Microsoft KB Archive/83726

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INT 12h and Windows Enhanced Mode

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Q83726

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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
A well-known method of providing security features for MS-DOS is to install an interrupt handler into the upper area of conventional memory. INT 12h is used to inform MS-DOS that this portion of memory is no longer available for use with MS-DOS applications. For this technique to work with Windows enhanced mode, it is necessary to write a virtual device (VxD) to map the affected memory pages into virtual machines.

MORE INFORMATION
When the value in location 40:13 of the BIOS data segment is decreased, MS- DOS will reserve memory immediately below the 384 kilobyte adapter segment. The return value from issuing an INT 12h is the new amount of available memory. Several manufacturers of security software use this technique to install software prior to starting MS-DOS.

This technique fails with Windows in enhanced mode because Windows does not map the corresponding pages in virtual machines onto the physical locations of those pages. If any interrupt handlers reside in the affected area, the missing mapping will keep the handlers from being invoked properly.

The EBIOS virtual device, provided with the Windows Device Development Kit (DDK), demonstrates using the VxD services PhysIntoV86 and Assign_Device_V86_Pages to add page mappings to Windows. These techniques have been incorporated into the RIPLMEM VxD, which was originally designed to address a problem related to starting Windows on diskless workstations. RIPLMEM is distributed as an application note and is in the Software/Data Library. For more information on RIPLMEM, query on the following words in the Microsoft Knowledge Base:

"prod(win3) and riplmem" Additional query words: 3.0 boot

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