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Filename : CBTRAP.ASM

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; Install a custom Interrupt 1b (Ctrl-Break exception) handler
;
; Public domain by Bob Stout
;
; Requires MASM 5.1 or later or equivalent
;
; Assemble with: MASM /Mx /z ...
; TASM /jMASM /mx /z ...

% .MODEL memodel,C ;Add model support via command
;line macros, e.g.
;MASM /Dmemodel=LARGE

.DATA?
_origvec dd ?

.DATA

public cbrcvd

cbrcvd dw 0

.CODE

;
; This is our actual ISR
;
myint1b:
mov ax,-1
mov cbrcvd,ax
iret

;
; Call this to install our ISR
;
ins1b PROC USES AX BX DS ES
mov ax,351bh ;get old vector...
int 21h
mov word PTR _origvec,bx
mov word PTR _origvec+2,es ;...and save it

push cs ;get myint1b segment in DS
pop ds
mov dx, OFFSET myint1b ;install myint1b in int 1bh
mov ax,251bh
int 21h
ret
ins1b ENDP

;
; Call this to uninstall our ISR
;
redo1b PROC USES AX BX DS
mov dx, word PTR _origvec ;restore original vector
mov ds, word PTR _origvec+2
mov ax,251bh
int 21h
ret
redo1b ENDP

end