;Notes -- The Critical Error Handler (int 24h) can be freely set and
; reset by any program that wants to. DOS makes it easy to
; do so by automatically restoring the previous handler upon
; exit. Thus you don't have to worry about cleaning up the
; vector before exiting. You cannot use Int 27h to install
; a critical interrupt handler as a TSR, because DOS also
; restores the old handler with 27h just as it does with a
; normal exit. If you want to, you can change the vector that
; DOS restores to point to your own routine. Upon building the
; PSP, DOS stores the old critical error handler address at offset,
; 12h in the PSP. (More specifically, it is stored in Offset/Segment
; format, (4 bytes) beginning at offset 12h of the PSP. so.....
; if you want to change the vector that DOS restores, put your own
; handler's segment at PSP + 14h and its offset at PSP 12h.
;
; This (resetting the PSP vector) is only necessary if your program
; goes TSR, and not if it is a stand-alone program.
;
;The rest of this program deals with the more normal case, installing a
; custom critical error handler for just the currently running program.




CSEGSEGMENTPARAPUBLIC 'CODE'
ASSUME CS:CSEG,DS:CSEG,ES:CSEG,SS:CSEG
ORG100H

PROGRAMPROCNEAR

START:
MOVDX,OFFSET NEWINT24;load the address of our handler
MOVAX,2524H;into the interrupt vector table
INT21H;and execute the interrupt change

;-------------------------------------------------------------------------
; Here I put the main part of my program,
; doing whatever it is supposed to do
;
; NOTE: You do NOT have to save & reset the INT 24h
; vector at the end of the program.
; It is automatically restored by DOS
; from the PSP upon exit.
;
; NOTE: Your program should check the ERRFLAG code
; after performing any operation which can
; produce a critical error (disk, printer, or
; device request) and should take the appropriate
; action (inform the user, abort the program, or
; whatever)
;--------------------------------------------------------------------------

EXIT:MOVAX,4C00H
INT21H
PROGRAMENDP


ERRFLAGDB 0;a flag which will be incremented by
; one each time there is a
; a critical error. The main program
; must decide what to do when it
; finds this flag set.
; i.e. continue, alert user, etc.
; (and of course reset the flag to
; zero when finished)

;--------------------------------------------------------------------------
; Here is the interrupt procedure to intercept the critical error handler
;--------------------------------------------------------------------------

NEWINT24PROC FAR
ASSUMECS:NOTHING,DS:NOTHING,ES:NOTHING;don't know where they
; will be upon entry
STI;enable the interrupts which are
; automatically disabled upon entry
INCERRFLAG;set my own error flag for some later
; action by main program
XORAL,AL;tell DOS to ignore the error
IRET;send it back to where it came

NEWINT24ENDP

;--------------------------------------------------------------------------
; End of the interrupt procedure to intercept the critical error handler
;--------------------------------------------------------------------------

CSEGENDS
ENDPROGRAM