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7800 Pac-man Construction Set - Editing the Title


jwierer

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First of all Happy Independence day to everybody in the United States of America!

 

I've known where the title screen is stored for a long time, but never bothered editing it, because using my sprite editor (see earlier post) it's kind of hard to really envision what a 64x30 title graphic looks like. With the non-Jr Pac-man games, you read a nibble, draw 2 pixels, shift down a row, read a nibble, draw 2 pixels. I expected when I drew it on screen I would see a single contiguous stream of data that says Ms. Pac-man. Wishful thinking... It starts off looking good, but then turns to garbage.

blogentry-5778-1246740658_thumb.jpg

Wisely, when GCC built the title graphic it was designed for optimizing ROM space so empty tiles aren't stored. Instead sections of graphics are placed strategically at different locations to create the graphic. Of course when you place them in the correct locations it starts to take shape.

blogentry-5778-1246740668_thumb.jpg

What you'll notice about this image is missing sections, highlighted in pink. These are actually what I would call "shared" in that GCC reused sections of the title screen in portions where the tiles are effectively the same set of pixels. You can see the color shaded sections of title screen that are reused. Once you know this you can decode the entire title graphic.

blogentry-5778-1246740679_thumb.jpg

So yes, it is technically possible to edit the title screen just by modifying the binary, but you'll have to take special care around working on the shared sections, which makes it prohibitively difficult. The "Ms." portion is available in all Ms. Pacman and Pacman variants, except Pacman Collection, for editing which makes it the most obvious place to hack.

 

Happy 4th,

Jeff

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