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In formats which only support a limited number of sections, such as a. If the output format supports any number of sections, but with numbers and not names as is the case for Oasys , the name should be supplied as a quoted numeric string. A section name may consist of any sequence of characters, but a name which contains any unusual characters such as commas must be quoted.
The address is an expression for the VMA the virtual memory address of the output section. If you do not provide address , the linker will set it based on region if present, or otherwise based on the current value of the location counter.
If you provide address , the address of the output section will be set to precisely that. If you provide neither address nor region , then the address of the output section will be set to the current value of the location counter aligned to the alignment requirements of the output section.
The alignment requirement of the output section is the strictest alignment of any input section contained within the output section. For example,. The address may be an arbitrary expression; section Expressions in Linker Scripts. For example, if you want to align the section on a 0x10 byte boundary, so that the lowest four bits of the section address are zero, you could do something like this:. The most common output section command is an input section description.
The input section description is the most basic linker script operation. You use output sections to tell the linker how to lay out your program in memory.
You use input section descriptions to tell the linker how to map the input files into your memory layout. An input section description consists of a file name optionally followed by a list of section names in parentheses. The file name and the section name may be wildcard patterns, which we describe further below see section Input section wildcard patterns. The most common input section description is to include all input sections with a particular name in the output section.
In the first example, they will be intermingled. You can specify a file name to include sections from a particular file. You would do this if one or more of your files contain special data that needs to be at a particular location in memory. For example: data. This is not commonly done, but it may by useful on occasion. If you did not, the linker will attempt to open the file as an input file, as though it appeared on the command line. Note that this differs from an INPUT command, because the linker will not search for the file in the archive search path.
In an input section description, either the file name or the section name or both may be wildcard patterns. File name wildcard patterns only match files which are explicitly specified on the command line or in an INPUT command. The linker does not search directories to expand wildcards. If a file name matches more than one wildcard pattern, or if a file name appears explicitly and is also matched by a wildcard pattern, the linker will use the first match in the linker script.
You can change this by using the SORT keyword, which appears before a wildcard pattern in parentheses e. When the SORT keyword is used, the linker will sort the files or sections into ascending order by name before placing them in the output file. The map file shows precisely how input sections are mapped to output sections.
This example shows how wildcard patterns might be used to partition files. You can use this to place common symbols from a particular input file in one section while common symbols from other input files are placed in another section. For example:. In this case, the linker will use a different special section name for other types of common symbols.
This permits you to map the different types of common symbols into memory at different locations. This notation is now considered obsolete. The following example is a complete linker script.
Each keyword is followed by an expression in parentheses providing the value to store see section Expressions in Linker Scripts. The value of the expression is stored at the current value of the location counter. After storing the bytes, the location counter is incremented by the number of bytes stored. When both host and target are 32 bits, an expression is computed as 32 bits. If the object file format of the output file has an explicit endianness, which is the normal case, the value will be stored in that endianness.
When the object file format does not have an explicit endianness, as is true of, for example, S-records, the value will be stored in the endianness of the first input object file.
You may use the FILL command to set the fill pattern for the current section. It is followed by an expression in parentheses. Any otherwise unspecified regions of memory within the section for example, gaps left due to the required alignment of input sections are filled with the two least significant bytes of the expression, repeated as necessary.
A FILL statement covers memory locations after the point at which it occurs in the section definition; by including more than one FILL statement, you can have different fill patterns in different parts of an output section. If both are used, the FILL command takes precedence. The linker will not create output section which do not have any contents. This is for convenience when referring to input sections that may or may not be present in any of the input files.
If you use anything other than an input section description as an output section command, such as a symbol assignment, then the output section will always be created, even if there are no matching input sections. In this section we will describe the remaining section attributes. Each output section may have a type. The type is a keyword in parentheses. The following types are defined:. The linker normally sets the attributes of an output section based on the input sections which map into it.
You can override this by using the section type. The address expression which may appear in an output section description sets the VMA see section Output section address.
You can change that by using the AT keyword. The expression lma that follows the AT keyword specifies the load address of the section.
This feature is designed to make it easy to build a ROM image. The run-time initialization code for use with a program generated with this linker script would include something like the following, to copy the initialized data from the ROM image to its runtime address.
Notice how this code takes advantage of the symbols defined by the linker script. If a section is assigned to one or more segments, then all subsequent allocated sections will be assigned to those segments as well, unless they use an explicitly : phdr modifier. You can use :NONE to tell the linker to not put the section in any segment at all.
Any otherwise unspecified regions of memory within the output section for example, gaps left due to the required alignment of input sections will be filled with the two least significant bytes of the value, repeated as necessary. You can also change the fill value with a FILL command in the output section commands; see section Output section data.
At run time, some sort of overlay manager will copy the overlaid sections in and out of the runtime memory address as required, perhaps by simply manipulating addressing bits. This approach can be useful, for example, when a certain region of memory is faster than another. The sections are all defined with the same starting address. The load addresses of the sections are arranged such that they are consecutive in memory starting at the load address used for the OVERLAY as a whole as with normal section definitions, the load address is optional, and defaults to the start address; the start address is also optional, and defaults to the current value of the location counter.
Since the sections all run at the same address, it normally does not make sense for one section to refer directly to another. See section Other linker script commands. Any characters within secname which are not legal within C identifiers are removed. C or assembler code may use these symbols to move the overlaid sections around as necessary. At the end of the overlay, the value of the location counter is set to the start address of the overlay plus the size of the largest section. Here is an example.
C code to copy overlay. The above example could have been written identically as follows. The linker's default configuration permits allocation of all available memory. You can use it to describe which memory regions may be used by the linker, and which memory regions it must avoid.
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