Untitled Document: mmo section mapping

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#### 3.5.3 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 ‘`0x0000…00`’ to ‘`0x01ff…ff`’ and ‘`0x2000…00`’ to ‘`0x20ff…ff`’ are entered in sections named `.text` and `.data` respectively. If an area is not otherwise described, but would together with a neighboring lower area be less than ‘`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 `.MMIX.sec.n`. Here, n is a number, a running count through the mmo file, starting at 0.

A loadable section specified as:

     .section secname,"ax"
     TETRA 1,2,3,4,-1,-2009
     BYTE 80

and linked to address ‘`0x4`’, is represented by the sequence:

     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.

Note that the lop_spec wrapping does not include the section contents. Compare this to a non-loaded section specified as:

     .section thirdsec
     TETRA 200001,100002
     BYTE 38,40

This, when linked to address ‘`0x200000000000001c`’, is represented by:

     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

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.

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