The MS-DOS version of elvis uses Thomas Wagner's do_exec() function to swap
itself out while running external programs.  His original distribution is
available at...

    ftp://ftp.demon.co.uk/pub/mirrors/simtelnet/msdos/pgmutil/exec33b.zip

In elvis, the following files are derived from his work:

    osmsdos/doexec.h	This is a copy of his "exec.h" file.

    osmsdos/doexec.lib	This is a library containing the spawncl.obj and
			checkpcl.obj files from his distribution, plus an
			exec.obj file compiled from his exec.c source file.

The following is the "readme.doc" file from his original distribution.
-------------------------------------------------------------------------------

                  An EXEC function with memory swap
                   Version 3.3b, released 93-11-29

                      Public Domain Software by
                            Thomas Wagner
                       Ferrari electronic GmbH
                  (NOTE: Address change, see below)


This archive contains the sources for an 'EXEC' function that allows
calling external programs, while optionally swapping out the memory
image to EMS, XMS, or file. When swapping out, only a few K of main
memory remain resident. The code and data stub is about 1k, the actual
memory usage depends on memory fragmentation, and especially on the
size of the environment variables. The resident memory usage
typically ranges from 2K to 7K.

The Routines are compatible with 
   Turbo C (Versions 1.x, 2.x, and C++ 1.0)
   Borland C++ (Version 2.0 and above),
   Microsoft C (Versions 5.1 and above), 
   Watcom C (Version 8.0), 
   Turbo Pascal (Versions 4.x to 6.x).

EMS (LIM 3.0 or above) or XMS is used automatically if there is
enough space left, otherwise a temporary file is created. If the
"TEMP=" or "TMP=" environment variable is present, the temporary file
is created in the directory specified by this variable, otherwise it
is created in the current directory.

For detailed usage and parameter information, see the file "exec.h" (C) 
or "exec.pas" (Pascal).

The general format is

   retcode = do_exec (filename to execute,
                      program parameter and redirection string,
                      spawn options,
                      memory needed (0xffff to always swap, 0 to never swap),
                      environment pointer/flag)

for example:

   rc = do_exec ("cl", "-c -Od exec.c >&errout", USE_ALL, 0xffff, NULL);

or, for Pascal:

   rc := do_exec ('tpc', '/$D+ exec >&errout', USE_ALL, $ffff, false);

Redirection for standard input, standard output, and standard error
is optionally handled by parsing the command parameter string for the
standard redirection combinations:

   stdin:   <file
   stdout:  >file    or >>file   to append
   stderr:  >&file   or >&>file  to append

Redirection is supported by default, to disable it you must change
the define in both spawn.asm and exec.c/exec.pas.

If the command to be executed is a BAT file, the command processor
will be invoked automatically. The command processor will also be
invoked if the command is empty. The COMSPEC environment variable is
used to locate the command processor, any parameters present on the
COMSPEC line are inserted into the parameter string.

An example:

   Given   COMSPEC=C:\DOS\COMMAND.COM /E:960
           PATH=C:\DOS;C:\CMD
           File B.BAT resides in C:\CMD
           do_exec is called with ('b', 'one two >out', ...)

   Then the command executed is
           C:\DOS\COMMAND.COM
   with the parameter string
           /E:720 /C C:\CMD\B.BAT one two
   and standard output redirected to file 'out'.



                        CONTENTS
                        ========

This archive contains the following files:

    README.DOC      This file
    LIESMICH.DOC    German version of this file

    GETLANG.EXE     A helper program to extract a single-language
                    version from the dual-language source. All C and
                    Assembler sources (Pascal sources only partially)
                    are commented in both English and German,
                    which makes the code hard to read. For easier
                    reading, you can use GETLANG to eliminate one
                    of the languages.

         Usage:   GETLANG language compiler <infile >outfile
            Where    language is 'E' for English or 'D' for German
                     compiler is 'C' for C files, 'A' for Assembler,
                     or 'P' for Pascal.

         Samples: GETLANG e a <spawn.asm >spawne.asm
                  GETLANG e c <extest.c >exteste.c

    DEUTSCH.BAT     Batch-File to execute GETLANG for all source
                    files, German version
    ENGLISH.BAT     Batch-File to execute GETLANG for all source
                    files, English version

    SPAWN.ASM       The main swap/exec function

        This file is common to the C and Pascal versions.
        It must be assembled with Turbo-Assembler for use with
        Pascal. The C version can be assembled with TASM (specify 
        /JMASM51 on the command line) or MASM 5.1.

