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From: tcs@mailer.jhuapl.edu (Carl Schelin)
Newsgroups: comp.os.msdos.programmer
Subject: Abort, Retry, Fail (Summary)
Message-ID: <1991Apr19.131352.255@aplcen.apl.jhu.edu>
Date: 19 Apr 91 13:13:52 GMT
Sender: news@aplcen.apl.jhu.edu (USENET News System)
Organization: Johns Hopkins University - Applied Physics Lab
Lines: 50

A few days ago I asked for assistance in solving a problem I was having. 
The response was overwhelming. I want to thank everyone who sent me e-mail 
and especially the answer that solved the last question.

Most everyone pointed to setvect() to take over the critical error handler 
(int 24) which worked with mixed results. While it would work fine on A: 
upon bypassing it, it would choke on B: and loop until booted. If it 
encountered an unlinked network drive, it would drop to Dos after invoking 
the handler.

Since Borland C++'s Programmer's Guide (page 281) said "A function of type 
interrupt will automatically save (in addition to SI, DI, and BP) the 
registers AX through DX, ES, and DS. These same registers are restored on 
exit from the interrupt handler.", I was under the mistaken impression that 
I could do nothing to the registers since the values saved would be 
restored after the handler returned. I was wrong as Frank Whaley's code 
segment pointed out:

void interrupt
criterr(bp, di, si, ds, es, dx, cx, bx, ax)
unsigned bp, di, si, ds, es, dx, cx, bx, ax;
{
    /*  return 'fail'  */
    ax = (ax & 0xFF00) + 3;
}

Once I added "ax = ..." to the handler, everything returned and I was able 
to continue. 

As a final bit of information, here are the values returned by dx when you 
generate int 21 with AX = 0x4409:

               VDISK     = 0x0800
               BERNOULLI = 0x0802
               LOCAL     = 0x0840
               LOCAL     = 0x0842      /* for Compaq Dos v4.01 */
               NETWORK   = 0x1756
               EXTERN    = 0x4840      /* external 3.5" for example */
               SUBST     = 0x8840

Joined and Assigned drives take the characteristics of the original drive 
in this case.

I've sent this information to Ralf Brown (INTER291 interrupt list). It may 
be a correction to his list.

Once again, thanks everyone and especially Frank Whaley (few@gupta.com).

Carl Schelin
tcs@mailer.jhuapl.edu