Understanding Operating Systems

Definition of Operating System

• An Operating System (OS) provides a virtual machine on top of the hardware, making it more convenient than the raw hardware interface
• An OS is “All of the code you didn’t write”

Convenience of OS

• Simpler
• More reliable
• More secure
• More portable
• More efficient

What do Operating Systems Do?

• Manage physical and virtual resources
• Provide users with a well-behaved environment
• Define a set of logical resources (objects) and a set of well-defined operations on those resources (interface)
• Provide mechanisms and policies for the control of resources
• Control how different users and programs interact

Resources to be Managed

• CPU(s)
• Memory
• Storage Devices (disks, tapes, etc.)
• Input Devices (keyboard, mouse, cameras, etc.)
• Output Devices (printers, displays, speakers, etc.)
• Networks

Computer-System Operation

• I/O (Input/Output) devices and the CPU can execute concurrently
• Each device controller is in charge of a particular device type
• Each device controller has a local buffer
• CPU moves data from main memory to local buffers
• CPU also moves data to main memory from local buffers
• Main memory = RAM
• I/O is from the device to local buffer of controller
• Device controller informs CPU that it has finished its operation by causing an interrupt

What’s in an OS?

• Logical Structure
• Windowing & Graphics
• Virtual Memory
• File System
• Memory Management
• Networking
• Access Control
• Process Management
• Generic I/O
• Device Drivers
• Naming
• Interrupts
• Cache
• Physical Memory
• Hardware Devices
• Applications
• OS Interface
• Physical Machine Interface
• Machine Independent Services
• Machine Dependent Services
• Quake
• Word Processor
• System Utils
• Shells
• Windowing & Graphics

Major Issues in Operating Systems

1

• Structure: how is an OS organised?
• Sharing: how are resources shared among users?
• Naming: how are resources named by users or programs
• Protection: how is one user/program protected from another
• Security: how to authenticate, control access, and secure privacy
• Performance: why is it so slow?
• Reliability & Fault Tolerance: how do we deal with failures?
• Extensibility: how do we add new features?

2

• Communication: how can we exchange information?
• Concurrency: how are apparently parallel activities created and controlled?
• Scale & Growth: what happens as demands & resources increase?
• Persistence: how to make data outlast the processes that created them
• Compatibility: can we ever do anything new?
• Distribution: accessing the world of information
• Accounting: who pays the bills, and how do we control resource usage?

A Component View of an OS

Four Managers in an OS

Process Management 1 of 3

• Distinguish between a Program and a Process
• A process is a program in execution
• It is a unit of work within the system
• Sometimes called a job
• A Program is a passive entity, whereas a Process is an active entity
• Process needs resources to accomplish its task:
  • CPU, memory, I/O, files
  • Initialization data

Process Management 2 of 3

• Process termination requires reclaim of any reusable resources, so that other processes can use them
• The OS assists with this
• Process which has a single “flow” or thread of execution is called a “single-threaded”
• We have one program counter, specifying the memory location of next instruction to execute
• Single-threaded Process executes instructions sequentially:
  • one instruction at a time (starting with the first), until completion

Process Management 3 of 3

• Multi-threaded process has a multiple “flows” or threads of execution
• One program counter per thread; for eg: 4 threads in a process needs 4 program counters
• A thread’s program counter specifies memory location of next instruction to execute in that thread
• Sometimes, we say “Thread Counter”