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370 lines
14 KiB
Plaintext
@section mmo backend
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The mmo object format is used exclusively together with Professor
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Donald E.@: Knuth's educational 64-bit processor MMIX. The simulator
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@command{mmix} which is available at
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@url{http://mmix.cs.hm.edu/src/index.html}
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understands this format. That package also includes a combined
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assembler and linker called @command{mmixal}. The mmo format has
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no advantages feature-wise compared to e.g. ELF. It is a simple
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non-relocatable object format with no support for archives or
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debugging information, except for symbol value information and
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line numbers (which is not yet implemented in BFD). See
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@url{http://mmix.cs.hm.edu/} for more
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information about MMIX. The ELF format is used for intermediate
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object files in the BFD implementation.
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@c We want to xref the symbol table node. A feature in "chew"
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@c requires that "commands" do not contain spaces in the
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@c arguments. Hence the hyphen in "Symbol-table".
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@menu
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* File layout::
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* Symbol-table::
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* mmo section mapping::
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@end menu
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@node File layout, Symbol-table, mmo, mmo
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@subsection File layout
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The mmo file contents is not partitioned into named sections as
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with e.g.@: ELF. Memory areas is formed by specifying the
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location of the data that follows. Only the memory area
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@samp{0x0000@dots{}00} to @samp{0x01ff@dots{}ff} is executable, so
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it is used for code (and constants) and the area
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@samp{0x2000@dots{}00} to @samp{0x20ff@dots{}ff} is used for
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writable data. @xref{mmo section mapping}.
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There is provision for specifying ``special data'' of 65536
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different types. We use type 80 (decimal), arbitrarily chosen the
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same as the ELF @code{e_machine} number for MMIX, filling it with
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section information normally found in ELF objects. @xref{mmo
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section mapping}.
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Contents is entered as 32-bit words, xor:ed over previous
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contents, always zero-initialized. A word that starts with the
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byte @samp{0x98} forms a command called a @samp{lopcode}, where
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the next byte distinguished between the thirteen lopcodes. The
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two remaining bytes, called the @samp{Y} and @samp{Z} fields, or
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the @samp{YZ} field (a 16-bit big-endian number), are used for
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various purposes different for each lopcode. As documented in
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@url{http://mmix.cs.hm.edu/doc/mmixal.pdf},
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the lopcodes are:
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@table @code
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@item lop_quote
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0x98000001. The next word is contents, regardless of whether it
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starts with 0x98 or not.
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@item lop_loc
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0x9801YYZZ, where @samp{Z} is 1 or 2. This is a location
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directive, setting the location for the next data to the next
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32-bit word (for @math{Z = 1}) or 64-bit word (for @math{Z = 2}),
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plus @math{Y * 2^56}. Normally @samp{Y} is 0 for the text segment
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and 2 for the data segment. Beware that the low bits of non-
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tetrabyte-aligned values are silently discarded when being
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automatically incremented and when storing contents (in contrast
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to e.g. its use as current location when followed by lop_fixo
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et al before the next possibly-quoted tetrabyte contents).
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@item lop_skip
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0x9802YYZZ. Increase the current location by @samp{YZ} bytes.
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@item lop_fixo
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0x9803YYZZ, where @samp{Z} is 1 or 2. Store the current location
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as 64 bits into the location pointed to by the next 32-bit
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(@math{Z = 1}) or 64-bit (@math{Z = 2}) word, plus @math{Y *
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2^56}.
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@item lop_fixr
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0x9804YYZZ. @samp{YZ} is stored into the current location plus
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@math{2 - 4 * YZ}.
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@item lop_fixrx
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0x980500ZZ. @samp{Z} is 16 or 24. A value @samp{L} derived from
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the following 32-bit word are used in a manner similar to
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@samp{YZ} in lop_fixr: it is xor:ed into the current location
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minus @math{4 * L}. The first byte of the word is 0 or 1. If it
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is 1, then @math{L = (@var{lowest 24 bits of word}) - 2^Z}, if 0,
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then @math{L = (@var{lowest 24 bits of word})}.
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@item lop_file
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0x9806YYZZ. @samp{Y} is the file number, @samp{Z} is count of
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32-bit words. Set the file number to @samp{Y} and the line
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counter to 0. The next @math{Z * 4} bytes contain the file name,
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padded with zeros if the count is not a multiple of four. The
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same @samp{Y} may occur multiple times, but @samp{Z} must be 0 for
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all but the first occurrence.
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@item lop_line
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0x9807YYZZ. @samp{YZ} is the line number. Together with
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lop_file, it forms the source location for the next 32-bit word.
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Note that for each non-lopcode 32-bit word, line numbers are
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assumed incremented by one.
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@item lop_spec
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0x9808YYZZ. @samp{YZ} is the type number. Data until the next
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lopcode other than lop_quote forms special data of type @samp{YZ}.
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@xref{mmo section mapping}.
