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register variables

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• Register variables
  • Register variables:

    The keyword register is used to define register local variables in C Only a local variable can be defined as register The keyword register is a non-binding hint given to the C compiler that the variable will be access very frequently The C compiler has the option to use one of the registers of the CPU as the storage location for the register variable !!! (Because storing the variable in a register will reduce the access time for that variable)

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  • Example: (compiled on an Intel platform)

    int main( int argc, char *argv[] ) { int a; a = 1234567; printf("%d\n", a); }

    translates the statement a = 1234567 into:

    movl $1234567, -4(%rbp) // == move.l #1234567, -4(a6)

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to compile the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc -S register1.c Check out the file:          ./register1.s

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    However:

    int main( int argc, char *argv[] ) { register int a; a = 1234567; printf("%d\n", a); }

    translates the statement a = 1234567 into:

    movl $1234567, %ebx // Similar to: move.l #1234567, d0 *** Uses register ebx for variable a !!!

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to compile the program:

    Right click on link(s) and save in a scratch directory To compile:   gcc -S register2.c Check out the file:          ./register2.s

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  • Final Notes:

    You cannot use the reference operator & on a register variable: & will return the address of a variable Variables stored in a register does not have a (memory) address !!! In some C compilers, the register keyword will only work for the type int (and maybe float and double) (Because registers can only store basic data types) A register declaration is only a "recommendation" to the C compiler, it may be ignored...