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Index > DOS > how to determine if .com runs under DOS

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zhak



Joined: 12 Apr 2005
Posts: 490
Location: Belarus
zhak
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?
Post 23 Feb 2010, 00:42
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17280
Location: In your JS exploiting you and your system
revolution
Perhaps you can call a harmless int 0x21 function (like get_version) and see if you get a proper response.
Post 23 Feb 2010, 08:23
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
revolution wrote:
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 Wink

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.

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug
Post 23 Feb 2010, 08:35
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17280
Location: In your JS exploiting you and your system
revolution
DOS386 wrote:
Bad idea ... "get_version" usually returns garbage Wink
Garbage is okay. At least that means something is taking notice of the function and returning a result.
Post 23 Feb 2010, 08:43
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bitshifter



Joined: 04 Dec 2007
Posts: 764
Location: Massachusetts, USA
bitshifter
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...
Post 23 Feb 2010, 08:52
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zhak



Joined: 12 Apr 2005
Posts: 490
Location: Belarus
zhak
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 )
Post 23 Feb 2010, 12:38
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nop



Joined: 01 Sep 2008
Posts: 165
Location: right here left there
nop
@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 Laughing
Post 25 Feb 2010, 04:35
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zhak



Joined: 12 Apr 2005
Posts: 490
Location: Belarus
zhak
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 )
Post 25 Feb 2010, 10:50
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