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From: paula@bcsaic.UUCP (Paul Allen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Environment settings from a C program
Message-ID: <2434@bcsaic.UUCP>
Date: Fri, 16-Oct-87 17:14:17 EDT
Article-I.D.: bcsaic.2434
Posted: Fri Oct 16 17:14:17 1987
Date-Received: Sat, 24-Oct-87 16:16:03 EDT
References: <700@hsi.UUCP> <2400@bcsaic.UUCP>
Reply-To: paula@bcsaic.UUCP (Paul Allen)
Organization: Boeing Computer Services AI Center, Seattle
Lines: 27
Keywords: ms-dos environ putenv getenv turboc

This is an update to my recent posting (2400@bcsaic.UUCP) describing the
back-door interface to command.com.  As usual, posting from memory has
gotten me into trouble!  I was describing interrupt 2EH, *not* interrupt
1EH!  From the current BYTE "Inside the IBM PCs" special issue, page
168, comes the following data:

Undocumented interrupt 2EH causes command.com to execute a command
pointed to by ds:si.  The first byte of the command is the number of
bytes in the command.  The command must end with a carriage return, just
as if it had been typed on the keyboard.  The article is not clear on
whether the cr is included in the byte count.  So, to change something
in command.com's environment, a small model program could simply
construct the command, put its ds-relative offset in si, and do an int
2EH.  One side-effect of this technique is that any output from the
command will go to command.com's stdout (the screen).  Another poster
described a way to change command.com's environment by following the
chain of parent PSP pointers until you find command.com's.  Passing "set
foo=bar" to int 2EH seems considerably easier to me.  Int 2EH has been
described in depth in Ray Duncan's column in Dr. Dobb's Journal.  There
may also have been information in BYTE's "Best of BIX" column.  I have
used it on MSDOS 2.1.  I seem to remember that one of Ray Duncan's
articles claimed it would work on everything since version 2.0, but you
ought to check that out for yourself.

Paul Allen
paula@boeing.com
...!uw-beaver!ssc-vax!bcsaic!paula