CMPSC 311 - Introduction to Systems Programming

UNIX Origins

  • Multics project started in 1964 by MIT, General Electric, and Bell Labs.
  • Though the project failed, it generated numerous valuable concepts.
  • Thompson and Ritchie worked on Multics and later ported "Space Travel" to PDP-7.
  • They built tools and a file system for PDP-7, leading to the creation of UNIX.
  • Key attributes of UNIX:
    • Multiuser: Supports multiple users simultaneously, each with their own terminal.
    • Multitasking: Supports multiple programs running concurrently.
    • Portability: Only the lowest software layers need reimplementation when moving to different hardware.

UNIX Variants

  • (A visual representation of UNIX variants was shown, but specific details aren't provided in the text.)

Linux Origins

  • GNU (GNU is Not UNIX) is a free software project initiated by Richard Stallman in 1983.
  • It produced essential tools like gcc, gdb, bash, and emacs.
  • GNU also developed an operating system kernel called Hurd, but it's not widely used.
  • Linux is an operating system kernel.
  • A Linux distribution comprises the Linux kernel, user tools, and a desktop environment.
  • Linus Torvalds announced the project:
    • He was developing a free operating system (a hobby project, not intended to be large or professional like GNU) for 386/486 AT clones.

Linux Software Layers

  • OS kernel: Direct interaction with hardware/firmware.
  • System calls: Interface to the kernel.
  • System libraries: Wrappers around system calls.
  • Programming language libraries: Extends system libraries.
  • System utilities: Application-independent tools (e.g., fsck, fdisk, ifconfig, mknod, mount, nfsd).
  • Command interpreter/shell: User interface (in a terminal program).
  • Application libraries: Application-specific tools.
  • Applications: Complete programs for general users.
  • Some applications have their own command shells and programming languages (e.g., Perl, Python).

Linux Distributions

  • Semi-Commercial systems emerged since 1991, including Red Hat, SUSE/Novell, Caldera (defunct, SCO), Debian, Mandrake/Mandriva, Slackware, Gentoo, Ubuntu, Knoppix, Fedora.
  • distrowatch.com provides a list of Linux distributions.
  • Android, based on Linux, appeared in 2003.

Open Source

  • Many software systems today are distributed as open source.
  • Open-source software is licensed to allow users to review, modify, and distribute the source code without cost.
  • Variations exist regarding the ability to charge for derived software or prohibiting charges for derivative works.
  • The concept of "free beer" (gratis) is contrasted with "free speech" (libre).

Operating Systems

  • Software that:
    1. Interacts directly with hardware.
      • The OS is trusted to do this; user-level programs aren't.
      • The OS needs to be ported to new hardware; user-level programs are portable.
    2. Manages hardware resources (allocates, schedules, protects).
      • It decides which programs can access specific files, memory, screen pixels, and when.
    3. Abstracts away hardware devices.
      • Provides high-level, convenient, portable abstractions (e.g., files vs. disk blocks).
  • The OS is essentially a program running directly on the hardware.

UNIX as an Abstraction Provider

  • The OS is the "layer below."
  • A module your program can call (using system calls).
  • Provides a powerful API (the POSIX API).
  • Examples of OS API:
    • File system: open(), read(), write(), close()
    • Network stack: connect(), listen(), read(), write()
    • Virtual memory: brk(), shm_open()
    • Process management: fork(), wait(), nice()

UNIX as a Protection System

  • The OS isolates processes from each other.
  • Permits controlled sharing through shared name spaces (e.g., file system names).
  • The OS isolates itself from processes.
  • Prevents processes from directly accessing hardware.
  • The OS is allowed to access the hardware.
  • User processes run in unprivileged (user) mode.
  • The OS runs in privileged (kernel) mode.
  • User-level processes invoke system calls to safely enter the OS.

Hardware Privilege Modes

  • A privilege mode is a hardware state that restricts the operations that code can perform (e.g., prevents direct hardware access).
  • Two main modes:
    • User mode: Normal programs running with low privilege (also system services in "user space").
    • Kernel mode: The operating system running.
  • Unrelated to superuser (root, administrator) privileges.

Device Drivers

  • A device driver is a software module that implements the interface to real or virtual hardware.
  • Examples: printers, monitors, graphics cards, USB devices.
  • Often provided by the device manufacturer.
  • Drivers commonly run within the OS kernel for performance reasons (in kernel space).
  • Historically, drivers were compiled directly into the kernel, requiring recompilation when a new device type was introduced.
  • Each system had a different kernel.

Recompiling Kernels?

  • Recompilation of the kernel is problematic:
    • Time-consuming.
    • Requires expertise.
    • Versioning issues.
  • Solution 1: User-space modules
    • Creating user-space programs that support the OS.
    • Leverages protection against buggy code.
    • Allows independent patching and upgrading.
    • Reduces dependency on the kernel version (mostly).
    • Problem: performance; interacting with user space is slower than in-kernel operations.
  • Solution 2: Kernel modules (loadable kernel modules)
    • Software modules that run in kernel space and can be loaded/unloaded on a running system.
    • Extends kernel functionality without recompilation.
    • The kernel provides generic interfaces (APIs) for modules to communicate with it.
    • Used by almost every modern OS (OSX, Windows, etc.).
  • Tip: Use the lsmod command to see running modules on a UNIX system.