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Operating Systems

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An operating system manages hardware resources and gives programs a clean, multiplexed view of them. Its job: schedule CPU time, allocate memory, mediate I/O, and isolate processes.

Overview

User-space code talks to the kernel through system calls. The kernel runs in privileged mode; everything else is sandboxed. Modern kernels are mostly monolithic (Linux, Windows NT) with loadable modules.

Processes & Threads

  • Process — isolated address space, own page tables, PID.
  • Thread — shares memory within a process; cheaper context switch.
  • IPC: pipes, FIFOs, sockets, shared memory, message queues, signals.
  • Synchronization: mutex, semaphore, condition variable, RW lock, atomics.
  • Hazards: deadlock, livelock, priority inversion, race condition.

Memory Management

  • Virtual memory + paging — typical page 4 KiB, huge pages 2 MiB / 1 GiB.
  • TLB caches recent translations.
  • Page faults — minor (allocate) vs major (disk).
  • Allocators: brk/sbrk, mmap, slab, jemalloc, tcmalloc.
  • OOM killer (Linux) reclaims pages under pressure.

Filesystems & I/O

  • POSIX filesystems: ext4, XFS, btrfs, ZFS, APFS.
  • Windows: NTFS, ReFS.
  • Inodes, dentries, journals, copy-on-write, snapshots.
  • Block / character / network devices.
  • Async I/O: epoll, kqueue, io_uring, IOCP.

Scheduling

  • Preemptive multitasking; time slices (jiffies, ticks).
  • Linux CFS & EEVDF; real-time policies SCHED_FIFO, SCHED_RR.
  • RTOS: FreeRTOS, Zephyr, VxWorks, QNX, RTEMS — used in embedded & safety systems.
  • Priority inheritance to prevent inversion.

OS Families

  • Unix-like: Linux distros, *BSD, macOS (Darwin), Solaris.
  • Windows: NT-based (10, 11, Server 2019/2022/2025).
  • Mobile: Android (Linux), iOS (Darwin).
  • RTOS: see above.
  • Industrial: Windows IoT, Linux with PREEMPT_RT, VxWorks.
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