On Wed, Jun 11, 1997 at 07:39:15AM -0400, Alan G. Smith wrote:
> Actually VxD's are only for Win 3.x and Win 95.  To write to hardware in NT
> you have to write a Kernel mode Physical Device Driver.

Yes and no. You do need a kernel mode driver, but this is only directly
accessible to Win32 programs that explicitly call it. If you want to
virtualize a piece of hardware, you also need a virtual device driver
for Win16 programs running under NT. What happens is that the Win16 program
does an IN or OUT instruction, NT traps it, and passes it to the virtual
device driver (which has registered itself to handle such traps for the
appropriate addresses). The VxD then makes calls to the real (kernel) device
driver to perform the I/O, and passes the result back to the 16 bit program.

This is how the serial and parallel ports are virtualized under NT - I have
a multi-port serial card in my PC under NT4.0, and I can map these very
different hardware ports via the standard serial port VxD to be accessed
from a 16 bit DOS program! The DOS program thinks it's accessing COM3 or COM4
as a standard 16450 UART, but in fact it's talking to a device driver
which in turn is talking to the processor on the intelligent multi-port
card.

All this stuff is well covered in the MSDN library CD.

Writing such a virtual device driver is not overly difficult, nor is a
simple kernel device driver for a dumb piece of hardware like the PICMaster
interface board. In fact there is a skeleton driver in the DDK that will
allow access to any I/O port you care to configure. Using this plus
a VxD would allow MPLAB to work. I toyed with doing this myself at
one time, but it seems you need Visual C++ to compile kernel
device drivers and I couldn't persuade myself to use it (I use Watcom
for casting spells under Windoze.) Microsoft used to supply a pre-compiled
version of the generic port driver, but it's only in source form on
the NT4.0 DDK so far as I can see.


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Clyde Smith-Stubbs   |HI-TECH Software,      |Voice: +61 7 3354 2411
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