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3.3.1 File layout

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:

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 section 3.3.3 mmo section mapping.

lop_quote

0x98000001. The next word is contents, regardless of whether it starts with 0x98 or not.

lop_loc

0x9801YYZZ, where `Z' is 1 or 2. This is a location directive, setting the location for the next data to the next 32-bit word (for Z = 1) or 64-bit word (for Z = 2), plus Y * 2^56. Normally `Y' is 0 for the text segment and 2 for the data segment.

lop_skip

0x9802YYZZ. Increase the current location by `YZ' bytes.

lop_fixo

0x9803YYZZ, where `Z' is 1 or 2. Store the current location as 64 bits into the location pointed to by the next 32-bit (Z = 1) or 64-bit (Z = 2) word, plus Y * 2^56.

lop_fixr

0x9804YYZZ. `YZ' is stored into the current location plus 2 - 4 * YZ.

lop_fixrx

0x980500ZZ. `Z' is 16 or 24. A value `L' derived from the following 32-bit word are used in a manner similar to `YZ' in lop_fixr: it is xor:ed into the current location minus 4 * L. The first byte of the word is 0 or 1. If it is 1, then L = (lowest 24 bits of word) - 2^Z, if 0, then L = (lowest 24 bits of word).

lop_file

0x9806YYZZ. `Y' is the file number, `Z' is count of 32-bit words. Set the file number to `Y' and the line counter to 0. The next Z * 4 bytes contain the file name, padded with zeros if the count is not a multiple of four. The same `Y' may occur multiple times, but `Z' must be 0 for all but the first occurrence.

lop_line

0x9807YYZZ. `YZ' is the line number. Together with lop_file, it forms the source location for the next 32-bit word. Note that for each non-lopcode 32-bit word, line numbers are assumed incremented by one.

lop_spec

0x9808YYZZ. `YZ' is the type number. Data until the next lopcode other than lop_quote forms special data of type `YZ'. See section 3.3.3 mmo section mapping.

Other types than 80, (or type 80 with a content that does not parse) is stored in sections named .MMIX.spec_data.n 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.

lop_pre

0x980901ZZ. The first lopcode in a file. The `Z' field forms the length of header information in 32-bit words, where the first word tells the time in seconds since `00:00:00 GMT Jan 1 1970'.

lop_post

0x980a00ZZ. Z > 32. This lopcode follows after all content-generating lopcodes in a program. The `Z' field denotes the value of `rG' at the beginning of the program. The following 256 - Z big-endian 64-bit words are loaded into global registers `$G' ... `$255'.

lop_stab

0x980b0000. The next-to-last lopcode in a program. Must follow immediately after the lop_post lopcode and its data. After this lopcode follows all symbols in a compressed format (see section 3.3.2 Symbol table format).

lop_end

0x980cYYZZ. The last lopcode in a program. It must follow the lop_stab lopcode and its data. The `YZ' field contains the number of 32-bit words of symbol table information after the preceding lop_stab lopcode.

Note that the lopcode "fixups"; lop_fixr, lop_fixrx and lop_fixo are not generated by BFD, but are handled. They are generated by mmixal.

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 section 3.3.2 Symbol table format. 0x10404020 0x4d206120 0x69016e00 0x81000000 0x980c0005 - lop_end; symbol table contained five 32-bit words.