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

FileSystem CRC

Name:

 FileSystem

Comment:

 Data structures to represent the Nachos file system.

A file system is a set of files stored on disk, organized into directories. Operations on the file system have to do with "naming" -- creating, opening, and deleting files, given a textual file name. Operations on an individual "open" file (read, write, close) are to be found in the OpenFile class (openfile.h).

We define two separate implementations of the file system. The "STUB" version just re-defines the Nachos file system operations as operations on the native UNIX file system on the machine running the Nachos simulation. This is provided in case the multiprogramming and virtual memory assignments (which make use of the file system) are done before the file system assignment.

The other version is a "real" file system, built on top of a disk simulator. The disk is simulated using the native UNIX file system (in a file named "DISK").

In the "real" implementation, there are two key data structures used in the file system. There is a single "root" directory, listing all of the files in the file system; unlike UNIX, the baseline system does not provide a hierarchical directory structure. In addition, there is a bitmap for allocating disk sectors. Both the root directory and the bitmap are themselves stored as files in the Nachos file system -- this causes an interesting bootstrap problem when the simulated disk is initialized.

Collaborators:

Responsibilities: 

#ifdef FILESYS_STUB             // Temporarily implement file system calls as
                                // calls to UNIX, until the real file system
                                // implementation is available
class FileSystem {
  public:
    FileSystem(bool format) {}

    bool Create(char *name, int initialSize) {
        int fileDescriptor = OpenForWrite(name);

        if (fileDescriptor == -1) return FALSE;
        Close(fileDescriptor);
        return TRUE;
        }

    OpenFile* Open(char *name) {
          int fileDescriptor = OpenForReadWrite(name, FALSE);

          if (fileDescriptor == -1) return NULL;
          return new OpenFile(fileDescriptor);
      }

    bool Remove(char *name) { return Unlink(name) == 0; }

};

#else // FILESYS
class FileSystem {
  public:
    FileSystem(bool format);            // Initialize the file system.
                                        // Must be called *after* "synchDisk"
                                        // has been initialized.
                                        // If "format", there is nothing on
                                        // the disk, so initialize the directory
                                        // and the bitmap of free blocks.

    bool Create(char *name, int initialSize);
                                        // Create a file (UNIX creat)

    OpenFile* Open(char *name);         // Open a file (UNIX open)

    bool Remove(char *name);            // Delete a file (UNIX unlink)

    void List();                        // List all the files in the file system

    void Print();                       // List all the files and their contents

  private:
   OpenFile* freeMapFile;               // Bit map of free disk blocks,
                                        // represented as a file         
   OpenFile* directoryFile;             // "Root" directory -- list of
                                        // file names, represented as a file
};