Super DS Hacker

FAST6191 · Oct 20, 2020

Masking
Basically a quick way of doing "if this then do that" for each individual bit in the number you want to edit at the cost of you having to spend 10 seconds thinking about it ahead of time. The way it does this usually uses boolean logic which is ways of taking multiple inputs and giving a known outcome depending upon the operation being done. Boolean logic is fundamental to digital electronics and thus most programming so probably best to have the intro to boolean logic on lock (you can probably skip 3+ input XOR in favour of NOR, NAND, OR, AND, NOT, as well as 2 input XOR). I would suggest electronics for this one rather than pure programming if you are doing some reading but if programming works for you then so it goes.

Some of the more basic approaches to masking will be very quick ways of doing things like blank every bit except this one.
That way if a processor or whatever does not have a "check this bit" option you don't have to do if this number = this other value I want 5000 times to account for each and every possible combination.
You might even chain a couple of them together with other boolean operations or things like shifts (the ARM processors of the GBA and DS are all about shifting, it is one of the few things they do really well actually) if a processor or operation prefers a simple 0 or 1 answer.

In cheats you will probably first meet this with control activated cheats. Here if you have a cheat you don't necessarily want your user to have to let go of whatever they are holding (say holding up to run) to hold R and press select. This way if you mask off everything but the R and select buttons in the control register/debounced area then you can have the player continue to hold their control buttons but have the cheat fire and activate whatever else it does.

Later on it will allow you to twiddle specific bits or a selection of them. The basic step most would think of when wanting to change bits is to add a number to them. Works great until there are already some bits there and thus you change other numbers.
If you do an OR operation with all 0 in the parts you don't care about but 1 in the parts you do then as OR will return a high value if only one of the inputs is high you have just achieved your goal without having to get all fiddly and say change this bit, now change this.
Quite useful in cheats if there are a long string of bits to indicate status effects (you presumably either have it or you don't, can then make sense to shove them all into one value rather than having to keep track of 15 values multiplied by however many characters, even more so if you are on an older system where your memory, or free memory, is more usefully measured in kilobytes) and can't then set a value all the time to remove or include them without overwrite other status effects and thus breaking a game or reducing enjoyment of it (paralyse might be lame but if I am poisoned I want to have to deal with it, your stats cheat makes me immune to all effects, possibly even positive ones, and it is not ideal).

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