The mmo file contents is not partitioned into named sections as with e.g. ELF. Memory areas is formed by specifying the location of the data that follows. Only the memory area `0x0000...00' to `0x01ff...ff' is executable, so it is used for code (and constants) and the area `0x2000...00' to `0x20ff...ff' is used for writable data. See mmo section mapping.
There is provision for specifying “special data” of 65536
different types. We use type 80 (decimal), arbitrarily chosen the
same as the ELF
e_machine number for MMIX, filling it with
section information normally found in ELF objects. See mmo section mapping.
Contents is entered as 32-bit words, xor:ed over previous contents, always zero-initialized. A word that starts with the byte `0x98' forms a command called a `lopcode', where the next byte distinguished between the thirteen lopcodes. The two remaining bytes, called the `Y' and `Z' fields, or the `YZ' field (a 16-bit big-endian number), are used for various purposes different for each lopcode. As documented in http://www-cs-faculty.stanford.edu/~knuth/mmixal-intro.ps.gz, the lopcodes are:
Other types than 80, (or type 80 with a content that does not
parse) is stored in sections named
where n is the `YZ'-type. The flags for such a
sections say not to allocate or load the data. The vma is 0.
Contents of multiple occurrences of special data n is
concatenated to the data of the previous lop_spec ns. The
location in data or code at which the lop_spec occurred is lost.
Note that the lopcode "fixups";
lop_fixo are not generated by BFD, but are handled. They are
This trivial one-label, one-instruction file:
:Main TRAP 1,2,3
can be represented this way in mmo:
0x98090101 - lop_pre, one 32-bit word with timestamp. <timestamp> 0x98010002 - lop_loc, text segment, using a 64-bit address. Note that mmixal does not emit this for the file above. 0x00000000 - Address, high 32 bits. 0x00000000 - Address, low 32 bits. 0x98060002 - lop_file, 2 32-bit words for file-name. 0x74657374 - "test" 0x2e730000 - ".s\0\0" 0x98070001 - lop_line, line 1. 0x00010203 - TRAP 1,2,3 0x980a00ff - lop_post, setting $255 to 0. 0x00000000 0x00000000 0x980b0000 - lop_stab for ":Main" = 0, serial 1. 0x203a4040 See Symbol-table. 0x10404020 0x4d206120 0x69016e00 0x81000000 0x980c0005 - lop_end; symbol table contained five 32-bit words.