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diff --git a/bfd/doc/mmo.texi b/bfd/doc/mmo.texi new file mode 100644 index 0000000..f9c2358 --- /dev/null +++ b/bfd/doc/mmo.texi @@ -0,0 +1,369 @@ +@section mmo backend +The mmo object format is used exclusively together with Professor +Donald E.@: Knuth's educational 64-bit processor MMIX. The simulator +@command{mmix} which is available at +@url{http://mmix.cs.hm.edu/src/index.html} +understands this format. That package also includes a combined +assembler and linker called @command{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 +@url{http://mmix.cs.hm.edu/} for more +information about MMIX. The ELF format is used for intermediate +object files in the BFD implementation. + +@c We want to xref the symbol table node. A feature in "chew" +@c requires that "commands" do not contain spaces in the +@c arguments. Hence the hyphen in "Symbol-table". +@menu +* File layout:: +* Symbol-table:: +* mmo section mapping:: +@end menu + +@node File layout, Symbol-table, mmo, mmo +@subsection File layout +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 +@samp{0x0000@dots{}00} to @samp{0x01ff@dots{}ff} is executable, so +it is used for code (and constants) and the area +@samp{0x2000@dots{}00} to @samp{0x20ff@dots{}ff} is used for +writable data. @xref{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 @code{e_machine} number for MMIX, filling it with +section information normally found in ELF objects. @xref{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 @samp{0x98} forms a command called a @samp{lopcode}, where +the next byte distinguished between the thirteen lopcodes. The +two remaining bytes, called the @samp{Y} and @samp{Z} fields, or +the @samp{YZ} field (a 16-bit big-endian number), are used for +various purposes different for each lopcode. As documented in +@url{http://mmix.cs.hm.edu/doc/mmixal.pdf}, +the lopcodes are: + +@table @code +@item lop_quote +0x98000001. The next word is contents, regardless of whether it +starts with 0x98 or not. + +@item lop_loc +0x9801YYZZ, where @samp{Z} is 1 or 2. This is a location +directive, setting the location for the next data to the next +32-bit word (for @math{Z = 1}) or 64-bit word (for @math{Z = 2}), +plus @math{Y * 2^56}. Normally @samp{Y} is 0 for the text segment +and 2 for the data segment. Beware that the low bits of non- +tetrabyte-aligned values are silently discarded when being +automatically incremented and when storing contents (in contrast +to e.g. its use as current location when followed by lop_fixo +et al before the next possibly-quoted tetrabyte contents). + +@item lop_skip +0x9802YYZZ. Increase the current location by @samp{YZ} bytes. + +@item lop_fixo +0x9803YYZZ, where @samp{Z} is 1 or 2. Store the current location +as 64 bits into the location pointed to by the next 32-bit +(@math{Z = 1}) or 64-bit (@math{Z = 2}) word, plus @math{Y * +2^56}. + +@item lop_fixr +0x9804YYZZ. @samp{YZ} is stored into the current location plus +@math{2 - 4 * YZ}. + +@item lop_fixrx +0x980500ZZ. @samp{Z} is 16 or 24. A value @samp{L} derived from +the following 32-bit word are used in a manner similar to +@samp{YZ} in lop_fixr: it is xor:ed into the current location +minus @math{4 * L}. The first byte of the word is 0 or 1. If it +is 1, then @math{L = (@var{lowest 24 bits of word}) - 2^Z}, if 0, +then @math{L = (@var{lowest 24 bits of word})}. + +@item lop_file +0x9806YYZZ. @samp{Y} is the file number, @samp{Z} is count of +32-bit words. Set the file number to @samp{Y} and the line +counter to 0. The next @math{Z * 4} bytes contain the file name, +padded with zeros if the count is not a multiple of four. The +same @samp{Y} may occur multiple times, but @samp{Z} must be 0 for +all but the first occurrence. + +@item lop_line +0x9807YYZZ. @samp{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. + +@item lop_spec +0x9808YYZZ. @samp{YZ} is the type number. Data until the next +lopcode other than lop_quote forms special data of type @samp{YZ}. +@xref{mmo section mapping}. + +Other types than 80, (or type 80 with a content that does not +parse) is stored in sections named @code{.MMIX.spec_data.