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53 lines
2.9 KiB
Markdown
53 lines
2.9 KiB
Markdown
# Ch 3: Memory and Objects
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Alright so we can do some basic "movement", but we left a big trail in the video
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memory of everywhere we went. Most of the time that's not what we want at all.
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If we want more hardware support we're going to have to use a new video mode. So
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far we've only used Mode 3, but modes 4 and 5 are basically the same. Instead,
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we'll switch focus to using a tiled graphical mode.
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First we will go over the complete GBA memory mapping. Part of this is the
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memory for tiled graphics, but also things like all those IO registers, where
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our RAM is for scratch space, all that stuff. Even if we can't put all of them
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to use at once, it's helpful to have an idea of what will be available in the
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long run.
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Tiled modes bring us three big new concepts that each have their own complexity:
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tiles, backgrounds, and objects. Backgrounds and objects both use tiles, but the
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background is for creating a very large static space that you can scroll around
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the view within, and the objects are about having a few moving bits that appear
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over the background. Careful use of backgrounds and objects is key to having the
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best looking GBA game, so we won't even be able to cover it all in a single
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chapter.
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And, of course, since most games are pretty boring if they're totally static
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we'll touch on the kinds of RNG implementations you might want to have on a GBA.
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Most general purpose RNGs that you find are rather big compared to the amount of
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memory we want to give them, and they often use a lot of `u64` operations, so
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they end up much slower on a 32-bit machine like the GBA (you can lower 64-bit
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ops to combinations of 32-bit ops, but that's quite a bit more work). We'll
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cover a few RNG options that size down the RNG to a good size and a good speed
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without trading away too much in terms of quality.
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To top it all off, we'll make a simple "memory game" sort of thing. There's some
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face down cards in a grid, you pick one to check, then you pick the other to
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check, and then if they match the pair disappears.
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## Drawing Priority
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Both backgrounds and objects can have "priority" values associated with them.
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TONC and GBATEK have _opposite_ ideas of what it means to have the "highest"
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priority. TONC goes by highest numerical value, and GBATEK goes by what's on the
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z-layer closest to the user. Let's list out the rules as clearly as we can:
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* Priority is always two bits, so 0 through 3.
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* Priority conceptually proceeds in drawing passes that count _down_, so any
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priority 3 things can get covered up by priority 2 things. In truth there's
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probably depth testing and buffering stuff going on so it's all one single
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pass, but conceptually we will imagine it happening as all of the 3 elements,
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then all of 2, and so on.
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* Objects always draw over top of backgrounds of equal priority.
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* Within things of the same type and priority, the lower numbered element "wins"
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and gets its pixel drawn (bg0 is favored over bg1, obj0 is favored over obj1,
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etc).
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