Re: ANN: new Pulsonix version 3 PCB software released

From: Robert Wessel (robertwessel2_at_yahoo.com)
Date: 03/16/04 

Date: 15 Mar 2004 19:22:50 -0800

"Meindert Sprang" <mhsprang@NOcustomSPAMware.nl> wrote in message news:<40544519$1@news.nb.nu>...
> "Jerry Avins" <jya@ieee.org> wrote in message
> news:40541909$0$2791$61fed72c@news.rcn.com...
>
> > Windows 95 and 98 let you control the serial and parallel ports without
> > hassle. Many programs that do physical I/O won't run on NT derivatives.
>
> The serial ports are completely accessible at register level in NT and 2000.
> For the parallel port are numerous drivers available that, once installed,
> allow any program to access the parallel port at register level.

Actually they're not. For *DOS* applications, a virtual device driver
provides the appearance of direct access. In short, what happens is
the application issues an I/O instruction, when generates a protection
fault. The VDD will have registered itself as a handler for some
range of ports (for example 3f8-3ff for COM1), and it gets a message
that an application has issued a byte/word/dword in/out to port
0xABCD, along with the contents of the registers. The VDD fakes the
operation, updates the registers, returns, and the application is
(more-or-less) none the wiser.

But, as you mentioned, there are drives (for example giveio.sys),
which will allow direct access, although they're certainly not there
by default.

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