If you have the actual device, you don't need an emulator. Unless you are deconstructing existing TI-89 programs, you don't need a decompiler.
The processor in the TI-89 is a Motorola 68k. You should look for resources for that platform -- and serveral are available, as the 68k is not a new processor. You can code it in assembly or C. A quick Google search query returned this site that has the dedicated TI-89 information you're looking for: http://www.technoplaza.net/