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Other types than 80, (or type 80 with a content that does not
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parse) is stored in sections named @code{.MMIX.spec_data.@var{n}}
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where @var{n} is the @samp{YZ}-type. The flags for such a
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sections say not to allocate or load the data. The vma is 0.
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Contents of multiple occurrences of special data @var{n} is
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concatenated to the data of the previous lop_spec @var{n}s. The
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location in data or code at which the lop_spec occurred is lost.
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@item lop_pre
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0x980901ZZ. The first lopcode in a file. The @samp{Z} field forms the
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length of header information in 32-bit words, where the first word
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tells the time in seconds since @samp{00:00:00 GMT Jan 1 1970}.
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@item lop_post
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0x980a00ZZ. @math{Z > 32}. This lopcode follows after all
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content-generating lopcodes in a program. The @samp{Z} field
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denotes the value of @samp{rG} at the beginning of the program.
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The following @math{256 - Z} big-endian 64-bit words are loaded
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into global registers @samp{$G} @dots{} @samp{$255}.
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@item lop_stab
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0x980b0000. The next-to-last lopcode in a program. Must follow
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immediately after the lop_post lopcode and its data. After this
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lopcode follows all symbols in a compressed format
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(@pxref{Symbol-table}).
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@item lop_end
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0x980cYYZZ. The last lopcode in a program. It must follow the
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lop_stab lopcode and its data. The @samp{YZ} field contains the
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number of 32-bit words of symbol table information after the
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preceding lop_stab lopcode.
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@end table
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Note that the lopcode "fixups"; @code{lop_fixr}, @code{lop_fixrx} and
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@code{lop_fixo} are not generated by BFD, but are handled. They are
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generated by @code{mmixal}.
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This trivial one-label, one-instruction file:
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@example
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:Main TRAP 1,2,3
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@end example
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can be represented this way in mmo:
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@example
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0x98090101 - lop_pre, one 32-bit word with timestamp.
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<timestamp>
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0x98010002 - lop_loc, text segment, using a 64-bit address.
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Note that mmixal does not emit this for the file above.
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0x00000000 - Address, high 32 bits.
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0x00000000 - Address, low 32 bits.
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0x98060002 - lop_file, 2 32-bit words for file-name.
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0x74657374 - "test"
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0x2e730000 - ".s\0\0"
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0x98070001 - lop_line, line 1.
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0x00010203 - TRAP 1,2,3
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0x980a00ff - lop_post, setting $255 to 0.
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0x00000000
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0x00000000
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0x980b0000 - lop_stab for ":Main" = 0, serial 1.
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0x203a4040 @xref{Symbol-table}.
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0x10404020
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0x4d206120
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0x69016e00
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0x81000000
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0x980c0005 - lop_end; symbol table contained five 32-bit words.
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@end example
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@node Symbol-table, mmo section mapping, File layout, mmo
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@subsection Symbol table format
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From mmixal.w (or really, the generated mmixal.tex) in the
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MMIXware package which also contains the @command{mmix} simulator:
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``Symbols are stored and retrieved by means of a @samp{ternary
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search trie}, following ideas of Bentley and Sedgewick. (See
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ACM--SIAM Symp.@: on Discrete Algorithms @samp{8} (1997), 360--369;
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R.@:Sedgewick, @samp{Algorithms in C} (Reading, Mass.@:
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Addison--Wesley, 1998), @samp{15.4}.) Each trie node stores a
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character, and there are branches to subtries for the cases where
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a given character is less than, equal to, or greater than the
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character in the trie. There also is a pointer to a symbol table
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entry if a symbol ends at the current node.''
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So it's a tree encoded as a stream of bytes. The stream of bytes
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acts on a single virtual global symbol, adding and removing
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characters and signalling complete symbol points. Here, we read
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the stream and create symbols at the completion points.
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First, there's a control byte @code{m}. If any of the listed bits
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in @code{m} is nonzero, we execute what stands at the right, in
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the listed order:
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@example
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(MMO3_LEFT)
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0x40 - Traverse left trie.
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(Read a new command byte and recurse.)
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(MMO3_SYMBITS)
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0x2f - Read the next byte as a character and store it in the
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current character position; increment character position.
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Test the bits of @code{m}:
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(MMO3_WCHAR)
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0x80 - The character is 16-bit (so read another byte,
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merge into current character.
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(MMO3_TYPEBITS)
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0xf - We have a complete symbol; parse the type, value
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and serial number and do what should be done
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with a symbol. The type and length information
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is in j = (m & 0xf).
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(MMO3_REGQUAL_BITS)
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j == 0xf: A register variable. The following
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byte tells which register.
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j <= 8: An absolute symbol. Read j bytes as the
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big-endian number the symbol equals.
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A j = 2 with two zero bytes denotes an
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unknown symbol.
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j > 8: As with j <= 8, but add (0x20 << 56)
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to the value in the following j - 8
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bytes.
