Thread: RAM vs Cache & Registers

  1. #1
    Registered User
    Join Date
    Sep 2003

    RAM vs Cache & Registers

    Does the compiler typically automatically use RAM, or do you have to explicitly tell it to use RAM? Does using ram have something to do with dynamic allocation.

    Sorry if this seems newbish, I just really want to know and I can't seem to find a clear answer.

  2. #2
    Pursuing knowledge confuted's Avatar
    Join Date
    Jun 2002
    It will use registers for performing operations (there is no other option). The processor will automatically cache the things in RAM which it thinks will be accessed next. Any part of your program that doesn't fit in the registers (which will be most of it) will go to the RAM, or virtual memory. It's kinda OS dependent... but no, you don't need to worry about it when you're coding.

  3. #3
    Join Date
    Sep 2003
    Register & cache use is part of the optimization process, some compilers will try to put as much as possible in the register and let the memory as much as possible in the cache when it is necessary to limit the acces to the RAM.

  4. #4
    Registered User
    Join Date
    Jan 2003
    To further confuse you, the volitile keyword can be used to force the compiler to place something that it might be tempted to ignore or use a register for in ram. For example
      int max = 8;
      for(int i=0; i < max; ++i) ...
    here, as long as the compiler sees that max is never modified in the body of the for loop the compiler might very well simply compare i to 8 at every step of the loop, and might even unroll the loop for you (duplicateing the body of the loop and loop over both 4 times)

    volitile int max = 8;

    forces the compiler to name a location in ram "max" and read this location every time through the loop. Normally we don't care where things live, this is exactly the reason we are using a compiler rather than writing assembly language really, computers are really good at making these sorts of decisions.

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