@var{n}} +where @var{n} is the @samp{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 @var{n} is +concatenated to the data of the previous lop_spec @var{n}s. The +location in data or code at which the lop_spec occurred is lost. + +@item lop_pre +0x980901ZZ. The first lopcode in a file. The @samp{Z} field forms the +length of header information in 32-bit words, where the first word +tells the time in seconds since @samp{00:00:00 GMT Jan 1 1970}. + +@item lop_post +0x980a00ZZ. @math{Z > 32}. This lopcode follows after all +content-generating lopcodes in a program. The @samp{Z} field +denotes the value of @samp{rG} at the beginning of the program. +The following @math{256 - Z} big-endian 64-bit words are loaded +into global registers @samp{$G} @dots{} @samp{$255}. + +@item 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 +(@pxref{Symbol-table}). + +@item lop_end +0x980cYYZZ. The last lopcode in a program. It must follow the +lop_stab lopcode and its data. The @samp{YZ} field contains the +number of 32-bit words of symbol table information after the +preceding lop_stab lopcode. +@end table + +Note that the lopcode "fixups"; @code{lop_fixr}, @code{lop_fixrx} and +@code{lop_fixo} are not generated by BFD, but are handled. They are +generated by @code{mmixal}. + +This trivial one-label, one-instruction file: + +@example + :Main TRAP 1,2,3 +@end example + +can be represented this way in mmo: + +@example + 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 @xref{Symbol-table}. + 0x10404020 + 0x4d206120 + 0x69016e00 + 0x81000000 + 0x980c0005 - lop_end; symbol table contained five 32-bit words. +@end example +@node Symbol-table, mmo section mapping, File layout, mmo +@subsection Symbol table format +From mmixal.w (or really, the generated mmixal.tex) in the +MMIXware package which also contains the @command{mmix} simulator: +``Symbols are stored and retrieved by means of a @samp{ternary +search trie}, following ideas of Bentley and Sedgewick. (See +ACM--SIAM Symp.@: on Discrete Algorithms @samp{8} (1997), 360--369; +R.@:Sedgewick, @samp{Algorithms in C} (Reading, Mass.@: +Addison--Wesley, 1998), @samp{15.4}.) Each trie node stores a +character, and there are branches to subtries for the cases where +a given character is less than, equal to, or greater than the +character in the trie. There also is a pointer to a symbol table +entry if a symbol ends at the current node.'' + +So it's a tree encoded as a stream of bytes. The stream of bytes +acts on a single virtual global symbol, adding and removing +characters and signalling complete symbol points. Here, we read +the stream and create symbols at the completion points. + +First, there's a control byte @code{m}. If any of the listed bits +in @code{m} is nonzero, we execute what stands at the right, in +the listed order: + +@example + (MMO3_LEFT) + 0x40 - Traverse left trie. + (Read a new command byte and recurse.) + + (MMO3_SYMBITS) + 0x2f - Read the next byte as a character and store it in the + current character position; increment character position. + Test the bits of @code{m}: + + (MMO3_WCHAR) + 0x80 - The character is 16-bit (so read another byte, + merge into current character. + + (MMO3_TYPEBITS) + 0xf - We have a complete symbol; parse the type, value + and serial number and do what should be done + with a symbol. The type and length information + is in j = (m & 0xf). + + (MMO3_REGQUAL_BITS) + j == 0xf: A register variable. The following + byte tells which register. + j <= 8: An absolute symbol. Read j bytes as the + big-endian number the symbol equals. + A j = 2 with two zero bytes denotes an + unknown symbol. + j > 8: As with j <= 8, but add (0x20 << 56) + to the value in the following j - 8 + bytes. + + Then comes the serial number, as a variant of + uleb128, but better named ubeb128: + Read bytes and shift the previous value left 7 + (multiply by 128). Add in the new byte, repeat + until a byte has bit 7 set. The serial number + is the computed value minus 128. + + (MMO3_MIDDLE) + 0x20 - Traverse middle trie. (Read a new command byte + and recurse.) Decrement character position. + + (MMO3_RIGHT) + 0x10 - Traverse right trie. (Read a new command byte and + recurse.) +@end example + +Let's look again