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title Control-Break handler for Lattice C programs
name break
include dos.mac
; Control-Break Interrupt Handler for Lattice C programs
; running on IBM PCs (and ROM BIOS compatibles)
;
; This module allows C programs running on the IBM PC
; to retain control when the user enters a Control-Break
; or Control-C. This is accomplished by taking over the
; Int 23H (MS-DOS Control-Break) and Int 1BH (IBM PC
; ROM BIOS Keyboard Driver Control-Break) interrupt
; vectors. The interrupt handler sets an internal
; flag (which must be declared STATIC INT) to TRUE within
; the C program; the C program can poll or ignore this
; flag as it wishes.
;
; The module follows the Lattice C parameter passing
; conventions, and also relies on the Lattice file DOS.MAC
; for the definition of certain constants and macros.
;
; The Int 23H Control-Break handler is a function of MS-DOS
; and is present on all MS-DOS machines, however, the Int 1BH
; handler is a function of the IBM PC ROM BIOS and will not
; necessarily be present on other machines.
;
if lprog
args equ 6 ;offset of arguments, Large models
else
args equ 4 ;offset of arguments, Small models
endif
cr equ 0dh ;ASCII carriage return
lf equ 0ah ;ASCII line feed
pseg
public capture,release ;function names for C
;
; The function CAPTURE is called by the C program to
; take over the MS-DOS and keyboard driver Control-
; Break interrupts (1BH and 23H). It is passed the
; address of a flag within the C program which is set
; to TRUE whenever a Control-Break or Control-C
; is detected. The function is used in the form:
;
; static int flag;
; capture(&flag)
;
capture proc near ;take over Control-Break
push bp ;interrupt vectors
mov bp,sp
push ds
mov ax,word ptr [bp+args]
mov cs:flag,ax ;save address of integer
mov cs:flag+2,ds ;flag variable in C program
;pick up original vector contents
mov ax,3523h ;for interrupt 23H (MS-DOS
int 21h ;Control-Break handler)
mov cs:int23,bx
mov cs:int23+2,es
mov ax,351bh ;and interrupt 1BH
int 21h ;(IBM PC ROM BIOS keyboard driver
mov cs:int1b,bx ;Control-Break interrupt handler)
mov cs:int1b+2,es
push cs ;set address of new handler
pop ds
mov dx,offset ctrlbrk
mov ax,02523H ;for interrupt 23H
int 21h
mov ax,0251bH ;and interrupt 1BH
int 21h
pop ds ;restore registers and
pop bp ;return to C program
ret
capture endp
;
; The function RELEASE is called by the C program to
; return the MS-DOS and keyboard driver Control-Break
; interrupt vectors to their original state. Int 23h is
; also automatically restored by MS-DOS upon the termination
; of a process, however, calling RELEASE allows the C
; program to restore the default action of a Control-C
; without terminating. The function is used in the form:
;
; release()
;
release proc near ;restore Control-Break interrupt
;vectors to their original state
push bp
mov bp,sp
push ds
mov dx,cs:int1b ;set interrupt 1BH
mov ds,cs:int1b+2 ;(MS-DOS Control-Break
mov ax,251bh ;interrupt handler)
int 21h
mov dx,cs:int23 ;set interrupt 23H
mov ds,cs:int23+2 ;(IBM PC ROM BIOS keyboard driver
mov ax,2523h ;Control-Break interrupt handler)
int 21h
pop ds ;restore registers and
pop bp ;return to C program
ret
release endp
;
; This is the actual interrupt handler which is called by
; the ROM BIOS keyboard driver or by MS-DOS when a Control-C
; or Control-Break is detected. Since the interrupt handler
; may be called asynchronously by the keyboard driver, it
; is severely restricted in what it may do without crashing
; the system (e.g. no calls on DOS allowed). In this
; version, it simply sets a flag within the C program to
; TRUE to indicate that a Control-C or Control-Break has
; been detected; the address of this flag was passed
; by the C program during the call to the CAPTURE function.
;
ctrlbrk proc far ;Control-Break interrupt handler
push bx ;save affected registers
push ds
mov bx,cs:flag ;set flag within C program
mov ds,cs:flag+2 ;to "True"
mov word ptr ds:[bx],-1
pop ds ;restore registers and exit
pop bx
iret
ctrlbrk endp
flag dw 0,0 ;long address of C program's
;Control-Break detected flag
int23 dw 0,0 ;original contents of MS-DOS
;Control-Break Interrupt 23H
;vector
int1b dw 0,0 ;original contents of ROM BIOS
;keyboard driver Control-Break
;Interrupt 1BH vector
endps
end