Segment declarations

A segment declaration creates a new segment in the a.out or changes the attribute values of an existing segment. (An existing segment is one that you previously defined or one of the three built-in segments described below.)

A segment declaration has the following syntax:

segment_name = {segment_attribute_value}*;

For each segment_name, you can specify any number of segment_attribute_values in any order, each separated by a space. (Only one attribute value is allowed for each segment attribute.) The segment attributes and their valid values are as follows:

segment_type:

LOAD
NOTE

segment flags:

?[R][W][X]

virtual_address:

Vnumber

physical_address:

Pnumber

length:

Lnumber

alignment:

Anumber

• text (LOAD, ?RX, virtual_address, physical_address, length, and alignment values set to defaults per CPU type)

• data (LOAD, ?RWX, virtual_address, physical_address, length, and alignment values set to defaults per CPU type)

• note (NOTE)

Note the following when entering segment declarations:

• A number can be hexadecimal, decimal, or octal, following the same rules as in the C language.

• No space is allowed between the V, P, L, or A and the number.

• The segment_type value can be either LOAD or NOTE.

• The segment_type value defaults to LOAD.

• The segment_flags values are R for readable, W for writable, and X for executable. No spaces are allowed between the question mark and the individual flags that make up the segment_flags value.

• The segment_flags value for a LOAD segment defaults to ?RWX.

• NOTE segments cannot be assigned any segment attribute value other than a segment_type.

• Implicitly declared segments default to segment_type value LOAD, segment_flags value ?RWX, virtual_address, physical_address, length, and alignment values set to defaults per CPU type.

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  NOTE: ld calculates the addresses and length of the current segment based on the previous segment's attribute values. Also, even though implicitly declared segments default to ``no length limit'', any machine memory limitations still apply.

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• LOAD segments can have an explicitly specified virtual_address value and/or physical_address value, as well as a maximum segment length value.

• If a segment has a segment_flags value of ? with nothing following, the value defaults to not readable, not writable and not executable.
  

• The alignment value is used in calculating the virtual address of the beginning of the segment. This alignment only affects the segment for which it is specified; other segments still have the default alignment unless their alignments are also changed.

• If any of the virtual_address, physical_address, or length attribute values are not set, ld calculates these values as it builds the a.out.

• If an alignment value is not specified for a segment, it is set to the built-in default. (The default differs from one CPU to another and may even differ between kernel versions. You should check the appropriate documentation for these numbers).

• If both a virtual_address and an alignment value are specified for a segment, the virtual_address value takes priority.

• If a virtual_address value is specified for a segment, the alignment field in the program header contains the default alignment value.

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  CAUTION: If a virtual_address value is specified, the segment is placed at that virtual address. For the UNIX system kernel this creates a correct result. For files that start via exec, this method creates an incorrect a.out file because the segments do not have correct offsets relative to their page boundaries.

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