IOCtl Calls in Protected-Mode Microsoft Windows

• Microsoft Windows Device Driver Kit (DDK) for Windows versions 3.0 3.1

Generally, Microsoft Windows translates standard MS-DOS IOCtl calls in the two protected modes (standard mode and enhanced mode). Therefore, in most cases an application can make IOCtl calls regardless of the mode in which Windows is running. However, if the buffer passed to the IOCtl call contains pointers to additional data, the calling application must translate those pointers and the data referenced by those pointers into a form that can be used in real mode, before making the IOCtl call.

MS-DOS functions are executed only in real mode, and thus can only access data that is located in the first 1 MB of memory. To work within this limitation, the memory referenced by the pointers contained in the buffer must be allocated below 1 MB. In addition, the buffer must contain the segment address of the data, rather than the protected-mode selector.

For example, consider IOCtl call Write Control Data to Character Device Driver, Interrupt 21h Function 44h Subfunction 03h. The parameters to this call are as follows:

   AH = 44H
   AL = 03H
   BX = handle
   CX = number of bytes to write
   DS:DX = segment:offset address of data buffer

If an MS-DOS device driver defines a special interface that requires pointers to additional data be passed in the main data buffer, the caller must ensure the pointers passed in the data buffer can be used in real mode. The caller should allocate the memory referenced by these pointers using the GlobalDOSAlloc() function.

GlobalDOSAlloc() returns a double-word. If the allocation is successful, the high-order word contains the segment:paragraph address of the allocated buffer, while the lower-order word contains the selector to the buffer. The selector is used to access the buffer in protected mode; the segment:paragraph address can be used in real mode. Use the GlobalDOSFree() function to free memory allocated by GlobalDOSAlloc().