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< a class = "header" href = "#io-registers" id = "io-registers" > < h1 > IO Registers< / h1 > < / a >
< p > The GBA has a large number of < strong > IO Registers< / strong > (not to be confused with CPU
registers). These are special memory locations from < code > 0x04000000< / code > to
< code > 0x040003FE< / code > . GBATEK has a < a href = "http://problemkaputt.de/gbatek.htm#gbaiomap" > full
list< / a > , but we only need to learn
about a few of them at a time as we go, so don't be worried.< / p >
< p > The important facts to know about IO Registers are these:< / p >
< ul >
< li > Each has their own specific size. Most are < code > u16< / code > , but some are < code > u32< / code > .< / li >
< li > All of them must be accessed in a < code > volatile< / code > style.< / li >
< li > Each register is specifically readable or writable or both. Actually, with
some registers there are even individual bits that are read-only or
write-only.
< ul >
< li > If you write to a read-only position, those writes are simply ignored. This
mostly matters if a writable register contains a read-only bit (such as the
Display Control, next section).< / li >
< li > If you read from a write-only position, you get back values that are
< a href = "http://problemkaputt.de/gbatek.htm#gbaunpredictablethings" > basically
nonsense< / a > . There
aren't really any registers that mix writable bits with read only bits, so
you're basically safe here. The only (mild) concern is that when you write a
value into a write-only register you need to keep track of what you wrote
somewhere else if you want to know what you wrote (such to adjust an offset
value by +1, or whatever).< / li >
< li > You can always check GBATEK to be sure, but if I don't mention it then a bit
is probably both read and write.< / li >
< / ul >
< / li >
< li > Some registers have invalid bit patterns. For example, the lowest three bits
of the Display Control register can't legally be set to the values 6 or 7.< / li >
< / ul >
< p > When talking about bit positions, the numbers are < em > zero indexed< / em > just like an
array index is.< / p >
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