lotto@talcott.UUCP (Jerry Lotto) (11/23/85)
We are doomed to repeat this over and over as more people find
out how useful environments can be. I hope to add something new
to the conversation as well.
In MS-DOS 3.1, COMMAND.COM can be invoked with command line arguments
by using the SHELL feature in CONFIG.SYS. As most people know already,
the syntax is SHELL=d:\path\COMMAND.COM /P d:\path /otherswitches.
The /P switch tells COMMAND that this is a "P"ermanent copy and is
the inverse of the "C"ommand switch which tells it to execute a command
(also on the line) and then exit.
The installation code for COMMAND (starting at CB0 under debug) reveals
some interesting features. After arranging a few interrupt vectors and
determining the location to load things, COMMAND quickly determines
what size the environment is to be. Around D04 to D16, it figures
out the amount of available memory, the maximum size of the environment
3E (62) paragraphs, and the default size 0A (10) paragraphs. Changing
this 0A will result in a larger default, up to the 3E limit. If COMMAND
has not inherited an environment from somewhere else, (determined from
the segment style address at offset 2C in the PSP), it points to an
internal copy at C10. The string PATH=, nul, COMSPEC=/COMMAND.COM is
already in place. The / is NOT a typo, this IS what is there. Then,
COMMAND gets the switch character by well known means (reserved function
37) and sees if it is a /. If so, the environment pathchar is changed at
a hard coded offset of 0E from the start. Sound like anticipation of
being called from Xenix?
After figuring out what drive all this is coming from and doing the
appropriate bookeeping, COMMAND then finishes reading off the command
line. Possible switches are:
F - looks like "forever". All I can figure out is that it causes
DOS to ask Abort, Retry, Ignore. Without this switch, DOS does a
function 51 (you guessed it, reserved) and then an IRET. Maybe DOS
execs a copy of COMMAND as an error handler for some situations.
D - Appears similar to C but saves less information about the current
process.
E - must be followed by a : and a decimal number not exceeding the limit
set around D04. All COMMAND does is replace the memory location to
which the 0A was moved with your number. Supported or not, it is a
much cleaner method of expansion (up to 1K) than a direct patch.
Further arguments are treated as various device/file names and COMMAND
attempts to create handles to them.
The teaberry shuffle with the switchar is interesting to say the least,
and without taking to much more of your time, let me say that further
inspection reveals similar doings again and again.
I doubt that the F and D switches will end up being as useful as E, but
if anyone takes the time to investigate further, I am always interested
in the results.
--
Gerald Lotto - Harvard Chemistry Dept.
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