mySE30_driver_stub.a

; 
; This is what I call a "stub driver" and will be used by the 
; never-been-used 0xA0 video mode
;

                dc.w 0                  ; no Ctrl, no status and no lock
                dc.w 0
                dc.w 0
                dc.w 0
                dc.w 0                  ; Open routine right at the beginning
                dc.w 0                  ; no prime

; ---------------------------------------------------------------------------

                move.l  #$F0,d1
                or.b    7(sp),d1
                ror.l   #8,d1
                movea.l d1,a1
                move.l  8(sp),d0
                cmpi.l  #3,d0
                bgt.s   VIA_exitErr
                beq.s   VIA_toggleMainAltScr
                subq.w  #1,d0
                beq.s   VIA_setMainScr
                bmi.s   VIA_setMainScrNoVsync
                bclr    #6,($50F00000).l

VIA_exitGood:                           
                                        
                moveq   #0,d0

VIA_exit:                              
                rts
; ---------------------------------------------------------------------------

VIA_exitErr:                            
                moveq   #$FFFFFF9C,d0   ; return -100
                bra.s   VIA_exit
; ---------------------------------------------------------------------------

VIA_toggleMainAltScr:                   
                bset    #6,($50F00000).l ; VIA1 bit-6, mainscreen (hardcoded, bah!)
                bclr    #6,($50F00000).l ; and then alternate screen. Does this flipflop mean something?
                bra.s   VIA_exitGood
; ---------------------------------------------------------------------------

VIA_setMainScr:                         
                bset    #6,($50F00000).l ; VIA1 base address (hardcoded!)
                bra.s   VIA_exitGood
; ---------------------------------------------------------------------------

VIA_setMainScrNoVsync:                  
                bset    #6,($50F01E00).l ; 1 = mainscreen
                bset    #6,($50F00000).l ; 1 = vertical synchronization interrupt disabled
                move.l  #$80,d0
                rts