Or you divide the Main Loop into 2 sections/part.
The First section/part does the movement
The Second section/part does the collision check
something like
Code:
a=1
;main loop
do
a=-a
if a=1
PlayerMovement()
Enemymovement()
Playerbullets()
Enemybullets()
else
CollisionCheck()
endif
flip ; or whatever the blitz uses to redraw the screen.
loop
Sadly i'm familiar only with the PC's version of Blitzbasic ...