Alias names can be added to existing memory regions created with the MEMORY Command command. Each name corresponds to at most one memory region.
REGION_ALIAS(alias, region)
The REGION_ALIAS
function creates an alias name alias for the
memory region region. This allows a flexible mapping of output sections
to memory regions. An example follows.
Suppose we have an application for embedded systems which come with various
memory storage devices. All have a general purpose, volatile memory RAM
that allows code execution or data storage. Some may have a read-only,
non-volatile memory ROM
that allows code execution and read-only data
access. The last variant is a read-only, non-volatile memory ROM2
with
read-only data access and no code execution capability. We have four output
sections:
.text
program code;
.rodata
read-only data;
.data
read-write initialized data;
.bss
read-write zero initialized data.
The goal is to provide a linker command file that contains a system independent
part defining the output sections and a system dependent part mapping the
output sections to the memory regions available on the system. Our embedded
systems come with three different memory setups A
, B
and
C
:
Section | Variant A | Variant B | Variant C |
.text | RAM | ROM | ROM |
.rodata | RAM | ROM | ROM2 |
.data | RAM | RAM/ROM | RAM/ROM2 |
.bss | RAM | RAM | RAM |
The notation RAM/ROM
or RAM/ROM2
means that this section is
loaded into region ROM
or ROM2
respectively. Please note that
the load address of the .data
section starts in all three variants at
the end of the .rodata
section.
The base linker script that deals with the output sections follows. It
includes the system dependent linkcmds.memory
file that describes the
memory layout:
INCLUDE linkcmds.memory SECTIONS { .text : { *(.text) } > REGION_TEXT .rodata : { *(.rodata) rodata_end = .; } > REGION_RODATA .data : AT (rodata_end) { data_start = .; *(.data) } > REGION_DATA data_size = SIZEOF(.data); data_load_start = LOADADDR(.data); .bss : { *(.bss) } > REGION_BSS }
Now we need three different linkcmds.memory
files to define memory
regions and alias names. The content of linkcmds.memory
for the three
variants A
, B
and C
:
A
Here everything goes into the RAM
.
MEMORY { RAM : ORIGIN = 0, LENGTH = 4M } REGION_ALIAS("REGION_TEXT", RAM); REGION_ALIAS("REGION_RODATA", RAM); REGION_ALIAS("REGION_DATA", RAM); REGION_ALIAS("REGION_BSS", RAM);
B
Program code and read-only data go into the ROM
. Read-write data goes
into the RAM
. An image of the initialized data is loaded into the
ROM
and will be copied during system start into the RAM
.
MEMORY { ROM : ORIGIN = 0, LENGTH = 3M RAM : ORIGIN = 0x10000000, LENGTH = 1M } REGION_ALIAS("REGION_TEXT", ROM); REGION_ALIAS("REGION_RODATA", ROM); REGION_ALIAS("REGION_DATA", RAM); REGION_ALIAS("REGION_BSS", RAM);
C
Program code goes into the ROM
. Read-only data goes into the
ROM2
. Read-write data goes into the RAM
. An image of the
initialized data is loaded into the ROM2
and will be copied during
system start into the RAM
.
MEMORY { ROM : ORIGIN = 0, LENGTH = 2M ROM2 : ORIGIN = 0x10000000, LENGTH = 1M RAM : ORIGIN = 0x20000000, LENGTH = 1M } REGION_ALIAS("REGION_TEXT", ROM); REGION_ALIAS("REGION_RODATA", ROM2); REGION_ALIAS("REGION_DATA", RAM); REGION_ALIAS("REGION_BSS", RAM);
It is possible to write a common system initialization routine to copy the
.data
section from ROM
or ROM2
into the RAM
if
necessary:
#include <string.h> extern char data_start []; extern char data_size []; extern char data_load_start []; void copy_data(void) { if (data_start != data_load_start) { memcpy(data_start, data_load_start, (size_t) data_size); } }