        To assemble:
            tasm /DPASCAL spawn,spawnp;     For Turbo Pascal, near calls
            tasm /DPASCAL /DFARCALL spawn,spawnp;  
                                            For Turbo Pascal, far calls
            ?asm spawn;                     For C (default small model)
            ?asm /DMODL=xxx spawn;          For C (model 'xxx')
         Example:
            masm /DMODL=large spawn;            Large model C
            tasm /DMODL=medium /JMASM51 spawn;  Medium model C

    SPAWNP.OBJ      SPAWN assembled for use with Pascal, near calls
    SPAWNCS.OBJ     SPAWN assembled for use with C (small model)
    SPAWNCL.OBJ     SPAWN assembled for use with C (large model)
        
        The C files have been assembled with the /MX switch
        for case-sensitive external linking.

        Note for Turbo Pascal: You can use the near call version of
        SPAWN even when compiling with "force far calls" by enclosing
        the external definitions of do_spawn and prep_swap in file
        exec.pas with {$F-} and {$F+}.
        To avoid confusion when generating multiple language
        versions, the Pascal OBJ-File was named "spawnp.obj".

    CHECKPAT.ASM   Utility function to check and resolve a path 

        This file is common to the C and Pascal versions.
        It must be assembled with Turbo-Assembler for use with
        Pascal. The C version can be assembled with TASM (specify 
        /JMASM51 on the command line) or MASM 5.1.

        To assemble:
            tasm /DPASCAL checkpat,checkpap;  For Turbo Pascal, near calls
            tasm /DPASCAL /DFARCALL checkpat,checkpap;  
                                              For Turbo Pascal, far calls
            ?asm checkpat;                    For C (default small model)
            ?asm /DMODL=xxx checkpat;         For C (model 'xxx')
         Example:
            masm /DMODL=large checkpat;            Large model C
            tasm /DMODL=medium /JMASM51 checkpat;  Medium model C

    CHECKPAP.OBJ    CHECKPAT assembled for use with Pascal, far calls
    CHECKPCS.OBJ    CHECKPAT assembled for use with C (small model)
    CHECKPCL.OBJ    CHECKPAT assembled for use with C (large model)
    CHECKPAT.PAS    Wrapper unit for checkpat (Pascal only)   

        The C files have been assembled with the /MX switch
        for case-sensitive external linking.
        The Pascal version must be assembled with the FARCALL switch
        when used with the CHECKPAT.PAS unit. At least Turbo Pascal
        version 5.5 seems to automatically generate a far call if an
        external routine is defined in the interface part of the unit.

    EXEC.PAS        Interface routines and documentation for Turbo Pascal
    EXEC.C          Interface routines for C
    EXEC.H          Interface definitions and documentation for C
    COMPAT.H        MS-C/TC Compatibility definitions for C

        These files prepare the parameters for the main spawn
        function, and handle the file search and environment 
        processing.

    EXTEST.C        C Test program for EXEC
    EXTEST.PAS      Turbo Pascal Test program for EXEC

        The EXTEST program tests the functionality of the do_exec
        function. It expects you to input a DOS-command and its
        parameters, separated by a comma. Entering an empty line
        will spawn a copy of COMMAND.COM without parameters.

   MAKEPAS          Make-file for Turbo Pascal (Borland Make) 
   MAKETC           Make-file for Borland C++ (Borland Make) 
   MAKEMS           Make-file for Microsoft C (MS NMAKE) 


The Turbo Pascal version of EXEC.PAS includes replacement functions
for the environment access functions 'envcount', 'envstr', and
'getenv', plus an additional function, 'putenv'. This function allows
you to add strings to the environment for the spawned process. The
definition is

        procedure putenv (envstr: string);

with 'envstr' containing a string of the form 'ENVVAR=value'. The '='
is required. To delete an environment string, use 'ENVVAR='. Please
use the environment functions from the EXEC unit only, do not mix them
with calls to the DOS unit functions.


                        SUPPORT
                        =======

This software is in the Public Domain. This means that there is no
restriction whatsoever on private or commercial use. No registration
fees have to be paid, and no licenses are necessary for use. It also
means that the author can not be held liable for any damages caused
by the use of this software. You have the source, please check it out
before use.