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Then comes the serial number, as a variant of
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uleb128, but better named ubeb128:
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Read bytes and shift the previous value left 7
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(multiply by 128). Add in the new byte, repeat
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until a byte has bit 7 set. The serial number
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is the computed value minus 128.
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(MMO3_MIDDLE)
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0x20 - Traverse middle trie. (Read a new command byte
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and recurse.) Decrement character position.
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(MMO3_RIGHT)
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0x10 - Traverse right trie. (Read a new command byte and
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recurse.)
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@end example
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Let's look again at the @code{lop_stab} for the trivial file
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(@pxref{File layout}).
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@example
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0x980b0000 - lop_stab for ":Main" = 0, serial 1.
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0x203a4040
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0x10404020
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0x4d206120
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0x69016e00
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0x81000000
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@end example
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This forms the trivial trie (note that the path between ``:'' and
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``M'' is redundant):
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@example
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203a ":"
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40 /
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40 /
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10 \
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40 /
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40 /
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204d "M"
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2061 "a"
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2069 "i"
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016e "n" is the last character in a full symbol, and
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with a value represented in one byte.
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00 The value is 0.
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81 The serial number is 1.
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@end example
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@node mmo section mapping, , Symbol-table, mmo
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@subsection mmo section mapping
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The implementation in BFD uses special data type 80 (decimal) to
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encapsulate and describe named sections, containing e.g.@: debug
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information. If needed, any datum in the encapsulation will be
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quoted using lop_quote. First comes a 32-bit word holding the
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number of 32-bit words containing the zero-terminated zero-padded
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segment name. After the name there's a 32-bit word holding flags
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describing the section type. Then comes a 64-bit big-endian word
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with the section length (in bytes), then another with the section
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start address. Depending on the type of section, the contents
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might follow, zero-padded to 32-bit boundary. For a loadable
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section (such as data or code), the contents might follow at some
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later point, not necessarily immediately, as a lop_loc with the
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same start address as in the section description, followed by the
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contents. This in effect forms a descriptor that must be emitted
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before the actual contents. Sections described this way must not
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overlap.
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For areas that don't have such descriptors, synthetic sections are
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formed by BFD. Consecutive contents in the two memory areas
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@samp{0x0000@dots{}00} to @samp{0x01ff@dots{}ff} and
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@samp{0x2000@dots{}00} to @samp{0x20ff@dots{}ff} are entered in
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sections named @code{.text} and @code{.data} respectively. If an area
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is not otherwise described, but would together with a neighboring
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lower area be less than @samp{0x40000000} bytes long, it is joined
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with the lower area and the gap is zero-filled. For other cases,
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a new section is formed, named @code{.MMIX.sec.@var{n}}. Here,
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@var{n} is a number, a running count through the mmo file,
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starting at 0.
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A loadable section specified as:
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@example
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.section secname,"ax"
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TETRA 1,2,3,4,-1,-2009
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BYTE 80
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@end example
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and linked to address @samp{0x4}, is represented by the sequence:
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@example
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0x98080050 - lop_spec 80
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0x00000002 - two 32-bit words for the section name
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0x7365636e - "secn"
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0x616d6500 - "ame\0"
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0x00000033 - flags CODE, READONLY, LOAD, ALLOC
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0x00000000 - high 32 bits of section length
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0x0000001c - section length is 28 bytes; 6 * 4 + 1 + alignment to 32 bits
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0x00000000 - high 32 bits of section address
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0x00000004 - section address is 4
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0x98010002 - 64 bits with address of following data
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0x00000000 - high 32 bits of address
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0x00000004 - low 32 bits: data starts at address 4
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0x00000001 - 1
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0x00000002 - 2
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0x00000003 - 3
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0x00000004 - 4
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0xffffffff - -1
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0xfffff827 - -2009
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0x50000000 - 80 as a byte, padded with zeros.
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@end example
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Note that the lop_spec wrapping does not include the section
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contents. Compare this to a non-loaded section specified as:
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@example
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.section thirdsec
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TETRA 200001,100002
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BYTE 38,40
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@end example
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This, when linked to address @samp{0x200000000000001c}, is
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represented by:
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@example
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0x98080050 - lop_spec 80
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0x00000002 - two 32-bit words for the section name
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0x7365636e - "thir"
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0x616d6500 - "dsec"
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0x00000010 - flag READONLY
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0x00000000 - high 32 bits of section length
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0x0000000c - section length is 12 bytes; 2 * 4 + 2 + alignment to 32 bits
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0x20000000 - high 32 bits of address
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0x0000001c - low 32 bits of address 0x200000000000001c
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0x00030d41 - 200001
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0x000186a2 - 100002
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0x26280000 - 38, 40 as bytes, padded with zeros
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@end example
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For the latter example, the section contents must not be
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loaded in memory, and is therefore specified as part of the
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special data. The address is usually unimportant but might
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provide information for e.g.@: the DWARF 2 debugging format.
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