The Xbox's MCPX is the first 512 (0x200) bytes of code executed by the processor.

For Xbox revisions,
V1.0:
MCPX has decrpytion algorithm for 2BL

V1.1:
Crappy flashROM hashing algorithm added. MCPX is bursting at the seams :)

V1.2 (identical to V1.3):
Similar V1.1?

Other Xboxs:
Green Debug:
Much the same as V1.0 (2BL decrpytion), but encryption key different, and labelled X2, not X3 (MCPX-X3)

White Debug:
Probably same as green debug.


Feel free to update this, and increase the scenes limited understanding of the MCPX.
MCPX can be extracted with special hardware or a customised 2BL that won't clear the MCPX temporary copy in memory.