AM> Does anyone know how can I modify a Dos box through a DLL under windows
 AM> (3.1) so that all i/o to a comport (in the dos box) is sent directly to
 AM> the DLL? Do I have to use harware hooks or something? I really need help
 AM> on this. Thanx in advance.
You cannot do this from a DLL or any other ring 3 (user level) application 
code.  The only way that you can do "port trapping" is by writing a custom 
virtua; device driver (VxD).
VxD's are 32 bit code that modify the underlying operating system, the part 
known as the Virtual Machine Manager (VMM).
In order to write a VxD for Win 3.1, you will need the Device Driver Kit 
(DDK) from Microsoft.  The DDK is a commercial product that MS no longer 
distribute separately, but only as part of the MS Developer Network 
Professional or Universal Subscription.  Since the Win 3.1 DDK was sold 
separately at one time, you might find it through a second hand source.
An alternative might be to find an early edition of Theilen and Woodruff's 
book on VxDs, which included a product called the "VxD lite" on the enclosed 
disk.  Note that MS persuaded the publisher to remove this package from later 
revisions of the book.  The book itself is a rewrite of the DDK 
"documentation", with all of the implied limitations.
A good book on VxD's is Karen Hazzah's "Writing Windows Drivers and VxDs".  
The second edition is better, although it focusses more on writing for Win 
.
If your plan is to virtualize the i/o ports associated with a standard COM 
port, you will also have to deal with the fact that there is already a 
built-in VxD that does this, and handles contention issues surrounding these 
ports.
Any of the above will require you to write your driver in ASM.  The DDK 
includes an appropriate version of MASM that will work with the include files 
supplied in the DDK.  If you want to write in C (OB C comment follows), you 
can use the VToolsD library from Vireo (www.vireo.com).  Since the library 
retails for about $500, it is about the same price as the MSDN subscription.  
I recommend it to professional VxD developers because of the quality of 
support provided by Vireo.
Good Luck!
    -Ron  (ron-bass@ti.com)
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