Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!amdcad!ames!umd5!mimsy!oddjob!gargoyle!ihnp4!ihlpa!vkar
From: vkar@ihlpa.ATT.COM (Thayalan)
Newsgroups: comp.sys.ibm.pc
Subject: Environment Size
Message-ID: <6770@ihlpa.ATT.COM>
Date: 7 Jan 88 22:32:03 GMT
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 52


<<Maybe I'm being dense, but how does one determine the size of the
<<environmment space (ie, how much room there is for environment
<<variables) from within a program?  I can read the environment just
<<fine, I just don't know how much free space there is.
<<
<<(Assuming I ever get this answered, is anyone interested in a Turbo
<<Pascal unit to handle the environment space?)
<<
<<Kenneth Herron


Yes you can find out how much memory DOS has allocated for the environment.
There are two ways to determine this. The cleanest way is to go through
the whole environment, starting from the beginning, byte by byte until
you find two consecutive zero bytes. This way you can determine the 
number of bytes in the environment block, but remember that the memory
is allocated in terms of paragraphs (16bytes). So if you found that the
environment size was 28 bytes long then the environment block is 32 bytes
(ie. 2 paragraphs). BTW, the location of the environment can determined
by looking at the PSP ( program segment prefix ) at location 2cH: this
contains the starting segment address of the environment. 

The second approach is to look at the memory one paragraph before (below)
the environment block. This is the memory control block for the environment
block. The memory control block is created by the operating system, for
every block of memory allocated by it. And the memory control block
contains the size of the environment block. You might not want to use
this approach since this is not officially documented by MS. If you
want more information about this you can read about it in Advanced MS-DOS
by Ray Duncan (page 179).

An important point to note is that DOS will allocate only the minimum
amount of memory neccessary to hold the environment when you execute a
program. In other words, you may specify that you need 1k byte for
the environment of command.com -- you specify this in config.sys file
in the SHELL= line. Then when you invoke another program from command.com
the operating system will create an environment block for this program.
and this block will not necessarily be 1k byte long. It will only be as
big as the actual information in the  environment ( unused portions
will not be passed along). This implies that if a program needs to
modify the environment then it has to allocate memory and copy
the environment received from the parent and then modify this
area. High level-languages have library routines to do this 
 ( C compilers have the "putenv" function to do this for you).
If you program in assembly language then you'll have to create a
an assembly language procedure to do this.
The PC Magazine ( Jan 26,1988 issue) has a good article by
Ray Duncan on this issue.

K. Thayalan
ihlpa!vkar