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USER-GENERATED EXCEPTIONS IN GENERAL
------------------------------------
MS-DOS users can generate exceptions by means of two separate
mechanisms, Ctrl-C and Ctrl-Break. Although these are often treated
the same, they are actually handled in subtly different manners. The
difference in the way these are processed allows a great deal of
flexibility in allowing users to interrupt an executing program.
When a program is executed, the Ctrl-C Interrupt 23h is set up to
point to a default error handler. This handler is called whenever a
Ctrl-C character is detected in the keyboard input buffer. When a
program terminates in any way, MS-DOS resets the Interrupt 23h vector
to its default state. Note that the Ctrl-C character in the input
buffer is only recognized - and an Interrupt 23h generated - when
retrieving characters from the buffer and if BREAK ON is set.
The Ctrl-Break Interrupt 1Bh works somewhat differently, though
usually in concert with Interrupt 23h. Whenever the ROM BIOS detects
the Ctrl-Break key combination, the keyboard buffer is flushed and a
Ctrl-C key combination is stuffed in place of the previous contents.
This Ctrl-C will later be detected and processed by Interrupt 23h.
Ctrl-Break processing therefore offers more immediate response than
Ctrl-C processing if the default action is overridden.
Several caveats are in order here, however. First is the fact that,
unlike Interrupt 23h, MS-DOS does not restore the default state of
Interrupt 1Bh upon program termination. Second is that while Ctrl-C
processing is standardized among the various machines utilizing both
MS-DOS and PC-DOS, Ctrl-Break processing is much less standardized.
Finally, since processing either ultimately relies on trapping
Ctrl-C, either may be ignored for a long period because of the way
that Ctrl-C is detected.
HANDLING USER-GENERATED EXCEPTIONS
----------------------------------
DOS's default Ctrl-C handler is triggered whenever the Ctrl-C
character is detected in the input buffer. DOS's response is to
simply close all files which were opened using handle functions and
to terminate the program. The limitations of this approach and the
desirability of providing your own exception processing is obvious.
An equally obvious solution to the default Ctrl-C handler's problems
is to explicitly do your own Ctrl-C exception processing. CCTRAP.ASM
installs and de-installs you own customized exception handler. Note
that the code is written to accept the address of a function specified
with an explicit segment and offset.
Also note that an explicit de-installation function is provided
despite the fact that DOS restores the default Int 23h vector upon
program termination. The reason this is provided is that you should
always de-install a Ctrl-C interrupt trap before you spawn a child
process. Within your program, if you need to spawn such a process
through any mechanism other than spawning a subordinate shell (more
on this in a second), you should explicitly de-install your interrupt
handlers and re-install them when the subordinate process returns. As
noted, this is unnecessary when the subordinate process is a DOS
shell such as COMMAND.COM, since the shell will reset the interrupts
to their defaults during execution.
Ctrl-Break processing is much more problematical, though potentially
more powerful. The first problem to deal with is how to assure that
the default Int 1Bh Ctrl-Break handler will be restored upon program
termination. The de-installation function therefore becomes mandatory
in this context rather than optional as in the case of the Int 23h
handler. CBTRAP.ASM shows a sample Ctrl-Break handler. Since Ctrl-Break
processing is much less standardized than Ctrl-C processing, the
safest way to deal with it is to simply set a flag, "cbrcvd", which
informs your program that a Ctrl-Break has been received. Your
program may then poll this flag and take appropriate actions at
"safe" times within your program.
WHERE THE CARET-C COMES FROM
----------------------------
There's still nothing new here and nothing to prevent the ugly "^C"
being printed to the screen. This is because it is actually printed
by the BIOS during Int 9 processing, long before DOS ever sees it.
What this means is that even though the code in CCTRAP.ASM and
CBTRAP.ASM is fine, it still only provides a framework for solving
our problem.
TRAPFLAG.ASM is the final trick to banish the "^C". The actual ISR
has to muck around quite a bit with the keyboard hardware, as is to
be expected of an Int 09h replacement. Whenever a Ctrl-C or Ctrl-
Break is detected, it is trapped, and our exception handler called in
place of the original Int 09h handler, after discarding the trapped
key codes.
Referring to TRAPFLAG.ASM, note that since I need to trap both Ctrl-C
and Ctrl-Break, I adopt the flag approach introduced in CBTRAP.ASM
for dealing with both Ctrl-C and Ctrl-Break processing. Now, rather
than supplying an explicit exception vector, I merely set a global
flag to inform me if either exception has occurred and accept the
responsibility of processing the exceptions within the body of my
program. I've added an extra bit of versatility here by posting
different non-zero values to the flag, "ccrcvd" depending on whether
the exception was a Ctrl-C or Ctrl-Break.
TRAPDEMO.C is a short C program demonstrating the use of the combined
Ctrl-C/Ctrl-Break handler. Using this approach, your carefully
crafted screens need never more be cluttered with the "^C" uglies.
SYSTEM-GENERATED EXCEPTIONS
---------------------------
It's usually desirable, in any professional-looking program, to
explicitly trap the Int 24h critical error interrupt to process
system-generated exceptions. CERRINST.ASM is a portable critical
error handler installation program functionally equivalent to the
[_]hardxxx() package in Borland C++ and Microsoft C++ compilers, and
the ceror_xxx() package in Zortech C++.
It's obvious that writing code to intercept DOS critical error
exceptions is just as simple as intercepting Ctrl-C or Ctrl-Break
exceptions. The real challenge in writing critical error handlers is
in interpretation of the nature of the exception.
The critical error handler requires more information in order to
decide what action to take than does a Ctrl-C handler. All of this
information is passed in the CPU's registers. Just like a typical
compiler vendor's critical error handler, CERRINST.ASM will simply
pass these registers and leave their interpretation to you. In
CERRINST.ASM, the information required for intelligent critical error
processing is posted in 4 global variables, cedevdvr, cetype,
ceerror, and cereturn.
Next you need to determine what your program requires of a critical
error handler. CERRTRAP.ASM is a skeletal, yet robust critical error
function which may be called from the handler in CERRINST.ASM.
CERRTRAP.ASM assumes you have set up the following specific error
handlers in the global variables provided:
FAT error (*FAT_err)();
Disk read error (*read_err)()
Disk write error (*write_err)()
Terminal error (*term_err)(),
Printer out of paper (*no_paper)(),
All other errors (*fixup_ret)(),
In the case of an unrecognized error, fixup_ret() is called. A simple
skeleton for this function would be:
#include <dos.h> /* for _osmajor */
extern int exerr; /* DOS extended error posted by CERRTRAP.ASM */
extern int rmvbl; /* removable media flag posted by CERRTRAP.ASM */
extern int locus; /* extened error locus posted by CERRTRAP.ASM */
extern int class; /* extened error class posted by CERRTRAP.ASM */
extern int suggest; /* suggested action posted by CERRTRAP.ASM */
int fixup_ret(void)
{
if (2 < _osmajor)
{
/* analyze DOS extended error information */
return appropriate_error_code;
}
/* cleanup */
return 2; /* abort */
}
In customing your specific critical error handler functions, there
are several important restrictions to keep in mind. The first is that
no DOS system services may be requested other than Interrupt 21h
functions 01h-0Ch (character I/O), 30h (get DOS version number), and
59h (get extended error information). All registers except AL must be
preserved since DOS sets them up for processing Retry returns prior
to invoking the critical error interrupt.
Finally, the handler must return with an IRET instruction, passing a
return code in AL to tell DOS what to do next. The available codes
and their actions under various DOS versions are:
0 - Ignore
1 - Retry
2 - Abort
3 - Fail (DOS 3.3 and later)
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