Actually I did glance at the MS-DOS 2.0 code already, and my
understanding is that they didn't even try addressing this, just relied
on COMSPEC (if defined, either through SET or by feeding it as an
argument to command.com
), and if no COMSPEC then using the default
That sounds okay when it's MS that does it, because it's their OS, but
an alternative shell does not have this luxury.
> HACK: when 4DOS.COM
is the shell and it doesn't have an environment
> from which to obtain its original program name, it grovels through
> all of memory to find the filename that was used to exec it; it
> wants to find the SHELL= line in the in-memory copy of CONFIG.SYS,
> and it knows that sysinit converts the SHELL= keyword to an 'S', so
> it expects to find an 'S' immediately before the filename, but since
> we are now storing line # info in the config.sys memory image, 4DOS
> fails to find the 'S' in the right spot.
> So, on the final pass of CONFIG.SYS, copy the command code (eg, 'S')
> over the line number info, since we no longer need that info anyway.
> This relies on the fact that getchr leaves ES:SI pointing to the
> last byte retrieved.
That is very interesting indeed, thanks for sharing! This at least
confirms that there is no "clean" way to do what I wanted... So I will
probably give up on that. MS compatibility was only a bonus, my primary
target is working with the DOS-C kernel, and there I do not have any
Nonetheless, thank you for your kind research.
> As for other flavors of DOS, most of them are open source, so I think
> it would be worth it to download some source code and search for
Under DOS-C it is a no-brainer: the kernel actually allocates a tiny
(96 bytes) environment and passes it to COMMAND.COM
. There, the entire
path is present.