Make blitter shift value a variable?

[Brick Nash]

Hi folks, quick blitter question:

I'm trying to make the shift value for source A in BLTCON0 a dynamic variable, but I can't quite figure out how to do it. I set up a variable called BOBSHIFT in my data section, and I tried doing this:

move.l #BOBSHIFT+$9f00000,$040

...but I think I need to add an offset or something. Either that or it isn't possible, but my brain is a little fatigued. It's just for moving my image right (ascending mode) in a smooth movement when I push right on the joystick, so nothing fancy, and I set up the image to have an extra 16 pixels at the right for space to shift into.

Thanks!

[DanScott]

you might need to do something like:

move.l BOBSHIFT,d0
or.l #$09f00000,d0
move.l d0,BLTCON0(a6)


This assumes that BOBSHIFT is a longword stored in memory and already has the 4bit shift value in the top bits....

if you want to have an actual x position, then you need to mask off the shift and move it into position.


moveq #0,d0
move.w Xpos,d0
and.w #15,d0
ror.w #4,d0
or.w #$9f0,d0
swap d0
move.l d0,BLTCON0(a6)

This masks the shift value, rotates the shift value to the correct place, "or" in the minterm, and then the swap moves that to the top word (with the bottom word containing zero) before moving into the blitter register

[Brick Nash]

Excellent, thank you! I'll have a try with these and see what I can do.

EDIT: Yep that worked! A few shifts to the left and or-ing it in did the trick nicely.

Thank you for the help!