🚀 go-pugleaf

If your memory resident program takes over some interrupt, one way for it to
determine if a copy of itself has already been made resident is to examine
the handler of any interrupt that it takes over:

		; replace "XX" with your favorite interrupt number

		org	100h
begin:		jmp	discard		;Jump over resident stuff

oldXX		dd	?		;
intXX:		nop			;Everything before the label
		nop			; "discard" will become resident
		nop			;
		jmp	far ptr cs:oldXX;

matlen		equ	$-intXX		;Match length for determing residency

discard:	xor	ax,ax		;ES ->
		mov	es,ax		; Segment zero
		les	di,es:[XX*4]	;ES:DI -> Current Int XX handler
		mov	si,offset intXX	;DS:SI -> Our new Int XX handler
		mov	cx,matlen	;Number of bytes that must match
		rep	cmpsb		;Compare the two handlers
		je	AlreadyThere	;Jump if they're the same

		; Our code has not yet been installed:
		; - Save old intXX vector in 'oldXX'
		; - Point intXX at our new routine 'intXX'
		; - Do anything else that may need to be done at install time

		mov	dx,offset discard	;Figure out
		add	dx,0Fh			; how many paragraphs
		mov	cl,4			; are to remain
		shr	dx,cl			; resident, then call
		mov	ax,3100h		; terminate/stay resident.
		int	21h

AlreadyThere:	; Our code has already been installed.

		mov	ax,4C00h		;Normal
		int	21h			; temination

Bill Frolik
hp-pcd!bill
Hewlett-Packard Portable Computer Division
Corvallis, Oregon