The mmo object format is used exclusively together with Professor Donald E. Knuth's educational 64-bit processor MMIX. The simulator mmix which is available at http://www-cs-faculty.stanford.edu/~knuth/programs/mmix.tar.gz understands this format. That package also includes a combined assembler and linker called mmixal. The mmo format has no advantages feature-wise compared to e.g. ELF. It is a simple non-relocatable object format with no support for archives or debugging information, except for symbol value information and line numbers (which is not yet implemented in BFD). See http://www-cs-faculty.stanford.edu/~knuth/mmix.html for more information about MMIX. The ELF format is used for intermediate object files in the BFD implementation.