I will try my best to eliminate any bugs reported to me, and to 
incorporate suggested enhancements and changes. However, my spare 
time is limited, so I can not guarantee continued or individual 
support. Please address all reports or questions to my business 
address: 

        Ferrari electronic GmbH
        attn: Thomas Wagner
        Ruhlsdorfer Strasse 138
        D-14513 Teltow
        Germany

        Phone: (+49 3328) 474 626
        Fax:   (+49 3328) 438 04-0
        BBS:   (+49 3328) 438 04-8

        Internet:    twagner@bix.com
        BIX:         twagner
        Compuserve:  100023,2042

But, please, if at all possible, do it in writing. Please do not 
phone unless it is absolutely vital (or you have a business 
proposal). I like to hear about any applications for EXEC, and 
if you are visiting Berlin, I also invite you to drop by for a 
talk. But I am usually not that happy when I am interrupted in my 
paid work by a phone call requesting support for a free product. 

I will try to answer all letters and Faxes I receive. However, I am
usually not the fastest in this respect, so please be patient. If you
don't hear for me for a while, send me a short reminder. The
preferred, and the fastest, method to reach me is through our BBS,
and through BIX, where I daily check my mailbox. Send mail to
'twagner' from BIX, or to 'twagner@bix.com" through the Internet. The
second best way is CompuServe e-mail, which I usually check several
times, but at least once, a week.

BIX (tm) is an electronic conferencing system. BIX can be (and is)
accessed from all parts of the world. Although accessing BIX from
outside the US isn't exactly cheap (don't ask me what I have to pay
each month), the wealth of information available there, and the fast
and extensive help the other members can give you on all kinds of 
hard- and software problems, makes it worth every Mark, Peseta, 
Franc, or Ruble you have to spend. New versions and updates of EXEC
will first appear on BIX. 

To get more info on joining BIX, call the BIX Customer Service at 
800-695-4775 (U.S.), or 617-354-4137 (elsewhere) from 12:00 to 23:00
EDT (-4 GMT). BIX access currently is $39 for three months (flat
fee), plus the applicable telecomm charges (Tymnet in the U.S. and
Canada, your local PTT's Packet Net charges from outside the U.S.).
International users living near a BT Tymnet node can access BIX
through international Tymnet at special low rates. Call the BIX
helpline for Tymnet access points and charges. Other international
users will need an account (NUI) with their local packet net. Please
enquire at your post/telecomm office for details.

Our BBS is available 24 hours, 7 days a week, and supports up to
14.400 bps, with V.42bis and MNP5 error correction and compression.


                        RESTRICTIONS
                        ============

The "no return" mode of EXEC is included for completeness only. It
has some disadvantages over the standard compiler functions for
executing without return. In particular, files are not closed, and
interrupt vectors used by the Run-Time-Library are not restored to
the DOS defaults. If possible, use the standard RTL functions for
this mode.

The assembler module "spawn" must not be the first module linked.
Either put it into a library, or specify spawn.obj as one of the last
objects in the link command or the Turbo project file. The spawn
module will overwrite about 1k at the start of the program image.
Although the contents of this area will be saved, they may not
contain parts of the spawn module itself, since this could destroy
the code being executed. The do_exec function will check for this
condition, and return an error code if the program code could be in
danger.

The calling program may not have interrupt handlers installed when
calling the do_exec function. This includes handlers for Control C
and critical errors. If you want to handle interrupts while the
program is swapped, you will have to modify the spawn module such
that the interrupt handlers are included in the resident part.

All open files will stay open during the EXEC call. This reduces the
number of handles available to the child process. The "C_FILE_INFO"
environment variable created by some C compilers on the standard C
spawn call is not supported. If the NO_INHERIT flag is set in
spawn.asm (default), all open files except for the first five
standard handles will be "hidden" from the child, and thus will not
be inherited. This increases the possible number of open files for
the child, although the system-wide open file limit (the config.sys
FILES= value) still must be high enough to support all open files.

Internal commands are not automatically processed. You can execute 
those by loading the command interpreter (by passing an empty string 
as the command name). For example:

(C)     retcode = do_exec ("dir", "*.*", USE_ALL, 0xffff, environ);
        if (retcode == RC_NOFILE)
           retcode = do_exec ("", "/c dir *.*", USE_ALL, 0xffff, environ);

(P)     retcode := do_exec ('dir', '*.*', USE_ALL, $ffff, true);
        if (retcode = RC_NOFILE)
           retcode := do_exec ('', '/c dir *.*', USE_ALL, $ffff, true);



                        CAUTIONS
                        ========

The functions should be compatible with DOS versions down to DOS
2.21, but they have been tested under DOS 3.3, DOS 4.0, DOS 5.0, 
DOS 6.0, and DR-DOS 5.0 only.

