CS110: Principles of Computer Systems

Autumn 2021
Jerry Cain
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Lecture 03: Filesystems II, Design Principles

• There are two major design principles relevant to our discussion of filesystems.
  • Modularity and Layering
    • ​Subdivision of a larger system into a collection of smaller subsystems, which themselves may be further subdivided into even smaller sub-subsystems.
    • Example: filesystems, which use a form of modularity called layering, which is the organization of several modules that interact in some hierarchical manner, where each layer typically only opens its interface to the module above it.
      • symbolic link layer
      • absolute path name layer
      • path name layer
      • file name layer
      • inode number layer
      • file layer
      • block layer

Lecture 03: Filesystems II, Design Principles

• There are two major design principles relevant to our discussion of filesystems.
  • Modularity and Layering
    • Subdivision of a larger system into a collection of smaller subsystems, which themselves may be further subdivided into even smaller sub-subsystems.
    • Example: g++, which chains together a series of components in a pipeline (which is another form of layering).
      • the preprocessor, which manages #includes, #defines, and other preprocessor directives to build a translation unit that is fed to...
      • the lexer, which reduces the translation unit down to a stream of tokens fed in sequence to...
      • the parser, which groups tokens into syntactically valid constructs then semantically verified by...
      • the semantic analyzer, which confirms that the syntactically valid constructs make sense and respect C++'s type system, so that x86 instructions can be emitted by...
      • the code generator, which translate your C++ code into equivalent machine code.

Lecture 03: Filesystems II, Design Principles

• There are two major design principles relevant to our discussion of filesystems.
  • Modularity and Layering
    • Subdivision of a larger system into a collection of smaller subsystems, which themselves may be further subdivided into even smaller sub-subsystems.
    • Example: computer networks, which rely on a programming model known as TCP/IP, so named because its two most important protocols (TCP for Transmission Control Protocol, IP for Internet Protocol) were the first to be included in the standard.
      • TCP/IP specifies how data should be packaged, transmitted, routed, and received.
      • The network stack implementation is distributed down through four different layers:
        • application layer (the highest layer in the stack)
        • transport layer
        • network layer
        • link layer (the lowest layer in the stack)
      • We learn application-layer networking in CS110 around Week 7.
      • CS144 teaches all four layers in detail and how each
        layer interacts with the one beneath it.

Lecture 03: Filesystems II, Design Principles

• There are two major design principles relevant to our discussion of filesystems.
  • Modularity and Layering
  • Naming and Name Resolution​
    • Names provide a way to refer to system resources, and name resolution is a means for converting between human-readable names and machine-friendly ones.
    • Examples:
      • Humans prefer absolute and relative pathnames to identify files, and computers work better with inode and block numbers. You'll spend a good amount of energy with Assignment 2 managing the discovery of inode numbers and file block contents given a file's name.
      • Humans prefer names like www.google.com, but computers prefer numbers like 74.125.239.51.  The Domain Name System—you might know it as DNS—is a distributed database that maps every domain name to one or more associated IP addresses.