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.