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========================================================= 7 0   P O I N T S   P R O J E C T =====================================================
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=============================================================== U S E F U L   T I P S ===========================================================
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For filesystem, use the disk named "test.img" in cse506 folder. Disk Image size is 1 MB

If new disk is to created, here are the following steps:

    1. Run the qemu command to create a 1 MB disk named "test.img" : qemu-img create test.img 1M 

    2. Run qemu with file = test.img

    3. For running qemu, run the executable file qstart.sh

For Shutdown : type shutdown on shell.
                            
For any queries, please contact SBUnix developers.

To format the disk change the value from false to true in init_disk.

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=============================================================== S Y S T E M   C A L L S ==========================================================
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    READ()
    Syntax       :  read(uint64_t fd_type, uint64_t addr, uint64_t length)
    Description  :  Attempts to read up to length number of bytes using file descriptor fd_type into the buffer starting at addr.
    Dependencies :  Always open a file using OPEN system call before reading and close the file after completion. Returns -1 for files with WRONLY flag set.
    return       :  returns length of bytes read into the buffer or -1 on error


    WRITE() 
    Syntax       :  write(int n, char *str, int len)
    Description  :  Attempts to write up to length number of bytes into file descriptor fd_type from the buffer starting at addr.
    Dependencies :  Always open a file using OPEN system call before writing and close the file after completion. 
                    Returns -1 for files opened with O_RDONLY flag set. Cannot do write on files in tarfs.
    return       :  returns length of bytes written into the buffer or -1 on error
    
    BRK()
    Syntax       :  brk(uint64_t no_of_pages)
    Description  :  Increase size of vma by no_of_pages.
    return       :  returns the virtual address in heap
    
    FORK()
    Syntax       :  fork()
    Description  :  Creates a child process. Returns pid = 0 for child and non zero for parent.
    return       :  returns the pid for parent process or zero to child 
    
    EXECVPE()
    Syntax       :  execvpe(char *file, char *argv[], char *envp[])
    Description  :  Executes a file along with any necessary arguments argv[] in the environment envp[].
    return       :  on failure returns return -1, on success doesnot return
    
    WAIT()
    Syntax       :  wait(uint64_t status)
    Description  :  Parent waits for a child process to exit.
    Dependencies :  Returns -1 if parent has no children.
    
    WAITPID()
    Syntax       :  waitpid(uint64_t fpid, uint64_t fstatus, uint64_t foptions)
    Description  :  Parent waits for a child process with fpid to complete.
    Dependencies :  Returns -1 if parent has no children. If particular child pid does not exit then waits for any of the children to exit.
    
    EXIT()
    Syntax       :  exit(int status)
    Description  :  Terminate the calling process immediately.

    YIELD()
    Syntax       :  yield()
    Description  :  Hands control over to next READY process.
    
    MMAP()
    Syntax       :  mmap(uint64_t addr, uint64_t nbytes, uint64_t flags)
    Description  :  Allocate nbytes of memory to Current Process at the addr location. 
                    If no addr is given, alloc from vma end of last vma list element.
    
    MUNMAP()
    Syntax       :  sys_munmap(uint64_t* addr, uint64_t length)
    Description  :  Iterate through vma list and free all anon-type vma. 
                    Check if there are any other vmas mappped on this addr if not then free the page.

    GETPID()
    Syntax       :  getpid()
    Description  :  returns the process id of currently running process.
    
    GETPPID()
    Syntax       :  getppid()
    Description  :  returns the parent's process id of currently running process.
    
    LISTPROCESS()
    Syntax       :  listprocess()
    Description  :  gives the list of all currently running process.
    
    OPENDIR()
    Syntax       :  opendir(uint64_t* entry, uint64_t* directory)
    Description  :  returns a directory struct with list of all inode/files in it.
    return       :  on error returns directory struct with values set to null.   
   
    READDIR()
    Syntax       :  readdir(DIR* node)
    Description  :  To list contents in directory, call readdir iteratively. 
    Dependencies :  Read directory is followed after open directory system call.
    return       :  returns the pointer to dirent structure . Returns NULL if reached end of directory or on error. 

    CLOSEDIR() 
    Syntax       :  closedir(uint64_t* node)
    Description  :  closes the list of all the files in the node Directory structure.
    Dependencies :  Close directory is followed after open directory system call.
    
    OPEN()
    Syntax       :  open(char* dir_path, uint64_t flags)
    Description  :  opens a files and returns a file descriptor. Open files from tarfs and disk depending on dir_path.
    Dependencies :  flags can be O_CREAT, O_RDONLY, O_WRONLY, O_APPEND, O_TRUNC, O_RDWR
    return       :  returns file descriptor number. open on a directory returns -1 as file descriptor.
    
    CLOSE()
    Syntax       :  close(int fd)
    Description  :  closes a file with fd file descriptor. Copies all file's vma's onto disk before closing only.
    
    SLEEP()
    Syntax       :  sleep(int msec)
    Description  :  Changes the state of current task to sleep for msec milliseconds.
    
    CLEAR()
    Syntax       :  clear()
    Description  :  Clears the screen.
    
    SEEK()
    Syntax       :  seek(uint64_t fd_type, int offset, int whence) 
    Description  :  moves the current file pointer by offset number of bytes in the file.
    Dependencies :  whence can be SEEK_SET, SEEK_CUR, SEEK_END. This determines the offset from start , current pointer or end of file.

    MKDIR()
    Syntax       :  mkdir( uint64_t dir)
    Description  :  Creates new directory using path given by dir. Returns -1 for already existing directory
    Dependencies :  Creates one level of directory at a time.

    SHUTDOWN()
    Syntax       :  shutdown() 
    Description  :  exit all processes.