Operating Systems

What is an operating system ?

• Bridge between users and computer hardware.
• A program controlling the execution of all other programs

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• It allocates resources for apps considering fairness and efficiency.
• It controls all execution of programs.
• It abstracts to eliminate complexity.

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1. Switch on computer
2. Bootstrap Program
3. OS starts

Design goals of an OS:

• User goals:
  • convenient
  • easy to learn
  • safe
  • fast
• System goals:
  • easy to design, implement, maintain.
  • flexible.
  • reliable.
  • efficient.
  • prevent malicious or accidental bad behavior.
  • protection between apps, users (isolation is key)
  • Reliable
  • Portable (runs on small devices)
  • Security
  • Energy efficient.

Running multiple programs at the same time:

• the OS manages switching processes from execution.
• It is possible with multiple cores.
• Not possible with one core. alternative: switch between process 1 and 2.

Tasks Of an OS

• Managing computer hardware and connected peripherial equipment.
• Need for access to shared resources.
• OS enables sharing resources in two different ways:
  • In time: CPU time
  • In space: memory space, permanent storage

How a modern computer works

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CPU

Central processing unit / Processor

• Fetch the instructions of a program from memory
• Decode the instruction (e.g., add two numbers)
• Execute the instruction

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Main Memory

Random Access Memory or RAM

A large array of bytes that CPU can access directly:

• instructions must be in main memory
• Temporary (volatile) storage of the processes
• Program is static, process is active = in execution

Storage devices

1. Registers
2. Cache
3. Main Memory
4. Non volatile memory
5. Hard-disk drives
6. Optical disk
7. Magnetic tapes

Key-Word	Definition
Speed or access time	time it takes to access the stored information
Cost per bit	monetary cost per bit
Size	storage capacity in bits
Volatility	if data can be stored permanently, if it is volatile it is not permanent

Key-Word	Definition
process	program in execution, a unit of work within the system
program counter	the status of the current activity of a process (memory address of the next instruction to be executed

IO (Input/Output)

controlling the peripheral hardware: • keyboard, mouse, screen, hard disk, printer, etc. •Interrupts (processor temporarily stops to execute OS code according to the interrupt’s type) • Hardware interrupts vs. software-generated interrupts (called trap or exception) • Some peripherals can access the memory directly without needing to interrupt the CPU •Direct memory access (DMA)

Main tasks of an OS:

Process managment

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Memory Managment

Assigning parts of memory resources among competing processes Deciding which processes (or parts thereof) and data to move into and out of memory Keeping track of which parts of memory are currently being used and by whom Provide isolation among processes Allocating and deallocating memory space as needed

swapping: storing the contents of memory which momentarily is not needed (because its process is currently not running) on hard disk, so the memory can be used by another process

virtual memory: using the hard disk to pretend that there is more memory than there physically is it enables users to run programs that are larger than actual physical memory

File Managment

files: long sequences of bits or (usually) bytes with meta-information: name, size, owner, creation date.

• How to find information?
• Bookkeeping to keep track of which part of which file is stored in which sector of the hard disk
• Files are organized hierarchically into “directories” (popularly also called “folders”)
• How to monitor the free blocks?
• How to ensure that one user cannot read another user’s data?
• Protection among users via different access rights to files

File system examples:

• NTFS on Windows
• ext2, ext3, ext4: filesystem on Linux systems, ext4: also on Android

Protection

isolation of processes

• Each process has its own memory space.

• Not allowed to access outside its address space.

• If it attempts to access, an exeption is raised.

• Proper authorization from the operating system (kernel vs user mode)

• A process can execute only within its own address space!

• Timer to ensure that no process can gain control of the CPU without eventually relinquishing control

• Distinguish between users (authentication of users, which user can do what with what? e.g., reading/writing files, accessing I/O equipment)

Two modes of operation: user vs. kernel mode

How to prevent buggy or malicious applications to harm the computer?

• Hardware support to distinguish between the execution of operating-system code and userdefined code (a protection mechanism) Kernel mode (mode bit: 0)
• executing on behalf of the OS, privileged instructions (e.g., setting the mode bit)

User mode (mode bit:1)

• executing on behalf of the user, some actions are not possible to execute
• raise a trap to the OS if attempted to execute such actions
• System call: user program asks the OS to perform some tasks on the user program’s behalf via system calls

Virtual Machines

software that simulates an entire computer.

• An entire operating system can be run within a VM. • Multiple VMs can run on a single "real" machine. • sharing the processor efficiently is done by hypervisor software.

VMs are useful for:

• Developing/Testing apps for multiple OSes without having multiple systems