Update: the old code had a bug if the EXE was made resident using the HOT_RES keyword (thanks to Jan Bredenbeek for pointing it out), updated code below.
Today I’ve had a short discussion about creating a QDOS executable that can both be LRESPRed (to install a thing) and EXecuted (to, erm, execute it). One important detail is that LRESPR must only work in the main BASIC, as otherwise the system will crash once the daughter BASIC is terminated. There are several ways to do it, but I’d like to share mine:
base bra.s start
nop
nop
dc.w $4AFB
dc.w 4
dc.b 'Test'
start
; Trick: on an EXE call a6 must point to either a BRA ($60) or JMP ($4E).
; both have bit 6 set ($40). On Basic a6 points to the MSB of sb_buffb.
; Bit 6 will never be set as long as QLs have less than 1GB of RAM :-)
btst #6,(a6)
beq.s lrespr ; Definitely LRESPR
cmp.w #$4afb,6(a6) ; This check is somewhat optional...
beq.s job_start ; but in case QLs ever have >1GB... :)
lrespr
moveq #sms.info,d0
trap #1
tst.l d1 ; Job ID = main BASIC
beq.s basic_start
; LRESPRed from daughter SBASIC/MultiBasic
moveq #err.nimp,d0
rts
; LRESPred from main BASIC
basic_start
; ...
moveq #0,d0
rts
; EXECed as normal job
job_start
; ...