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computer science operating system

CS Primer - Operating Systems

Intro to Operating Systems

  • Chapter two of OSTEP
  • Fetch --> Decode --> Execute
  • In charge of making sure the system operates correctly and efficiently
  • Virtual machine
    • Takes a physical resource (processor, memory, disk, etc.) and transforms it into an easy-to-use virtual form
  • Sys calls
    • OS APIs
  • A resource manager
  • Policies determine which programs should run
  • "You should be using Emacs. If you are using vi, there is probably something wrong with you. If you are using something that is not a real code editor, that is even worse." 💀
  • Protocols for protecting writes
    • Journaling
    • Copy-on-write
  • Goals
    • Abstractions
    • Performance
    • Protection between apps
    • Reliability
    • Energy efficiency
    • Security
  • System call vs procedure call
    • System call transfers control into the OS while raising the hardware privilege level
  • User apps run in "user mode"
    • Hardware restricts what apps can do

Stack Overflow

  • Stack is a memory region allocated by the OS
    • Overflowing it, you try to write to memory outside the region where you don't have permission
  • OS raises a segfault due to the illegal memory address
    • It doesn't know what the cause of the segfault is which is why you don't get an actual stack overflow error
  • Historically the heap was started at low addresses, stack at high addresses
    • The big gap in the middle maximized the space available before collision
    • Doesn't matter anymore thanks to enormous virtual address space
  • What contributes to the size of a stack frame?
    • Data
    • Return address
    • Rounding up to alignment boundaries
      • Usually 16 bytes on modern systems
      • Helps with performance
  • You can't map all the virtual address space to actual hardware
  • Address Space Layout Randomization
  • Programs can request an increase to their stack limit with setrlimit
    • You can raise your soft limit up to the hard limit but only root can raise hard limits
    • getrlimit to check limits
  • Threads get their own stack to avoid collissions
  • vmmap
    • Virtual size --> how much address space is reserved for the stack
    • Resident size --> how much physical RAM is being used
  • Demand paging
    • Declaring an array doesn't allocate physical RAM, you have to write to the memory
    • OS maps physical memory when you touch it

Byte write

  • File systems don't track individual bytes on disk
    • Space is allocated in fixed-size chunks (blocks)
    • Blocks are usually 4096 bytes (4 KiB)
    • Writing 1 byte still gets a 4 KiB block
  • Larger blocks === less bookkeeping overhead but more wasted space
  • /dev/urandom is an interface to the kernel's random number generator
  • File descriptor is a "handle" to an open file
    • More efficient for accessing a file (open once)
    • Keeps state (position tracking, etc.)

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