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From:	Andrew Cottrell <andspam DOT at DOT swiftdsl DOT dot DOT com DOT dot DOT au AT unknown DOT addr>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: error 4 with write to serial port
Date:	Fri, 23 Jan 2004 19:18:33 +1100
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>I have built a DOS program with DJGPP to access a serial port.
>This program works fine under plain DOS (ic IBM PC/DOS 6.3).
>However when I boot from a bootable diskette created with
>WinXP and start the program it returns error 4 with a write()
>to the serial port.
You don't mention the comms method you used. 

be aware that under XP using a 16 bit app you cannot access the
hardware, but with DJGPP beign 32 bit DPMI you can and do have access
to the hardware under XP. This is not to say that when performing
int21 or other intxx calls that they work correctly due to XP puting
in hooks and redirecting the call!!!!!

>What could be the reason? Any solution/circumvention??
Reasons:-
1) You may be using BIOS calls and as such these get redirected under
XP.

Solution:-
1) Use a comms library that directly writes to the UART (and uses the
FIFO).

There are a number of comms libraries in the V2TK directory with
varying features, so have a look a good look at your requirements
before you start.

I have data sniffer that I can send to you with full source code
(relased under GPL by me) that works under XP if you need to see how
it can be done.

Andrew

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