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<a name="mmo-section-mapping"></a>
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<p>
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<a name="mmo-section-mapping-1"></a>
<h4 class="subsection">3.5.3 mmo section mapping</h4>
<p>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.
</p>
<p>For areas that don’t have such descriptors, synthetic sections are
formed by BFD. Consecutive contents in the two memory areas
‘<samp>0x0000…00</samp>’ to ‘<samp>0x01ff…ff</samp>’ and
‘<samp>0x2000…00</samp>’ to ‘<samp>0x20ff…ff</samp>’ are entered in
sections named <code>.text</code> and <code>.data</code> respectively. If an area
is not otherwise described, but would together with a neighboring
lower area be less than ‘<samp>0x40000000</samp>’ 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</var></code>. Here,
<var>n</var> is a number, a running count through the mmo file,
starting at 0.
</p>
<p>A loadable section specified as:
</p>
<div class="example">
<pre class="example"> .section secname,"ax"
TETRA 1,2,3,4,-1,-2009
BYTE 80
</pre></div>
<p>and linked to address ‘<samp>0x4</samp>’, is represented by the sequence:
</p>
<div class="example">
<pre class="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.
</pre></div>
<p>Note that the lop_spec wrapping does not include the section
contents. Compare this to a non-loaded section specified as:
</p>
<div class="example">
<pre class="example"> .section thirdsec
TETRA 200001,100002
BYTE 38,40
</pre></div>
<p>This, when linked to address ‘<samp>0x200000000000001c</samp>’, is
represented by:
</p>
<div class="example">
<pre class="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
</pre></div>
<p>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.
</p>
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