hello guys!
I'm building a .com program that can be executed either under DOS, or not (i.e. loaded by a boot sector on a floppy). how should i determine if the program was started under DOS? I've implemented a PSP word 0 check (if it contains 0xcd, 0x20 then we're under DOS), but i'm not sure if this is enough. Maybe something else should be checked to be 100% sure?

Perhaps you can call a harmless int 0x21 function (like get_version) and see if you get a proper response.

Bad idea ... "get_version" usually returns garbage

But you can peek the date http://www.ctyme.com/intr/rb-2686.htm after confirming that INT $21 target is non-ZERO. Also you can check other values inside the PSP.

Garbage is okay. At least that means something is taking notice of the function and returning a result.

How about passing a special value in a register (from booter).
Use a register that DOS normally sets to zero, and make it nonzero.
Now program can check for special value and determine who it is...

hm... interesting idea about setting a register. i.e. some magic number can be passed in ax, or in a pair of registers (ax:cx, for example) to double-check and exclude possible collisions


OFFTOPIC: DOS386's signature always makes me smile )) I'm a QA engineer and I hate high-level languages, so this "bug" is double-funny )

@zhak offtopic
hehe i hate hlls too and even 100kb is tiny by todays bloatware standards! maybe dos386 should change his sig to 1Mb or 10Mb or even 100Mb

yes, nobody takes care of software package size today. i laughed very loud some time ago when i loaded intel's web site and saw a banner stating that new Core processors are COMPATIBLE WITH VISTA!!! hahaha its now OS should be supported by hardware, not vice versa.

but, on the other hand, this possibly doesn't allow engineers to relax and they keep inventing more powerful hardware.

anyway, sometimes i wish it were late 80's - early 90's when i write system code... things used to be a lot more easier that days )