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Alias names can be added to existing memory regions created with the MEMORY 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); } }
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