"The ultimate virus killer" assembly source code. Taken from the book "More Tricks & Tips for the Amiga"

virus-killer.asm

;VIRUS-KILLER V1.O; by SM'87
start:
	move.l 4,a6						;EXECBASE at a6
	moveq #0,d1						;Flags: no Virus here
	tst.l 46(a6)						;Test, for distorted Cool-Capture
	beq.s noSCA						;Wasn't SCA-Virus
	clr.l 46(a6)						;Cool-Capture clear
	addq.b #1,d1						;Bit 0 set
noSCA:
	cmpi.w #$FC,148(a6)					;Vertical Blank Interrupt normal?
	beq.s noVBI						;no?
	addq.b #2,d1						;Bit 1 set
	bra.s ClearTag						;KickTag-Pointer clear
noVBI:
	tst.l 550(a6)						;KickTag-Pointer changed?
	beq.s GoOn						;no?
	addq.b #4,d1						;Bit 2 set
ClearTag:
	clr.l 550(a6)						;Pointer cleared
	clr.l 554(a6)						;Pointer cleared
GoOn:
	move.b d1,Virusflag					;Flag reserved
	lea dosname,a1						;Address of Lib names
	moveq #0,d0						;Version is the same
	jsr -552(a6)						;Open Library
	move.l d0,dosbase					;Reserve Library-Base
	beq errfix						;Branch on Error
	move.l d0,a6						;Prepare for DOS call
	jsr -60(a6)						;Get Output-handle
	move.l d0,Outputhandle					;and store
	beq errfix						;Branch when error
	move.l #title,d2					;Title at d2
	move.l #titleend-title,d3				;Text-Length
	jsr writeout						;Text output
	tst.b Virusflag						;Virusflag test
	bne.s Virusfound					;Branch on Virus
	move.l #clean,d2					;Clear Message
	move.l #cleanend-clean,d3				;Length
	jsr writeout						;Text output
	bra errfix						;Program end
Virusfound:							;Virus is active
	btst #0,Virusflag					;Cool-Capture?
	beq.s notsca						;No
	move.l #scaV,d2						;Message
	move.l #scaVend-scaV,d3					;Length
	jsr writeout						;Text output
notsca:
	btst #1,Virusflag					;VB-Interrupt?
	beq.s notvbi						;No
	move.l #bbVvbi,d2					;Message
	move.l #bbVvbiend-bbVvbi,d3				;Length
	jsr writeout						;output
	bra.s bbfound						;next Message
notvbi:
	btst #2,Virusflag					;Kicktag?
	beq.s errfix						;No
	move.l #bbVtag,d2					;Message
	move.l #bbVtagend-bbVtag,d3				;Length
	jsr writeout						;output
bbfound:
	move.l #bbv,d2						;message
	move.l #bbvend-bbv,d3					;Length
	jsr writeout						;output
errfix:
	move.l 4,a6
	move.l dosbase,d0
	beq.s quit
	move.l d0,a1
	jsr -414(a6)
quit:
	moveq #0,d0
	rts
writeout:
	move.l outputhandle,d1
	jmp -48(a6)
dosname:dc.b "dos.library",0
title:	dc.b $c,$9b,"1;31;42m - Virus-Killer"
	dc.b " V1.0 - ",10,13
	dc.b "(c)  1988 by S. Maelger",10,13,10
	dc.b $9b,"0;31;40m"
titleend:align
clean:	dc.b "No symptoms for Virus"
	dc.b "-Infection found !",10,13,10
cleanend:align
scaV:	dc.b "Reset-Vector Cool-Capture has"
	dc.b " been used!",10,13
	dc.b "SCA-Virus suspected !",10,13
	dc.b "Virus in memory destroyed.",10,13,10
scaVend:align
bbVvbi:	dc.b "Vertical Blank Interrupt has"
	dc.b "been used!",10,13
bbVvbiend:align
bbV:	dc.b "Byte-Bandit-Virus suspected!",10,13
	dc.b "Virus in memory destroyed.",10,13,10
bbVend:align
bbVtag:	dc.b "KickTagPointer is no longer"
	dc.b " in operating system!",10,13
bbVtagend:align
dosbase:dc.l 0
outputhandle:dc.l 0
Virusflag:dc.b 0
	end