Processors

Pretty OK if GCC supports it - but:

* Got to have modern binary format support (ELF)

* Linus makes some presumptions (structure layout)

Memory managment

* Needs ability to support virtual memory

* ELKS and co. can get on without it

Differences within families

* e.g. MIPS 32/64 bit

* Endianness (Cray T3)

Caching