As you may or may not know, the Thunderbyte resident av utilities hook
 themselves to the device driver chain using the following device names:
 TBDRVXXX, TBFILXXX, TBDSKXXX, TBMEMXXX, TBCHKXXX and TBLOGXXX.
 Now, by doing trial handle opens you can detect if those devices do or
 do not exist et voila, you have a method for testing residency. TBAV
 itself scans the actual device driver chain for the TB???XXX devices
 which is unlike this method, pretty much impossible to confuse, but also
 undocumented and thus it's not guaranteed to work under future versions
 of DOS! Yes, Frans Veldman calls vile and unsafe functions in his battle
 against replicating codefragments.

 Added note: Just recently I was looking at the EMM virus written by
 the author of the OneHalf family and found that it traces the device
 chain to detect thunderbyte residency. This means that this kind of
 detection isn't exactly new. Oh well, what the heck.

                .model tiny

                .code

                org 100h

start:
                mov ah, 09
                mov dx, offset startmsg
                int 21h
                mov cx,6
                mov dx, offset tbdrvxxx
detect_loop:
                mov ah,09
                int 21h
                mov ax, 3d00h
                add dx,9
                int 21h
                push dx
                mov dx, offset not_resident
                jc dont_add
                add dx, (resident-not_resident)
                mov bh,3eh
                xchg ax,bx
                int 21h
dont_add:
                mov ah, 09
                int 21h
                pop dx
                add dx,9
                loop detect_loop
                int 20h
startmsg        db 'Thunderbyte Residency Test by Rhincewind [Vlad]'
                db 0dh,0ah,0dh,0ah,'$'
tbdrvxxx        db 'TbDriver$'
                db 'TBDRVXXX',0
tbfilxxx        db 'TbFile$',0,0
                db 'TBFILXXX',0
tbdskxxx        db 'TbDisk$',0,0
                db 'TBDSKXXX',0
tbmemxxx        db 'TbMem$',0,0,0
                db 'TBMEMXXX',0
tbchkxxx        db 'TbCheck$',0
                db 'TBCHKXXX',0
tblogxxx        db 'TbLog$',0,0,0
                db 'TBLOGXXX',0
not_resident    db ' - Not Resident',0dh,0ah,'$'
resident        db ' - Resident',0dh,0ah,'$'

                end start