMARTDrive, is as
follows:

  ALL read/write operations to any SMARTDrive managed drive will always be done
  through the SMARTDrive-managed low- memory buffer regardless of any other
  parameters of the read/write. In other words, the read/write address for the
  operation passed to the hardware will ALWAYS be the address of a buffer
  inside SMARTDrive. The data is transferred from/to the real address for the
  operation, to/from the SMARTDrive buffer at an appropriate time (before or
  after the request) for the operation.

This is important because a Bus Master DMA hard disk controller does NOT use the
system DMA controller, (that's why it's called a BUS master card). For Windows
enhanced mode, this would normally be a problem. Since the Bus Master hardware
is not "virtualized," like the system DMA controller is, the correct linear to
physical address mapping isn't performed for the read/write operation.

This means that the read/write may be performed to the incorrect physical address
and the system will crash. By turning on double buffering in SMARTDrive (this is
done automatically by enhanced-mode Windows), all read/write operations to the
drive are transferred through a low-memory SMARTDrive buffer where the following
is true:

  linear address = physical address

As a result the Bus Master disk hardware needs to be virtualized because it's
been arranged, through SMARTDrive double buffering, for all read/write
operations to the disk to occur at an address where Linear=Physical. Most of the
new SCSI drive controllers are examples of such Bus Master Cards. If you don't
load SMARTDrive, these disks do NOT work with enhanced-mode Windows. If you load
SMARTDrive, they DO work. The reason is the double buffering that SMARTDrive is
told to do when enhanced mode is operating.

Double buffering has a performance impact on overall disk performance. This cost
is judged to be "worth the benefit" of getting the product to work on a broader
range of MS-DOS platforms.

If you want to learn more about the importance/relevance of Linear versus
Physical addresses, and how this impacts the operation of transfers to/from a
hardware device in the MS-DOS context, check the local bookstore for a book on
the operation of virtual memory, in paging operating systems, that uses an
operating system operating on the Intel 80386 processor as its standard "working
example." An example is "The 80386 Book," by Ross P. Nelson, published by
Microsoft Press.

Additional query words: 3.00 3.0 3.0a 3.00a SmartDrive.sys winmem win30

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Keywords          :  
Technology        : kbWin3xSearch kbZNotKeyword3 kbWin300 kbWin300a
Version           : WINDOWS:3.0,3.0a

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