tcs@mailer.jhuapl.edu (Carl Schelin) (04/19/91)
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.eduRalf.Brown@B.GP.CS.CMU.EDU (04/20/91)
In article <1991Apr19.131352.255@aplcen.apl.jhu.edu>, tcs@mailer.jhuapl.edu (Carl Schelin) wrote: }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. I did a little further digging, and the interrupt list now reads: INT 21 - DOS 3.1+ - IOCTL - CHECK IF BLOCK DEVICE REMOTE AX = 4409h BL = drive number (00h = default, 01h = A:, etc) Return: CF clear if successful DX = device attribute word bit 15: drive is SUBSTituted bit 12: drive is remote bit 9: direct I/O not allowed CF set on error AX = error code (01h,0Fh) (see AH=59h) Note: on local drives, DX bits not listed above are the attribute word from the device driver header (see AH=52h); for remote drives, the other bits appear to be undefined SeeAlso: AX=4400h,AX=4408h,AX=440Ah,INT 2F/AX=122Bh -- {backbone}!cs.cmu.edu!ralf ARPA: RALF@CS.CMU.EDU FIDO: Ralf Brown 1:129/3.1 BITnet: RALF%CS.CMU.EDU@CMUCCVMA AT&Tnet: (412)268-3053 (school) FAX: ask DISCLAIMER? Did | It isn't what we don't know that gives us trouble, it's I claim something?| what we know that ain't so. --Will Rogers