at the @code{lop_stab} for the trivial file +(@pxref{File layout}). + +@example + 0x980b0000 - lop_stab for ":Main" = 0, serial 1. + 0x203a4040 + 0x10404020 + 0x4d206120 + 0x69016e00 + 0x81000000 +@end example + +This forms the trivial trie (note that the path between ``:'' and +``M'' is redundant): + +@example + 203a ":" + 40 / + 40 / + 10 \ + 40 / + 40 / + 204d "M" + 2061 "a" + 2069 "i" + 016e "n" is the last character in a full symbol, and + with a value represented in one byte. + 00 The value is 0. + 81 The serial number is 1. +@end example + +@node mmo section mapping, , Symbol-table, mmo +@subsection mmo section mapping +The implementation in BFD uses special data type 80 (decimal) to +encapsulate and describe named sections, containing e.g.@: debug +information. If needed, any datum in the encapsulation will be +quoted using lop_quote. First comes a 32-bit word holding the +number of 32-bit words containing the zero-terminated zero-padded +segment name. After the name there's a 32-bit word holding flags +describing the section type. Then comes a 64-bit big-endian word +with the section length (in bytes), then another with the section +start address. Depending on the type of section, the contents +might follow, zero-padded to 32-bit boundary. For a loadable +section (such as data or code), the contents might follow at some +later point, not necessarily immediately, as a lop_loc with the +same start address as in the section description, followed by the +contents. This in effect forms a descriptor that must be emitted +before the actual contents. Sections described this way must not +overlap. + +For areas that don't have such descriptors, synthetic sections are +formed by BFD. Consecutive contents in the two memory areas +@samp{0x0000@dots{}00} to @samp{0x01ff@dots{}ff} and +@samp{0x2000@dots{}00} to @samp{0x20ff@dots{}ff} are entered in +sections named @code{.text} and @code{.data} respectively. If an area +is not otherwise described, but would together with a neighboring +lower area be less than @samp{0x40000000} bytes long, it is joined +with the lower area and the gap is zero-filled. For other cases, +a new section is formed, named @code{.MMIX.sec.@var{n}}. Here, +@var{n} is a number, a running count through the mmo file, +starting at 0. + +A loadable section specified as: + +@example + .section secname,"ax" + TETRA 1,2,3,4,-1,-2009 + BYTE 80 +@end example + +and linked to address @samp{0x4}, is represented by the sequence: + +@example + 0x98080050 - lop_spec 80 + 0x00000002 - two 32-bit words for the section name + 0x7365636e - "secn" + 0x616d6500 - "ame\0" + 0x00000033 - flags CODE, READONLY, LOAD, ALLOC + 0x00000000 - high 32 bits of section length + 0x0000001c - section length is 28 bytes; 6 * 4 + 1 + alignment to 32 bits + 0x00000000 - high 32 bits of section address + 0x00000004 - section address is 4 + 0x98010002 - 64 bits with address of following data + 0x00000000 - high 32 bits of address + 0x00000004 - low 32 bits: data starts at address 4 + 0x00000001 - 1 + 0x00000002 - 2 + 0x00000003 - 3 + 0x00000004 - 4 + 0xffffffff - -1 + 0xfffff827 - -2009 + 0x50000000 - 80 as a byte, padded with zeros. +@end example + +Note that the lop_spec wrapping does not include the section +contents. Compare this to a non-loaded section specified as: + +@example + .section thirdsec + TETRA 200001,100002 + BYTE 38,40 +@end example + +This, when linked to address @samp{0x200000000000001c}, is +represented by: + +@example + 0x98080050 - lop_spec 80 + 0x00000002 - two 32-bit words for the section name + 0x7365636e - "thir" + 0x616d6500 - "dsec" + 0x00000010 - flag READONLY + 0x00000000 - high 32 bits of section length + 0x0000000c - section length is 12 bytes; 2 * 4 + 2 + alignment to 32 bits + 0x20000000 - high 32 bits of address + 0x0000001c - low 32 bits of address 0x200000000000001c + 0x00030d41 - 200001 + 0x000186a2 - 100002 + 0x26280000 - 38, 40 as bytes, padded with zeros +@end example + +For the latter example, the section contents must not be +loaded in memory, and is therefore specified as part of the +special data. The address is usually unimportant but might +provide information for e.g.@: the DWARF 2 debugging format. |