By: Dave Walker
----------------------------------------------------------------------

Here's a simple model for invoking a child process. For simplicity, it
uses the parent's PSP and environment.  This is written in TASM format
using ideal mode,  but it shouldn't be too hard to convert if desired.
There are a couple of things that might need explanation:

  - On starting an EXE program,  DS and ES point to the PSP.  In other
    words, DS:0 = ES:0 = PSP:0.

  - The environment segment is stored  at  PSP:002Ch.  If you want the
    child to inherit the parent's environment,  just fetch [PSP:002Ch]
    and store it in the first word of the parameter block.

To  invoke  a second  copy  of the command  processor (not necessarily
COMMAND.COM, ie. 4DOS, etc) try replacing the childname with something
similar to this:

dosfunc     db      '/C dir *.exe',0    ;Do DIR for EXE files

The /C option tells the command processor  to execute the  command and
return to it's caller.  By not using  the /C option,  you can invoke a
shell like so:

shell       db      'prompt SHELL$g',0  ;Invoke DOS shell

Dave 1:396/1


;;
;;  Note from Uwe E. Schirm
;;  Some remarks on that I got from Mitch Ames:
;;

Tue 8 Dec 92 11:28
By: Mitch Ames
Re: CHILD.TXT from 80XXX
------------------------------------------------------------------------

US> CHILD.TXT

US> uses the parent's PSP and environment.  This is written in TASM format
US> using ideal mode, but it shouldn't be too hard to convert if desired.
US> There are a couple of things that might need explanation:

US>   - The environment segment is stored at PSP:002Ch.  If you want the
US>     child to inherit the parent's environment, just fetch [PSP:002Ch]
US>     and store it in the first word of the parameter block.

Better still, just store (or initialize to) zero.  If the environment
block pointer is zero, DOS will use the parent's environment
automatically.

US> To invoke a second copy of the command processor (not necessarily
US> COMMAND.COM, ie. 4DOS, etc.) try replacing the childname with something
US> similar to this:

US> dosfunc     db      '/C dir *.exe',0    ;Do DIR for EXE files

This is the command tail, not the child process's name (which is
copied from COMSPEC, eg "C:\COMMAND.COM").

US> The /C option tells the command processor to execute the command and
US> return to it's caller.  By not using the /C option, you can invoke a
US> shell like so:

US> shell       db      'prompt SHELL$g',0  ;Invoke DOS shell

This doesn't make a great deal of sense.  If you wanted to invoke a
DOS shell, you'd have to invoke comspec as the child process, with a
null tail (tail db 0,cr), or with appropriate parameters (tail db
length,"/e:1024").

To change the child shell's prompt you'd have to set up a copy of the
current environment, modify "PROMPT=$p$g" (for example) to
"PROMPT=SHELL$p$g", point the parameter block to the new environment
and then call Exec.

Mitch