Mirrored memory is a side effect of unconnected lines on the address bus thus making the content of those bits irrelevant. Code can take advantage of this to run faster or put tag values into addresses.
What does this have to do with speed?
On a GBA, VRAM is faster than ordinary RAM. Programs can do well to use it for tight inner loops.
Almost all programs use IWRAM for this. It's one of the things it's for. It's as fast if not faster than VRAM (depends on if the VRAM is being accessed by the PPU at the time).
Using STM (store multiple) to DMA registers? Again, go faster.
In retrospect, yeah this might actually be the case.
Save type masquerading might be code that helps when running on a development kit, but I admit that I can't think of what use it might have.
There are a handful of GBA games that lie about save type as anti-piracy and refuse to save or even boot if it finds the wrong one. It's also really the only anti-piracy technique that any other GBA games actually use, since it's quite effective against flash carts.
You generally need to mask potentially-tagged pointers before dereferencing them. (Ab)using virtual memory or unconnected lines lets you skip the mask, eliminating the cost in any code that's otherwise unconcerned about the tags. This in turn may let you save a byte or register here and there (and thus save memory bandwith / potentially spill fewer registers, maybe saving some performance).
GCs may abuse pointer tagging for keeping track of what they scaned. Ruby's VALUE type is pointer sized, and will point to Ruby objects such as strings and symbols - but it can also directly represent e.g. a 31-bit integer value ('Fixnum') on 32-bit systems without needing to be dereferenced, and without needing to consume a separate type field.
Also used in early revisions of the Amiga. (BCPL or Amiga Basic used pointer tags for something like garbage collect or frame pointers or something IIRC.) Mac had something called 32 bit clean which I imagined could have been pointer tagging.)
"32 bit clean" meant that the ROM didn't use pointer tagging; otherwise it was using the top 8 bits for its own purposes and wouldn't run on machines which used the full 32-bit address line.
What does this have to do with speed?
On a GBA, VRAM is faster than ordinary RAM. Programs can do well to use it for tight inner loops.
Almost all programs use IWRAM for this. It's one of the things it's for. It's as fast if not faster than VRAM (depends on if the VRAM is being accessed by the PPU at the time).
Using STM (store multiple) to DMA registers? Again, go faster.
In retrospect, yeah this might actually be the case.
Save type masquerading might be code that helps when running on a development kit, but I admit that I can't think of what use it might have.
There are a handful of GBA games that lie about save type as anti-piracy and refuse to save or even boot if it finds the wrong one. It's also really the only anti-piracy technique that any other GBA games actually use, since it's quite effective against flash carts.