Compiler compatibility has been tested for Borland C++ 2.0/3.1,
Microsoft C 6.0a, Visual C++ 1.0, and Turbo Pascal 5.5 only. I do not
have access to other compilers. Turbo Pascal 6.0 was reported to work
fine with EXEC. The DOS-extender mode of Turbo Pascal 7.0 (and any
other DOS extender) will not work with EXEC.

Spawning a command that exits and stays resident (TSR), like PRINT or
Sidekick, will fragment memory and prevent a return to the calling
program. This should, however, not crash the system. Allocated EMS
or XMS pages are released, a swap file is deleted.

If the program memory image is not contiguous, the swapping code has
to use undocumented DOS-internals. In particular, the swapper has to
modify DOS memory control blocks directly. In theory, this could lead
to incompatibilities with later versions of DOS, or with DOS
substitutes or emulators. In practice, no problems have been reported
with any DOS version, including DOS 6.0 and the DR-DOS versions.

If the NO_INHERIT flag is set to TRUE in spawn.asm, some undocumented
fields in the PSP are used and modified. This should work with all
DOS versions and clones, but you can set NO_INHERIT to FALSE if you
are afraid of potential problems (but not if you use handle-table
expansion).


                        Revision History
                        ================

Changes for Version 3.3a to 3.3b:

Under certain circumstances (the first MCB of a program was the PSP,
not the environment, and unallocated blocks followed), the restored
memory image was incorrect. This bug was reported and fixed by
Giorgio Angelotti.

Changes for version 3.3 to 3.3a:

Besides the address change, the only significant modification is a
better handling of redirection. Like with DOS, redirection parameters
may now be interspersed with other command line arguments.

The Pascal version was buggy in the handling of multiple redirection
and COMSPEC parameters. This was fixed.


Changes for version 3.2a to 3.3:

A new option for Turbo Pascal recognizes the free heap space and
suppresses swapping for this area. For many applications this will
speed up the swapping process, and reduce swap space requirements.
Since this is version dependent (Turbo Pascal version 6 uses a
different heap management scheme than its predecessors), the
preassembled version has this feature disabled. Please set PAS_FREE
in SPAWN.ASM to TRUE, and set TPAS_6 to TRUE or FALSE depending on
your version of Turbo Pascal to use it.

The Pascal "putenv" function was buggy. When putting a variable
already in the environment, it produced a duplicate entry instead of
replacing it. Bug reported and fixed by A. Bailey.

When determining program name and path, a program with a base name
(without extension) equal to the name of a directory was not found
under certain circumstances. The checkpath function was modified to
correctly handle this case. Bug reported by H. Lembke.

An extended handle table would cause the swap-back to fail on a
file-swap if NO_INHERIT was true. This was due to the restoration of
the handle table pointers before allocating and restoring the MCB the
handle table was in. The handle table pointer is now restored after
swapping everything back. Bug reported by H. Lembke.

EXEC would fail completely with an extended handle table if
NO_INHERIT was false. The MCB containing the handle table now is not
swapped, which will likely fragment memory, so setting NO_INHERIT to
false when extending the handle table is not recommended. 

The C do_exec function now correctly handles NULL pointers to the
command line and parameter strings.


Changes for version 3.2 to 3.2a:

A bug in checkpat.asm that caused incomplete path specs was fixed.

A bug in spawn.asm that caused redirected files to stay open even
after program termination was fixed.

Changes for version 3.1 to 3.2:

The call of a user-defineable function (through a function pointer or
a procedure variable) from do_exec immediately before executing the
external command was added. This function can output messages and
perform additional checks. The previously internal structure
containing the swap parameters was made global for access by this
function. For details see exec.h/exec.pas, and the example in
extest.c/extest.pas.

A bug in checkpat.asm that led to erroneous operation of the 'exists'
function when used with Turbo Pascal was fixed.

The Pascal version of extest was (finally) brought up to date, and
now corresponds closely to the C version. A sample for the use of the
user-defined call-back function was added in both versions.

The definition of the internal routines in exec.c was changed to
"static", initialisation of some variables in do_exec was corrected.


