🚀 go-pugleaf

Kevin,

I have several scanner drivers that receive communications via an arbitrary
interrupt, and I've had no problems communicating with them via Windows
enhanced mode.  I use the int86 and int86x() functions.  But there are some
gotchas:

If you need to pass a memory buffer to your driver, the Windows program
must use GlobalDosAlloc() to get some memory that your device driver can
access.  Put the data there.  GlobalDosAlloc() gives you both a selector
and a segment address for the same area in memory.

If you pass an address to your device driver using a segment register,
i.e., ES:DX or whatever, then put the SELECTOR address in ES, because
Windows will convert it to the segment address when you make the call.
Or so it appears.  (I don't believe this is documented, it just works.)
If you pass the address in one of the GP registers, best use the SEGMENT
address because I don't think it'll be touched.

If you are doing multiple indirect addressing, that is, passing your
device driver a pointer to a pointer to more memory, be certain that
your second pointer is a SEGMENT address, not the selector, since of
course your device driver will be called in real mode.

During the course of discovering these things, I created functions to
emulate int86x() that called the DPMI function which performs a real mode
interrupt.  Interestingly I found that there were occasions where my
DPMI function would not work, but a call to int86x() from the library
would.  Go figure.

As has been said other places here, DPMI is clearly not _fully_
implemented.  Just enough to tease you into using it sometimes.   :)

Mojo
Caere Corp.

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