Microsoft KB Archive/92407

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API Translation Buffers in Enhanced Mode Windows


The information in this article applies to:

  • Microsoft Windows Device Development Kit (DDK) for Windows, version 3.1


Because enhanced mode Windows provides an extended MS-DOS environment, at times it may invoke system code running in real mode or even V86 mode. This article describes the process that Windows employs to pass protected mode data to V86-mode addressable areas, and discusses some potential problems arising from this scheme.


Data that resides outside of the V86-addressable area must be copied into the affected VMs virtual 1 megabyte (MB) space to be accessible by code running in V86 mode. Enhanced mode Windows (to be more precise, the V86 memory manager virtual device driver) provides two possibilities to do this: local API translation buffers (accessible via the V86MMGR_Allocate_Buffer and V86MMGR_Free_Buffer services) and global API translation buffers. This article focuses on the global API translation buffers.

At initialization time, virtual device drivers (VxDs) may ask enhanced mode Windows to set up a global API translation buffer (APITB) via the V86MMGR_Set_Mapping_Info service call, passing in a required (minimum) and a desired (maximum) buffer size. As a result, Windows allocates one global APITB in the corresponding address locations of all virtual machines (VMs) according to the following algorithm:

The APITB is always aligned on 4K boundaries and is a multiple of 4K. It is never smaller than the largest minimum size requested by all VxDs and never larger than the largest maximum size requested. Unless the user sets "ReservePageFrame=yes" in SYSTEM.INI, Windows attempts to allocate the APITB in the 384K adapter segment; if necessary, Windows will shrink it to the minimum size. If there is no space in the 384K adapter segment, the APITB will be allocated in conventional memory.

Thus, in a sense, the term "global" has two meanings: It means that the buffer is visible to all virtual machines and that the buffer is being shared by all VxDs. Although any VxD can request participation in the APITB, participation should be restricted to VxDs that need to provide access to it asynchronously. The swap-file virtual device, as well as most network protocol virtual devices (such as the virtual NetBIOS device), utilize the APITB. Because (asynchronous) hardware interrupts from the hard disk as well as the network can be reflected into any VM, the translation buffer accessed by these virtual devices must be global to all VMs.

In the case of the virtual NetBIOS device, the NetHeapSize= parameter in SYSTEM.INI is used to determine the APITB size that the virtual NetBIOS device requests from Windows.

Note that the APITB is nothing more than a reserved area in the corresponding memory locations of all virtual machines. A virtual device driver claims parts of the buffer by submitting a V86MMGR_Map_Pages call and releases it by calling the V86MMGR_Free_Map_Page_Region service. When portions of the APITB are being claimed, Windows flags these portions as unusable for other VxDs and maps physical pages into those portions. Any request for buffer usage will be 4K aligned; that is, the memory manager will always claim multiples of 4K from the APITB, regardless of the actual size of the request.

This poses a potential problem with virtual device drivers that claim portions of the APITB for unpredictable amounts of time. For example, the virtual NetBIOS device claims a portion of the APITB when an asynchronous NetBIOS request is being submitted from a protected mode VM and releases it when the callback routine associated with the NetBIOS request is invoked. Under heavy network-traffic conditions, one NetBIOS request may cause a new portion of the APITB to be allocated while another request is pending; this process may cause the APITB to become fragmented, thus failing bigger NetBIOS calls although the overall number of pages available in the APITB may be enough to satisfy the request (the V86MMGR always looks for a contiguous number of pages to satisfy a buffer claim request and does not compact memory within the APITB). That scenario may cause the following problem to appear:

Although the NetHeapSize parameter in SYSTEM.INI is sufficiently large, a full screen message with the following contents pops up:

This application requires a larger buffer for transferring information over the network. You can increase the buffer size by modifying the NetHeapSize setting in your SYSTEM.INI file. Include or modify the following setting in the [386Enh] section in your SYSTEM.INI file: NetHeapSize=<suggested new value>

If the NetHeapSize value is still too low, this message will appear again, suggesting a new value.

In pathologic cases, this may happen repeatedly, even if the NetHeapSize parameter is set to a value significantly higher than the maximum size needed.

To resolve this problem, you should either limit all NetBIOS requests to NCBs smaller than 4K so that fragmentation does not become an issue, or make sure that all pending NetBIOS requests have completed before submitting a larger NetBIOS request.

The NetAsynchFallback= entry in SYSTEM.INI allows applications to bypass the global APITB and work on a local APITB. This forces calls originating in real mode to be synchronous, but if the call orginates in protected mode, NetAsynchFallback has no effect; it does not transform the call to synchronous. There is no method for forcing everything to be synchronous.

The NoWaitNetIO option in the [386enh] section of SYSTEM.INI can be set to translate everything from synchronous to asynchronous. If this option is set to "=OFF", which is the default, requests will not be translated. If it is set to "=ON", all synchronous requests will be translated to asynchronous requests.


Please refer to the application note WW0335 (Windows memory management) for related information.

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Last Reviewed: November 1, 1999
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