Archive for the ‘Embedded’ Category

Building applications for ERC32

October 2, 2007

A while ago, I answered a message, in the RTEMS users mailing list, about how to build raw applications for the ERC32 architecture. With raw applications I mean applications that are not dependent of RTEMS, which is the usual RTOS for this architecture. I am now writing about it just to make it easier for people to find it and to add some information to the explanation.

Before starting with the process, you need to get and build the RTEMS toolchain in order to compile your future applications (I used RTEMS 4.6.1). This is because the specific toolchain for ERC32 non-RTEMS applications is no longer maintained. Note, that you don’t need to install RTEMS itself, but you will need the source code. This video shows, step by step, how to download and build these tools for the i386 BSP, which should be almost identical for ERC32.

I’m not sure if what I’m going to explain is the best way to do it, but it has worked for me, and I have ran programs generated this way in a real board for a long time. I also think this may be useful for other architectures as well.

1. You need to get the following source files from the ERC32 BSP and copy them to your application’s directory:

    cpukit/score/cpu/sparc/asm.h
    cpukit/score/cpu/sparc/rtems/score/sparc.h
    cpukit/score/cpu/sparc/rtems/score/cpu.h
    c/src/lib/libbsp/sparc/shared/start.S
    c/src/lib/libcpu/sparc/reg_win/window.S
    c/src/lib/libcpu/sparc/syscall/syscall.S

2. Modify start.S so it does not call any RTEMS code, that is, commenting these lines:

/*
    call    __bsp_board_init
    nop

    set     (SYM(rdb_start)), %g6   ! End of work-space area
    st      %sp, [%g6]

    set     (SYM(Configuration)), %l1
    ld      [%l1+4], %l3            ! work_space_size
    sub     %sp, %l3, %sp           ! set %sp to area below work_space
    andn    %sp, 0x0f, %sp          ! align stack on 16-byte boundary
*/

3. Compile start.S, window.S and syscall.S. You can remove many things from the header files, but that’s up to you.

sparc-rtems4.6.1-gcc -DASM -c -o start.o start.S

4. Compile your C files (e.g. test.c).

sparc-rtems4.6.1-gcc -O4 -Wall -mcpu=cypress -c -o test.o test.c

5. Link your program. It will not work right now, as you need to generate two final files, my_bsp_specs and linkcmds.

sparc-rtems4.6.1-gcc -mcpu=cypress -Betc -specs my_bsp_specs -o test
window.o syscall.o test.o

Note the arguments -B and -specs. -specs is used to specify a file with options to override the default switches passed to cc1, cc1plus, as, ld… By default, RTEMS uses the file bsp_specs (found in c/src/lib/libbsp/sparc/erc32). You can edit it and see that it automaticaly links with the RTEMS libraries and adds its own start.S object file. The -B flag specifies where to find the executables, libraries, include files, and data files of the compiler itself. I have used an etc directory where I have stored my_bsp_specs. If any of the needed files is not found by the compiler it will automatically search its own paths.

So, what we need to do is to create our own file, my_bsp_specs, which will not use any RTEMS library and which will point to our compiled start.S object file.

Basically,

*endfile:
crtend.o%s crtn.o%s

*lib:
-T linkcmds

*startfile:
start.o crti.o%s crtbegin.o%s

*link:
-dc -dp -N -e start

The %s indicates to search the file in the compiler specific directories. I have removed the %s from start.o so it will search for it in the current directory.

Another important thing to note in this file is the -T linkcmds option. This indicates the linker script, which describes where the code and data for the application will reside in memory, to be used by the GNU linker. Fortunately, we can use the one that comes with RTEMS (found in c/src/lib/libbsp/sparc/erc32/startup), so copy it to the etc directory.

Finally, note that when linking, we have not specified the start.o file as it is already done in the my_bsp_specs file.

You might also find more information in the RTEMS BSP and Device Driver Development Guide.

Code size optimizations

September 28, 2007

During the last two years I have had the opportunity to write embedded software for the SPARC ERC32 architecture. The first piece of software we have written is an application that needs to fit in a 64 Kbytes PROM with a minimal running system that can execute periodic tasks, send packets over a network, load another application stored in an EEPROM that can be remotely patched and many other things. Before starting to write it from scratch we tried to fit RTEMS (a Real-Time Operating System) in the 64 Kbytes. We achieved a running RTEMS in 55 Kbytes, but there was only 10 Kbytes left (note that modern versions of RTEMS are smaller).

The following tips are the ones I have found useful to decrease my application size (they might not work in all architectures). Note that all the tips are C oriented and that gcc has been used.

I would really appreciate if you can share your tricks and I will be glad to include them in the list.

Compress application code

As the application is stored in a PROM, you will normally have a minimal code to perform some initializations and checks (e.g. RAM tests), and the rest of the code for the application itself. The main application will run from RAM, so you could compress the application using a simple compression algorithm such as LZSS, uncompress the application to RAM and run it. The decompression algorithm will be obviously uncompressed.

If you are working with an ERC32 processor check the mkprom-erc32 tool.

Use inline functions judiciously

As it is well-known, performance issues should not be treated while developing. So, writing a lot of inline functions might not have great performance effects, but could have big size penalties if you don’t use them judiciously.

Compiler optimization flags

If performance is not important in your application (i.e. you have no hard real-time requirement), you can use the size optimization options provided by your compiler. For example, in gcc, you can use the -Os option to decrease the executable size. We reduced 5 Kbytes.

Note, that this will avoid inline functions optimization, but you can still tell gcc to include them passing the -finline-functions option.

Split functions into separate files

If you are developing different applications that might share most of the code, you may think on writing an static library. The point is that having a reusable library might affect the executable size. This is because, probably, not all the functions compiled will be used in all the applications. When gcc compiles a source file, all the functions in it will be linked in the final binary even if your code does not use them. Of course, if non of the functions of the source file is being used, they will not be linked. Splitting functions into separate files might solve the problem.

There is one way to avoid this (I haven’t tried it) with gcc using the -ffunction-sections option. This creates a separate section for every function. The linker can then use the –gc-sections option to discard unused sections.

Separate debug and release code

While writing embedded software you may need to have debug and release code. Clear examples are the printing or logging functions that might not be necessary in production code. If this is the case, separate the debug and release code. In C, you could have a serial port printing function in debug and an empty macro in release. This can be achieved via a building system, separating the definitions in different files (which I recommend), or using macros.

Do not use packed structures extensibly

By definition, packed structures are not aligned, this means that, depending on the architecture, the compiler needs to generate extra code to get members of structures. So, using packed structures extensibly may increase your application size and may decrease the performance.

Another reason for not using packed structures is portability.

Enumerations vs. constant objects

As described in this article, constant objects, such as:

unsigned int const ITEM_ID = 0x01200020;

may increase your application size. So, if you really have a few bytes left, you might be interested in changing the definition by using an enumeration:

enum { ITEM_ID = 0x01200020 };