mirror of
https://github.com/autc04/Retro68.git
synced 2024-11-24 07:31:32 +00:00
366 lines
14 KiB
Plaintext
366 lines
14 KiB
Plaintext
@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://www-cs-faculty.stanford.edu/~knuth/programs/mmix.tar.gz}
|
|
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://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.
|
|
|
|
@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://www-cs-faculty.stanford.edu/~knuth/mmixal-intro.ps.gz},
|
|
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.
|
|
|
|
@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
|
|
@url{http://www-cs-faculty.stanford.edu/~knuth/programs/mmix.tar.gz}):
|
|
``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.
|