• Address Span: 0x2000000 to 0x203FFFF (256k)

This is a big pile of space, the use of which is up to each game. However, the external work ram has only a 16-bit bus (if you read/write a 32-bit value it silently breaks it up into two 16-bit operations) and also 2 wait cycles (extra CPU cycles that you have to expend per 16-bit bus use).

It's most helpful to think of EWRAM as slower, distant memory, similar to the "heap" in a normal application. You can take the time to go store something within EWRAM, or to load it out of EWRAM, but if you've got several operations to do in a row and you're worried about time you should pull that value into local memory, work on your local copy, and then push it back out to EWRAM.

• Address Span: 0x3000000 to 0x3007FFF (32k)

This is a smaller pile of space, but it has a 32-bit bus and no wait.

By default, 0x3007F00 to 0x3007FFF is reserved for interrupt and BIOS use. The rest of it is mostly up to you. The user's stack space starts at 0x3007F00 and proceeds down from there. For best results you should probably start at 0x3000000 and then go upwards. Under normal use it's unlikely that the two memory regions will crash into each other.