Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!bu-cs!madd
From: madd@bu-cs.BU.EDU (Jim Frost)
Newsgroups: comp.sys.ibm.pc
Subject: Re: EXEC call (was: Re: shell script program for pc)
Message-ID: <19318@bu-cs.BU.EDU>
Date: 27 Jan 88 01:10:10 GMT
References: <2638@cup.portal.com> <5706@eecae.UUCP>
Reply-To: madd@bu-it.bu.edu (Jim Frost)
Followup-To: comp.sys.ibm.pc
Organization: Boston University Distributed Systems Group
Lines: 65

In article <5706@eecae.UUCP> driscoll@eecae.UUCP (Mike Driscoll) writes:
>in article <2638@cup.portal.com>, ROBERT_C_TELLEFSON@cup.portal.com says:

[i'm deleting a lot of stuff on whether or not command.com has
anything to do with loading .EXE programs)]

>  I am quoting from the IBM DOS 3.30 Technical Reference, page 6-199,
>which has two notes about the functioning of the exec call:
>
>  ...
>
>2. The EXEC call uses the loader portion of COMMAND.COM to perform the
>   loading.
>
>An identical note is found in the IBM DOS 3.10 Tech. Ref.
>
>  I now quote from Ray Duncan's Advanced MS-DOS, Microsoft Press, 1986,
>page 191, where the return from EXEC is discussed.  The following 
>statement is made:
>
>  The EXEC function will fail if:
>
>    [First two reasons deleted]
>
>    The transient portion of COMMAND.COM in highest RAM (which contains
>    the actual loader) has been destroyed and there is not enough
>    memory to reload it (MS-DOS 2 version only).
                          ^^^^^^^^^^^^^^^^^^^^^

My question is, does anyone know for sure whether the loader sections
moved from command.com to internal between versions 2.x and 3.x?

>2.  If I remove command.com from the root directory of my hard drive
>    and install your company's shell with the SHELL= command in 
>    config.sys, does your shell just use EXEC without supplying
>    any loader support?  Do you alter the COMSPEC environment
>    variable to point to your shell?

Here is personal experience.  I wrote a shell (psh, a pascal shell
that acts a lot like csh) under MS-DOS 3.10.  I installed it using
shell= and didn't have command.com available to MS-DOS.  Everything
ran.  I never even thought about the .EXE thing until friends told me
different things wouldn't work on their PC's.  A lot of people I know
use DOS 2.11.  I didn't think that there might be a correllation until
somewhat recently so I haven't tested the possibility of version
dependency.

As for altering the COMSPEC environment variable, sometimes this is a
good idea and sometimes it isn't.  Many programs REALLY expect you to
have DOS-style environment.  Be wary of changing COMSPEC.  You may
break other programs.  If you do change comspec, make SURE that your
new shell accepts /c (and /C).

More info on shells and compatibility with existing tools:

Many MS-DOS utilites need properly formatted FCB's, so if you write a
shell you better put in FCB building routines.

If anyone has questions on building workable shells, ask.  I'd have
made my psh public but there were a few bugs I never got completely
worked out, and redirection is really tough using Turbo Pascal 3.0.
Now that I have 4.0 though....

jim frost
madd@bu-it.bu.edu