BuS

• Volatile memory


  Requires power to maintain the stored information. If power is lost, the stored data is also lost.
• Volatile memory


  • RAM, CPU caches, etc.
• Non-volatile memory


  Retains stored information even without power.
• Non-volatile memory


  • hard drives (HDD), solid state drives (SSD), etc.
• Volatile is faster than non-volatile
• The faster the memory, the more expensive per byte
• Memory hierarchy


  Latency, Cost per byte
• Memory types in order of increasing latency and decreasing cost per byte

  • CPU registers
  • CPU Caches (SRAM)
  • Main memory (DRAM)
  • Flash storage (SSD)
  • Magnetic storage (HDD)
  • Non-volatile memory (NVRAM)
• Every device might work differently, we can't expose them directly to users + we need to provide sharing/isolation safely
• The OS needs to show a clean interface of storage devices to user space
• File


  A logical unit of information stored on a disk in a persistent way. It has a name, attributes, and contains data.
• File naming rules


  • Special characters/spaces may or may not be allowed
  • Length: 8 characters in MS-DOS, usually 255 or more today
  • Case-sensitivity: UNIX file systems are usually case-sensitive, Windows file systems like NTFS are insensitive
  • Extension support: File names can be a pair <name.extension>, or just a name
• File structure types


  • Unstructured raw data
  • Record based
  • Tree
• From now on, we will only considered unstructured raw data, as this is what most operating systems use.
• File types


  • Regular files
  • Directories
  • Character special files
  • Block special files
• ELF files are a special type of regular file used for executables on UNIX systems
• File attributes


  • Permissions
  • Password
  • Creator
  • Owner
  • Hidden flag
  • Size
  • Creation time
  • Last modification
  • Last access
• File operations


  • Create
  • Delete
  • Get/Set attributes
  • Rename
  • Open
  • Close
  • Read/Write
  • Seek
• File descriptor


  Represents an open file. Contains access mode, position, file identifier.
• UNIX file descriptors 0, 1 and 2 have special meaning: standard input (stdin), output (stdout) and error (stderr) respectively.
• Directories store a list of files of any type, e.g., regular, directories, etc.
• Directory topologies


  • Flat
  • Hierarchical
• Path


  Represents the "address" of a file in a hierarchical directory system. Can be absolute or relative.
• Directory operations


  • Create
  • Delete
  • OpenDir
  • CloseDir
  • ReadDir
• Hard link


  An entry in a directory that links to an already existing file. A file can appear in multiple directories.
• Symbolic link


  A special type of file that contains a reference to another file or directory.
• Get cwd:


  pwd
• Change cwd:


  cd <new path>
• Special path characters


  • . = cwd
  • .. = parent directory
  • e.g., ../../bin/ls
• Hidden files
• Directory Operations


  Functions provided to users to manipulate directories
• Create


  Create an empty directory, with . and .. special files
• Delete


  Delete a directory, only if it is empty
• OpenDir


  Open a directory before reading its content, returns a file descriptor
• CloseDir


  Close a directory to free the file descriptor
• ReadDir


  Read the content of the directory, i.e., the list of files it contains, in a standardized format
• One last type of directory operations is heavily related to files: links.
• Hard Links


  An entry in a directory that links to an already existing file
• Symbolic Links (symlinks)


  A special file type that contains a path to another file
• Link


  Create a hard link