Use of the : operator in C [duplicate]

by

They’re bit-fields, an example being that unsigned int addr:9; creates an addr field 9 bits long.

It’s commonly used to pack lots of values into an integral type. In your particular case, it defining the structure of a 32-bit microcode instruction for a (possibly) hypothetical CPU (if you add up all the bit-field lengths, they sum to 32).

The union allows you to load in a single 32-bit value and then access the individual fields with code like (minor problems fixed as well, specifically the declarations of code and test):

#include <stdio.h>

struct microFields {
    unsigned int addr:9;
    unsigned int cond:2;
    unsigned int wr:1;
    unsigned int rd:1;
    unsigned int mar:1;
    unsigned int alu:3;
    unsigned int b:5;
    unsigned int a:5;
    unsigned int c:5;
};

union micro {
    unsigned int microCode;
    struct microFields code;
};

int main (void) {
    int myAlu;
    union micro test;
    test.microCode = 0x0001c000;
    myAlu = test.code.alu;
    printf("%d\n",myAlu);
    return 0;
}

This prints out 7, which is the three bits making up the alu bit-field.

More Related Contents:

  1. Can you tell me specific what this program is doing? [closed]
  2. What does “static” mean in C?
  3. What does double underscore ( __const) mean in C?
  4. Arrow operator (->) usage in C
  5. How do you pass a function as a parameter in C?
  6. C function syntax, parameter types declared after parameter list
  7. Effects of the extern keyword on C functions
  8. Why are hexadecimal numbers prefixed with 0x?
  9. How do you format an unsigned long long int using printf?
  10. An example of use of varargs in C
  11. What is the full “for” loop syntax in C?
  12. In C, what is the correct syntax for declaring pointers?
  13. Parenthesis surrounding return values in C
  14. Is the last comma in C enum required?
  15. Two semicolons inside a for-loop parentheses
  16. Why doesn’t C have binary literals?
  17. How can I throw an exception in C?
  18. What does a “CALLBACK” declaration in C do?
  19. 3 plus symbols between two variables (like a+++b) in C [duplicate]
  20. What does \x mean in C/C++?
  21. C fundamentals: double variable not equal to double expression?
  22. In a C function declaration, what does “…” as the last parameter do?
  23. Returning function pointer type
  24. What constitutes a “valid” C Identifier?
  25. Reversing a string in C
  26. Operator precedence (bitwise ‘&’ lower than ‘==’)
  27. GCC with -std=c99 complains about not knowing struct timespec
  28. Why are malloc() and printf() said as non-reentrant?
  29. What is time(NULL) in C?
  30. Why does malloc allocate a different number of bytes than requested?

Leave a Comment