The linker's default configuration permits allocation of all available
memory. You can override this by using the MEMORY command.
The MEMORY command describes the location and size of blocks of
memory in the target. You can use it to describe which memory regions
may be used by the linker, and which memory regions it must avoid. You
can then assign sections to particular memory regions. The linker will
set section addresses based on the memory regions, and will warn about
regions that become too full. The linker will not shuffle sections
around to fit into the available regions.
A linker script may contain many uses of the MEMORY command,
however, all memory blocks defined are treated as if they were
specified inside a single MEMORY command. The syntax for
MEMORY is:
MEMORY
{
name [(attr)] : ORIGIN = origin, LENGTH = len
...
}
The name is a name used in the linker script to refer to the
region. The region name has no meaning outside of the linker script.
Region names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each memory region
must have a distinct name within the MEMORY command. However you can
add later alias names to existing memory regions with the REGION_ALIAS
command.
The attr string is an optional list of attributes that specify whether to use a particular memory region for an input section which is not explicitly mapped in the linker script. As described in SECTIONS, if you do not specify an output section for some input section, the linker will create an output section with the same name as the input section. If you define region attributes, the linker will use them to select the memory region for the output section that it creates.
The attr string must consist only of the following characters:
If a unmapped section matches any of the listed attributes other than ‘!’, it will be placed in the memory region. The ‘!’ attribute reverses this test, so that an unmapped section will be placed in the memory region only if it does not match any of the listed attributes.
The origin is an numerical expression for the start address of
the memory region. The expression must evaluate to a constant and it
cannot involve any symbols. The keyword ORIGIN may be
abbreviated to org or o (but not, for example,
ORG).
The len is an expression for the size in bytes of the memory
region. As with the origin expression, the expression must
be numerical only and must evaluate to a constant. The keyword
LENGTH may be abbreviated to len or l.
In the following example, we specify that there are two memory regions available for allocation: one starting at ‘0’ for 256 kilobytes, and the other starting at ‘0x40000000’ for four megabytes. The linker will place into the ‘rom’ memory region every section which is not explicitly mapped into a memory region, and is either read-only or executable. The linker will place other sections which are not explicitly mapped into a memory region into the ‘ram’ memory region.
MEMORY
{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
}
Once you define a memory region, you can direct the linker to place specific output sections into that memory region by using the ‘>region’ output section attribute. For example, if you have a memory region named ‘mem’, you would use ‘>mem’ in the output section definition. See Output Section Region. If no address was specified for the output section, the linker will set the address to the next available address within the memory region. If the combined output sections directed to a memory region are too large for the region, the linker will issue an error message.
It is possible to access the origin and length of a memory in an
expression via the ORIGIN(memory) and
LENGTH(memory) functions:
_fstack = ORIGIN(ram) + LENGTH(ram) - 4;