RTEMS 4.5.1-pre3 On-Line Library

Linker Script Initialized Data

BSP and Device Driver Development Guide

4.5: Initialized Data

Now there's a problem with the initialized data: the .data section has to be in RAM as this data may be modified during the program execution. But how will the values be initialized at boot time?

One approach is to place the entire program image in RAM and reload the image in its entirety each time the program is run. This is fine for use in a debug environment where a high-speed connection is available between the development host computer and the target. But even in this environment, it is cumbersome.

The solution is to place a copy of the initialized data in a separate area of memory and copy it into the proper location each time the program is started. It is common practice to place a copy of the initialized .data section at the end of the code (.text) section in ROM when building a PROM image. The GNU tool objcopy can be used for this purpose.

The following figure illustrates the steps a linked program goes through to become a downloadable image.

+--------------+     +--------------------+
| .data    RAM |     | .data          RAM |
+--------------+     +--------------------+
| .bss     RAM |     | .bss           RAM |
+--------------+     +--------------------+         +----------------+
| .text    ROM |     | .text          ROM |         |     .text      |
+--------------+     +--------------------+         +----------------+
                     | copy of .data  ROM |         | copy of .data  |
                     +--------------------+         +----------------+

      Step 1                Step 2                       Step 3

In Step 1, the program is linked together using the BSP linker script.

In Step 2, a copy is made of the .data section and placed after the .text section so it can be placed in PROM. This step is done after the linking time. There is an example of doing this in the file $RTEMS_ROOT/make/custom/gen68340.cfg:

# make a PROM image using objcopy
m68k-rtems-objcopy \
--adjust-section-vma .data= \

`m68k-rtems-objdump --section-headers \
$(basename $@).exe \
| awk '[...]` \
$(basename $@).exe

NOTE: The address of the "copy of .data section" is created by extracting the last address in the .text section with an awk script. The details of how this is done are not relevant.

Step 3 shows the final executable image as it logically appears in the target's non-volatile program memory. The board initialization code will copy the ""copy of .data section" (which are stored in ROM) to their reserved location in RAM.

BSP and Device Driver Development Guide