> what did they use for the BCPL parts?

 Nice question, offhand, I have no answer. If you are really interested,
you might want to ask Olaf Barthels, who successfully compiled AmigaOS
under NetBSD/Amiga with the greenhill C-Compiler. The compiler does not
compile the entire stuff. A large object file is generated and linked
together with other (assembler) objects under AmigaDOS using an old
linker. Olaf managed to compile all and everything using SAS/C, so he no
longer needs NetBSD.

-- 
Markus Illenseer