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I disassembled Turbo


Captain Cozmos

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Well, almost disassembled.

This one is a challenge because it is really jump tabled and memory dependent.
I thought Smurf was bad but then once I started decoding the Smurf puzzle it all fell into place.  So I have to work on this one.

Now, I said I was taking a break from disassembles for a bit, or a byte, but I did this one to see how the steering wheel works in the grand scheme of things and it did not disappoint.

I also learned that Turbo sets up several picture frames in VRAM at one time then shifts the Name Table register to give the illusion that the buildings are moving.  Zaxxon does this exact same thing to display it's scrolling background.


Remember when I explained, in brief detail, about how Montezuma's Revenge animated fire and what not.  This was done by shifting pattern Register 4


Basically, you set up a screen in vram at the Name Table address.  (View Screen)
You have a set of patterns say 32h,50h,90h....
Well, you can have an alternate set of patterns buried in VRAM that may be slightly different but have the same numbers.
So when you shift Register 4, the Name Table points to the new set of 32h,50h,90h....  This gives the appearance, on the "1" screen, of movement.

 

This can be achieved on a small scale because a few patterns here and a few patterns there are no big deal when you have an NMI that screws with you all the time.
But, if you want to alternate whole screens, 768 byte screens at every refresh, you do have 16k at your disposal.

 

So you have one set of patterns and as many screens that will fit within the 16k along with everything else and all you have to do is change Register 2 which will point to the new screen.
Remember 1k is 1024 bytes while 1 screen on a Colecovision is 768.

So now that you have been educated, go out and tackle the world you knuckle heads.

 

O3 Cozmos







 

Edited by Captain Cozmos
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