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FileHeader CRC

Name:

 FileHeader

Comment:

 The following class defines the Nachos "file header" (in UNIX terms, the "i-node"), describing where on disk to find all of the data in the file. The file header is organized as a simple table of pointers to data blocks.

The file header data structure can be stored in memory or on disk. When it is on disk, it is stored in a single sector -- this means that we assume the size of this data structure to be the same as one disk sector. Without indirect addressing, this limits the maximum file length to just under 4K bytes.

There is no constructor; rather the file header can be initialized by allocating blocks for the file (if it is a new file), or by reading it from disk.

Collaborators:

Responsibilities: 

class FileHeader {
  public:
    bool Allocate(BitMap *bitMap, int fileSize);// Initialize a file header,
                                                //  including allocating space
                                                //  on disk for the file data
    void Deallocate(BitMap *bitMap);            // De-allocate this file's
                                                //  data blocks

    void FetchFrom(int sectorNumber);   // Initialize file header from disk
    void WriteBack(int sectorNumber);   // Write modifications to file header
                                        //  back to disk

    int ByteToSector(int offset);       // Convert a byte offset into the file
                                        // to the disk sector containing
                                        // the byte

    int FileLength();                   // Return the length of the file
                                        // in bytes

    void Print();                       // Print the contents of the file.

  private:
    int numBytes;                       // Number of bytes in the file
    int numSectors;                     // Number of data sectors in the file
    int dataSectors[NumDirect];         // Disk sector numbers for each data
                                        // block in the file
};