Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!amdahl!oliveb!sun!burgundy!jborza
From: jborza%burgundy@Sun.COM (Jim_Borza)
Newsgroups: comp.sys.ibm.pc
Subject: Re: problems trying to intercept msdos function requests
Summary: Try this!
Message-ID: <89938@sun.uucp>
Date: 15 Feb 89 22:19:32 GMT
References: <3204@ttrdc.UUCP>
Sender: news@sun.uucp
Lines: 72

In article <3204@ttrdc.UUCP>, levy@ttrdc.UUCP (Daniel R. Levy) writes:
> Hello again, and thanks to all the folks who replied to my earlier request
> for info about talking PC software.
> 
> With just enough understanding of MS-DOS to be dangerous, I am now trying to
> do something screwey, that is, intercept calls to the MS-DOS function
> requests (that are made through interrupt 21h).  This is so that I can do
> something even screwier (that is, to make a UNIX-structured floppy look to
> programs running on the PC like a MS-DOS file system, as much as is possible
> given the braindamage of the MS-DOS file system).  This is a research project
> I've committed to for a class I am taking at Northwestern, so I can't back
> out of it.
> 
> My problem is that I can't even seem to make a do-nothing intercept program do
> what I want. [...]
> 
> CATCH:
> 
> ; make the real system call
> 
> 	int	88h
> 	iret
> 
> CODE	ends
> 
> STACK	segment stack
> 	assume	ss:STACK
> 	dw	64 dup(?)
> STACK	ends
> 
> 	end	START
> -- 
> Daniel R. Levy   

Mr. Gopal (an earlier respondent) was correct in looking at the FLAGS
register as a potential source of your problem.  DOS returns significant
information in the FLAGS register.  The problem occurs because an int xx
instruction pushes the FLAGS register on the stack and then pushes the
seg:offset.  When the DOS call returns from its interrupt, your CATCH
routine overwrites the FLAGS register, restoring it to the value it
had prior to the int 21h call.  You will need to "capture" the value
returned in the FLAGS register before issuing your iret at the end of
the CATCH procedure.  The value you pushed for FLAGS will, at the con-
clusion of the int 88h, be at sp-6; the value returned by DOS will be in
FLAGS.

One way around this may be to manipulate the registers so that the FLAGS
returned by DOS is on the stack at the right place (sp-6) or another way
might be to code CATCH so it doesn't monkey with FLAGS on its return. An
example:

CATCH	proc	FAR
	int	88h
	ret	2
CATCH	endp

This should return to the point of interrupt with a FAR return rather
than an iret.  The only difference is the FLAGS register.  The value
of 2 in the ret instruction tells the 8086 to bump the stack pointer
by 2 in order to ignore the FLAGS word you pushed initially.  Of course,
since this is a "do nothing" routine, it's simple.  God help us all if
you really want to "do something"!  :-o <-- (Mr. Bill)

I think the iret from DOS will re-enable interrupts but you might want
to code an "sti" following the int 88h for safety.

I haven't tried this but I think it's a good place to start.



Jim Borza - Sun Microsystems
Disclaimer?  Sure, why not?