Changes for version 3.0a to 3.1:

The capability to process BAT files and to handle redirection was
added.  The program search order now exactly matches the DOS order
with the exception of internal commands (there is no safe way to
determine whether a command is internal or external). Redirection is
optional.  The interface to do_exec did not change (redirection is
handled by parsing the parameter string), but do_spawn takes three
new parameters if redirection is enabled.

A routine was added (checkpat.asm) that checks and resolves a path
name in addition to splitting it. This routine does some thorough
checks on the supplied path and filename, and will handle critical
errors (invalid drive, drive not ready) without user intervention.
This routine is used to process the program file name, the command
processor filename, and the temporary path. This routine is not
dependent on the other EXEC/SPAWN routines, and thus might be
useful for other applications.

Several new error codes allow better analysis of error conditions.

The temporary file path will now always be a complete path, so that a
change of the default drive and directory while spawned can no longer
cause the swap file to get lost.

Any parameters on the COMSPEC= environment string will be inserted
into the command parameters when calling the command processor. This
allows specification of environment size and other parameters when
spawning.

The check for file existance was moved to checkpat.asm, and changed
from a 'find first' operation to 'get file attributes'. This seems to
be marginally faster, and avoids compiler dependencies.

The GETLANG program was corrected to use stderr to output the usage
information.


Changes for version 3.0 to 3.0a:

A minor bug in EXEC.C was fixed: a '<' was missing in a German
comment, so that GETLANG E would gobble up a large part of the file.

A problem (feature? bug?) with the Turbo C/Borland C "stat" function
always returning directories with a "read-only" attribute prevented
the "TEMP" directory from being used. The Write permission for the
directory is no longer checked.

The preallocation of the swap file introduced in Version 3.0 to make
sure that the disk can hold the swap file causes a severe slowdown
when the swap drive is a Novell Network drive. Two new "method" flags
were introduced to circumvent this problem:

      NO_PREALLOC - never preallocate
      CHECK_NET   - check for network drive, don't preallocate if net

If the file is not preallocated, the disk is not checked for
sufficient space at all. Using the "get disk free space" call usually
takes even longer than preallocation. This is not a big problem
though, swapping simply will fail with error code 0x502 when the disk
is full on swapping out.

Thanks to Tim Frost ('<' bug) and Tron Hvaring (stat and Novell
problems) for their reports.


Changes for version 3.0:

This is a major rewrite of the code in spawn.asm. The interface for
do_exec and do_spawn has changed, the exec and extest files have been
modified accordingly.

The main enhancement is support for XMS.

The code in spawn.c was modularized a tad, and a lot of comments were
added. This should help in better understanding the code. Old bugs
(versions 2.4 and 2.5 were buggy in handling non-swapping spawns)
were eliminated.

The allocation of swap space has been separated from the actual
swapping. Although not used in the current do_exec routine, this
could be used to issue an informative message about where the
swapping will go to, and to try other paths for the temporary file if
allocation fails. You might elect to add support for this in the
do_exec routine, just be careful not to modify the memory layout
between the calls to prep_swap and do_swap.

The MCB blocks are no longer modified in preparation of the swap.
Instead of using an internal chain with undocumented fields in the
MCB, the MCB chain is now parsed multiple times. Since the chain
normally consists of just a few elements, this will not severely
impact execution performance. MCBs are now swapped out and restored
in their original form, including the "unused" fields that are not
really unused in DOS 4.0 and later versions.

Saving fragmented memory will now use less EMS space, since the
blocks are packed tight. Previous versions started a new page for
every MCB.

Memory blocks located "below" the PSP (including the environment
block, unless it is needed for an environment copy) will now be
swapped, too. This may increase available memory if DOS memory is
fragmented.


Things not done in this release:

The MCB chain still is modified directly. Although some users have
suggested trying to call DOS to allocate the blocks, this has proven
impractical in some tests. If memory is fragmented, getting DOS to
allocate a block at an exact location is rather tedious. I will keep
the suggestion in mind, but the current method works reliably with
all known (and unknown) versions of DOS and its clones.

Another good suggestion not implemented in this version is the save
and restore of the interrupt vector table to remove TSRs installed
while the program was swapped out. It would be rather complex to find
and release all memory blocks belonging to the TSR, and without
releasing the TSRs memory, the program can not be swapped back in
anyway, so restoring the interrupt table wouldn't help.