[ RCOS.java Home | RCOS Home | David's Home ]

BitMap CRC

Name:

 BitMap

Comment:

 The following class defines a "bitmap" -- an array of bits, each of which can be independently set, cleared, and tested.

Most useful for managing the allocation of the elements of an array -- for instance, disk sectors, or main memory pages. Each bit represents whether the corresponding sector or page is in use or free.

Collaborators:

Responsibilities: 

class BitMap {
  public:
    BitMap(int nitems);         // Initialize a bitmap, with "nitems" bits
                                // initially, all bits are cleared.
    ~BitMap();                  // De-allocate bitmap

    void Mark(int which);       // Set the "nth" bit
    void Clear(int which);      // Clear the "nth" bit
    bool Test(int which);       // Is the "nth" bit set?
    int Find();                 // Return the # of a clear bit, and as a side
                                // effect, set the bit.
                                // If no bits are clear, return -1.
    int NumClear();             // Return the number of clear bits

    void Print();               // Print contents of bitmap

    // These aren't needed until FILESYS, when we will need to read and
    // write the bitmap to a file
    void FetchFrom(OpenFile *file);     // fetch contents from disk
    void WriteBack(OpenFile *file);     // write contents to disk

  private:
    int numBits;                        // number of bits in the bitmap
    int numWords;                       // number of words of bitmap storage
                                        // (rounded up if numBits is not a
                                        //  multiple of the number of bits in
                                        //  a word)
    unsigned int *map;                  // bit storage
};