Microsoft KB Archive/63937

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VDS.exe Demonstrates DMA in Windows 3.0

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Q63937

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The information in this article applies to:


 * Microsoft Windows Device Development Kit (DDK) for Windows, version 3.0

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SUMMARY
VDS.exe is a sample that contains the Virtual DMA Specification supported by Windows enhanced mode and standard mode. DMA requires special handling in the different modes of Windows version 3.0. This article focuses on the virtualization of the DMA ports that is done by the enhanced-mode Windows.

In real mode, DMA can be programmed and performed as usual, as in a normal DOS application.

This article addresses the following topics:


 * 1) DMA in enhanced mode (including bus master DMA, the Virtual DMA Specification [VDS], and the DMABUFFERSIZE SYSTEM.INI switch)
 * 2) DMA in standard mode

MORE INFORMATION
The following files are available for download from the Microsoft Download Center. Click the file names below to download the files:

"Vds.exe" For more information about how to download files from the Microsoft Download Center, please visit the Download Center at the following Web address

"http://www.microsoft.com/downloads/search.asp" and then click How to use the Microsoft Download Center.

VDS.exe contains the following files:

  Filename       Description ---  DMA_API.DOC    Virtual DMA Specification in Microsoft Word format WINDOC.STY    Style sheet used by DMA_API.doc DMA_API.TXT   The same specification in text format These services are necessary for applications that program DMA in Windows 3.0 enhanced mode.

DMA in Enhanced Mode
In enhanced mode, standard (on AT-type machines) DMA channels are virtualized. The Virtual DMA Device (VDMAD) translates (or maps) the linear address to the physical address. As long as an application program can obtain a linear address that is less than 24 bits, it can program the standard DMA channels using the linear address as a physical address. For example, programs (such as MS-DOS-based applications) running in Virtual 86 mode can program the standard DMA channels as they can in real mode. The linear address in the Virtual 86 mode is calculated as (segment<<4)+offset. When this address is used on the standard DMA channels, it is translated (mapped) by the Virtual DMA Device. In other words, the translation is done transparently through I/O virtualization.

However, there are two situations when an application cannot rely on VDMAD to do the translation:


 * 1) A program running in protected mode, such as a Windows program, cannot usually obtain linear addresses that are less than 24 bits.
 * 2) When dealing with a Bus Master DMA controller, because its I/O ports are not standard, there is no virtualization of its ports. Even if the application can obtain a linear address less than 24 bits, VDMAD cannot perform translation on ports that it doesn't know about.

In these cases, the application must use the Virtual DMA Specification (VDS) services. VDS services are provided by enhanced-mode Windows as an API through INT 4Bh. VDS provides services to lock a linear DMA region and return a physical address for the region. The application can specify whether an automatic remap should or should not be attempted when necessary. The returned physical address can then be used to program the DMA channels. VDS also provides a service to Scatter/Gather lock a linear region. This service is typically used on DMA devices that can perform Scatter/Gather.

To use VDS lock region services, an application needs to know either the virtual address selector(segment):offset or the linear address. The linear address in protected mode can be obtained using GetSelectorBase (see below for more information). Please be advised that due to a problem in Windows 3.0, it is sometimes necessary to perform GlobalPageLock on the DMA regions before using the VDS locking services. [See the Windows SDK reference for GlobalPageLock.]

Note that a program should disable the DMA translation using the VDS enable/disable translation service before it uses the physical address to program the standard DMA channels. It should enable the translation after the DMA channel is programmed.

The following is information about the DMABUFFERSIZE switch in SYSTEM.INI. In enhanced-mode Windows, when an application relies on VDMAD to do the translation, VDMAD may use a buffer it has allocated during initialization time to remap the DMA region. This buffer has a default size of 16K. The size may be changed to a larger one by setting the switch DMABUFFERSIZE in SYSTEM.INI to a value greater than 16. If a DMA region that requires remapping has a size greater than that of the allocated remapping buffer, Windows will crash and ask the user to set the DMABUFFERSIZE switch to a greater value. It is the application's responsibility to inform the user about this and have the user modify the SYSTEM.INI switch beforehand.

DMA in Standard Mode
VDS services are also provided in Windows standard mode. Windows-based applications can utilize the same services to do DMA in standard mode as in enhanced mode. MS-DOS-based applications running under standard-mode Windows may not have access to VDS services. Because MS-DOS-based applications are running in real mode, they should do the DMA programming as they would in the MS-DOS environment.

It is also possible for applications running in standard mode Windows to perform DMA without using VDS. This is because in standard mode, the linear address is the same as the physical address, the memory is contiguous, and memory addressing is limited to 24 bits (the 16 MB address space limit of the 80286 processor). Also, there is no virtualization on the DMA I/O ports, or other I/O ports in standard mode. The Windows-based application should obtain the linear address of the DMA region and program the DMA directly.

To obtain the linear address, the application should call GetSelectorBase to obtain the base address first. GetSelectorBase is documented in the Windows SDK reference.

  DWORD dwBase ; WORD wSelector ; DWORD dwPhyAddr ;

wSelector = (WORD)(lpBuffer >> 16) ; // selector of the buffer address.

dwBase = GetSelectorBase (wSelector) ; dwPhyAddr = dwBase + (0x0000FFFF & lpBuffer) ; // base + offset Note that although dwPhyAddr is a 32-bit variable, only the lower 24 bits will be nonzero. The 80286 can only address up to 24 bits of address space. The DMA controller can usually take 24 bits. If it can only take up to 20 bits, GlobalDOSAlloc should be used to allocate the memory. GlobalDOSAlloc allocates memory within the first 1 MB range. Please see the SDK reference for information on GlobalDOSAlloc.

Memory allocated in standard mode is physically contiguous. However, it is the application program's responsibility to make sure the memory block does not overlap a segment boundary. Most DMA controllers wrap around when they reach a segment boundary. The application program should break the DMA transfer into two smaller transfers in this case: the first transfer ends at the segment boundary and the second one begins at the segment boundary.

Additional query words:

Keywords : kbfile kbsample kb16bitonly kbDDK kbWinOS3

Issue type : kbinfo

Technology :