First of all, we need a safe way to trigger timer B. Why not use the level 4 interrupt which occurs each time the electron beam reaches the bottom of the screen. Here we set the value for timer B and in that scan line, the interrupt will occur! So if we want a raster on scan line 100, we activate timer B with a value of 100 in the level 4 interrupt and wait for the timer B interrupt.
Uh oh....strange things are happening....
MFP 68901: Hi guys! Interrupt time!
68000 : No way, I'm tired...leave me alone!
MFP 68901: Ah, come one, just one little interrupt...
68000 : Forget it, go away!
MFP 68901: Ok, have it your way.
68000 : Fine.
MFP 68901: OVERRIDE!!!!!!
68000 : OUCH!!!
MMU : Hey? What's going on? Who is overriding there without
filing the proper request at my address bus?
MFP 68901: I am and you can't stop me.
68000 : Groan...
MMU : Stop it right now or I'll have to get involved here.
68000 : Yeah, stop this idiot!
MFP 68901: Fools!
MMU : SYNCHRONIZE!!
Shifter : AAAAAAHHHHH! My borders!!!
MMU : Ooops, wrong interrupt level.
MFP 68901: Wow! no borders, let's throw some rasters in there!
Shifter : NO!!! AAAAAAAAAAAAAGGGRRLLL!!
68000 : Hey guys, what about me?
MMU, MFP 68901: We're having fun. In fact, we haven't had this
much fun since the Union Demo.
Shifter : WHAT???!! UNION DEMO??!!!? AAAAAAAHAHAAAAAHAAHAAA!!!
68000 : Nurse!!
MMU : Yeah, border here, synchro there.....I'm having a
blast.
MFP 68901: Ok, what does this address do?
Shifter : AHAHAAAAAAAAAGGRRRLGGALAAAHAAHAHHH!!!!
68000 : Hey, look out!!
Gemdos critical error handler: PANIC!
Exception vector: ZAP!
68000, MMU, MFP 68901: Uh oh.....
Shifter : UNION DEMO!!! RAAAAAAHHH! LOST BOYS!! GROOOOOWWWLL!!
CAREBEARS!!! AAAAAAAAAAHHAHAHAHAHHAHAHGGGGGLLLlllpp..
ST : CRASH!!
Stefan : F.CK!!