NES Emulator Question

[Avilius]

Hello, I have a small question regarding an NES emulator.

What exactly is the "Cartridge Expansion ROM" as described here? Can someone get into the specifics on what it actually does?

http://en.wikibooks.org/wiki/NES_Programming/Memory_Map

[helios]

http://wiki.nesdev.com/w/index.php/ExROM
http://wiki.nesdev.com/w/index.php/MMC5

[Disch]

Everything between addresses $4020-$FFFF get forwarded to the cartridge. The cartridge can decide to put anything anywhere in that range... though usually (99% of the time) it puts ROM between $8000-FFFF, RAM at $6000-FFFF, and leaves $4020-$5FFF empty.

[Avilius]

How can you tell what's in the space?

[Disch]

The mapper.

In the 16 byte header, there's a mapper number. That number tells you the cartridge layout as well as the mapping hardware used to swap memory around.

[Avilius]

So a cartridge with no mapper hardware doesn't use that space?

[Disch]

Basically.. yes.

Very few mappers use the $4xxx range.