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  • Comments (45)
    • prabhu
    • November 26th, 2010

    Greate… Work yaar. Its really interesting to see here to post linux related concept. All the best….

  1. answer me

  2. They are an instruction to the compiler to emit instructions that will cause branch prediction to favour the “likely” side of a jump instruction. This can be a big win, if the prediction is correct it means that the jump instruction is basically free and will take zero cycles. On the other hand if the prediction is wrong, then it means the processor pipeline needs to be flushed and it can cost several cycles. So long as the prediction is correct most of the time, this will tend to be good for performance.

    Like all such performance optimisations you should only do it after extensive profiling to ensure the code really is in a bottleneck, and probably given the micro nature, that it is being run in a tight loop. Generally the Linux developers are pretty experienced so I would imagine they would have done that. They don’t really care too much about portability as they only target gcc, and they have a very close idea of the assembly they want it to generate.

    These are macros that give hints to the compiler about which way a branch may go. The macros expand to GCC specific extensions, if they’re available.

    GCC uses these to to optimize for branch prediction. For example, if you have something like the following

    if (unlikely(x)) {
    dosomething();
    }

    return x;
    Then it can restructure this code to be something more like:

    if (!x) {
    return x;
    }

    dosomething();
    return x;
    The benefit of this is that when the processor takes a branch the first time, there is significant overhead, because it may have been speculatively loading and executing code further ahead. When it determines it will take the branch, then it has to invalidate that, and start at the branch target.

    Most modern processors now have some sort of branch prediction, but that only assists when you’ve been through the branch before, and the branch is still in the branch prediction cache.

    There are a number of other strategies that the compiler and processor can use in these scenarios. You can find more details on how branch predictors at Wikipedia: http://en.wikipedia.org/wiki/Branch_predictor

    • seshu
    • December 24th, 2010

    hi can you guide me in installing C in windows vista. i am facing problems with older versions like unable to get full screen of work space etc.

      • prabhu
      • December 24th, 2010

      Use the key ctrl+ for maximize the window size.

      If not working:
      Just check with your operating system architecture and install corresponding package. I hope 16 bit turbo c will work with vista.

      Otherwise you can also use gcc in windows with some modification.

      • devendra
      • February 1st, 2011

      goto http://www.bloodshed.net/devcpp.html

      and get the devcpp ide and install it.

      its a great one to work in windows and its having more flexible environment too.

  3. I just want to say Hi to Everyone!

    • prem
    • January 18th, 2011

    thanks

    • devendra
    • February 1st, 2011

    Hello all,

    here i just wanted to say about some simple concepts about the universal bootloader.

    universal bootloader U-boot is mainly used in embedded systems as u all guys know it because of its more confiurability and many bootable options.

    u can think of u-boot as a miniature operating system which will detect board configurations and sets up board for initial kernel loading procedures.

    Thanks to all,

    I will post new data about u-boot as early as possible.

    Thanks sandipan for preparing this website 🙂 .

    • Thanks for your new topic..
      U are always welcome..

      Anyway will you elaborate a bit on MINIATURE OPERATING SYSTEM 🙂

      ……

    • devendra
    • February 1st, 2011

    Hello all,

    The u-boot source consists of a board specific code,
    bootup code,
    cpu instructions (not fully)
    libraries for the specific architecture
    networking code
    and
    user commands to access the board hardware

    The board specific code performs the initialisation of the hardware registers and devices like ethernet, memory and cpu chips.

    The libraries contains apis that are used by most of the functions in the u-boot environment.

    The networking code implements a simple networking protocols like tftp, ping and even nfs.

    through tftp u can load the kernel image and rootfs from a device connected to the board through the ethernet.

    the simple flashing of the board will contains copying of the kernel image in a temporary location, erasing the flash address with the size of kernel image, copying the kernel image to the flash using the cp.b command.

    just as the kernel image the rootfs will be flashed in the same manner.

    u can check the data what the memory contains using md 0x01010101.

    u-boot supports the command line arguments like u can specify some of ur commands in the u-boot.

    like setenv loadkernel=’tftp 0x600000; bootm 0x600000′
    and u can run this command loadkernel using
    run loadkernel.

    this command will start the kernel from the ram.

    thanks to all,
    devendra.

    • devendra
    • February 9th, 2011

    A simple userspace timer

    1 #include
    2 #include
    3 #include
    4
    5 main(int a, char **v)
    6 {
    7 int val;
    8 double diff;
    9 struct timeval tv, tv1;
    10 gettimeofday (&tv, 0);
    11 if ((a == 1) || (a > 2)) {
    12 printf(“usage ./timer timerval\n”);
    13 return;
    14 }
    15 val = atoi(v[1]);
    16
    17 while (1) {
    18 gettimeofday (&tv1, 0);
    19 diff = ((int64_t)(tv1.tv_sec – tv.tv_sec) *

    1000 + (int64_t)(tv1.tv_usec – tv.tv_usec)/1000.0);
    20
    21 if ((diff) >= (float)val) {
    22 printf(“%f\n”, diff);
    23 memcpy (&tv, &tv1, sizeof(struct timeval));
    24 printf(“event\n”);
    25 }
    26 }
    27 }

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