Maxim Shevertalov Jay Kothari William M. Mongan Operating Systems Lecture 1: Introduction Review of System Architecture and Concurrent Programming Maxim Shevertalov Jay Kothari William M. Mongan Lec 1 Operating Systems

Introduction What is an Operating System? How are they designed? September 4, 1997 Introduction What is an Operating System? How are they designed? Why study them? Lec 1 Operating Systems

What is an Operating System? September 4, 1997 What is an Operating System? Facilitation + Performance Think of Air Traffic Control: they are in charge of lots of airplanes, and sometimes they tell airplanes to wait their turn. Despite what we may sometimes think, their intent is not to prevent us from doing what we want by imposing complicated rules. Rather, they are there to facilitate aircraft in flight. On its own, the air traffic controller does not accomplish much; but, together with the aircraft he supports, he aims to maximize performance for all the participating airplanes (not any one!), while protecting them from one another. It’s all about coordination. Lec 1 Operating Systems

What is an Operating System? September 4, 1997 What is an Operating System? Can we use a computer system without an operating system? Lec 1 Operating Systems

What is an Operating System? September 4, 1997 What is an Operating System? Can we use a computer system without an operating system? We would have to directly interface with the computer hardware. Pros: speed?, control? Cons: What if we need to run on a different machine? Or change the network card? Install a new OS? What if new features like multiprocessors or a cluster is introduced to the program? The operating system mediates between application (the airplanes) and the shared resources (the runways and airspace) in a way that provides abstraction and generality for the applications. Lec 1 Operating Systems

What is an Operating System? Trusted software interposed between the hardware and application/utilities to improve efficiency and usability Most computing systems have some form of operating systems Hard to use computer systems without OS Applications and Utilities Operating System Computer Hardware Lec 1 Operating Systems

Why Study Them? Take a deeper at system architecture September 4, 1997 Why Study Them? Take a deeper at system architecture Modular, layered software design All of our software runs on top of it! It is the common denominator of everything we write. How often do you make a syscall? Or invoke some system library? Learn to build complex systems Engineering Issues Business Issues (OS as a government) Security (viruses, worms, etc.) Lec 1 Operating Systems

Computer System Organization Computer-system operation One or more CPUs, device controllers connect through common bus providing access to shared memory Concurrent execution of CPUs and devices competing for memory cycles Memory CPU Disk Controller USB Controller GPU Lec 1 Operating Systems

Operating System Design: Modularity September 4, 1997 Operating System Design: Modularity Process Control: Threads Synchronization and Scheduling Protection, Memory Management, Caching Demand Paging File Systems Networking and Distributed Systems Protection and Security Lec 1 Operating Systems

One Size Fits All? Requirements Dictate Design September 4, 1997 One Size Fits All? Requirements Dictate Design Lec 1 Operating Systems

Evolution of the OS In the beginning... Computers Cost millions of $$$ Were hard to operate (performed using console switches) Were hard to program (in particular I/O devices) Lec 1 Operating Systems

Utilizing Expensive Hardware Expensive computers could not idle; utilization should be very high Batch processing was devised Programming done offline and subroutine were created for common tasks I/O and processing overlap obtained via buffering and interrupts Lec 1 Operating Systems

Factors in OS Evolution Cost of computers (i.e., computer time) Cost of people (i.e., user time) user wait time computer utilization time sharing systems early batch systems personal systems Lec 1 Operating Systems

September 4, 1997 Major OS Functions Lec 1 Operating Systems

Major OS Functions Control access and provide interfaces To the OS and devices attached to the system Provide interfaces for human-machine and machine-machine transactions Manage resources Mediate resource usage among different tasks Implement policies Lec 1 Operating Systems

Major OS Functions Control access and provide interfaces To the OS and devices attached to the system Provide interfaces for human-machine and machine-machine transactions Manage resources Mediate resource usage among different tasks Implement policies Provide abstractions Hide the peculiarities of the hardware. Example: device independent I/O Consume resources OS runs on the system that is being managed… so it is going to consume resources! Lec 1 Operating Systems

What is the Access Problem? User wants to access OS, why? User wants to access devices connected to the system It all starts with recognizing the user’s intent to access! Lec 1 Operating Systems

Access Problem: Handling I/O Programmed I/O The CPU transfers the data from (or to) the device buffers. After issuing an I/O operation the CPU continuously checks (polls) for its completion Lec 1 Operating Systems

Access Problem: Handling I/O Interrupt-driven I/O (slow speed, character device) The CPU issues an I/O operation and goes on; Device notifies (interrupts) the CPU as data arrives; CPU processes the data Lec 1 Operating Systems

How Does the OS Service Interrupts? Lec 1 Operating Systems

Access Problem: Data Processing Once data is captured from device (e.g., keyboard) what is next? Imagine you want to write a word processor Capture some keystrokes – for arrow and other control keys Capture words (cooked or raw form?) Lec 1 Operating Systems

Accessing Devices vs. OS Services OS interposes itself between the hardware and applications/utilities/users Controlling access to OS ---> Controls device access User OS Device Lec 1 Operating Systems

How Do You Control Access to the OS? Provides a two level architecture: Trusted mode Untrusted mode The OS (at least the core part – called Kernel) runs in the trusted mode User applications/utilities run in the untrusted mode Lec 1 Operating Systems

System Call via Trap OS provides system calls for accessing OS services from applications System calls are special procedure calls Control transfer Switch protection domain System calls also: Validate the call Allow authorized actions to take place Lec 1 Operating Systems

How are Syscalls Processed? Lec 1 Operating Systems

UNIX Syscall Examples Process control File manipulation fork(), exec(), wait(), abort() File manipulation chmod(), link(), stat(), creat() Device manipulation open(), close(), ioctl(), select() Information maintenance time(), acct(), gettimeofday() Communications socket(), accept(), send(), recv() Lec 1 Operating Systems

How Does an OS Manage Resources? Lec 1 Operating Systems

Process: Avatar of the Application A process represented process table entry Lec 1 Operating Systems

Simple Process Model Below is a two-state process model Running (on the CPU) Not running (waiting to get the CPU or at I/O) Lec 1 Operating Systems

Resource Management: Processor Time Processor time is the primary resource managed by the OS How it is managed depends on the type of OS: Batch vs Time-sharing Uniprogramming vs Multiprogramming Lec 1 Operating Systems

Example: Uniprogramming (only one program running on the system) What is the CPU Utilization? Lec 1 Operating Systems

Example: Multiprogramming (multiple programs simultaneously running) Lec 1 Operating Systems

Comparing System Utilization Lec 1 Operating Systems

Process Scheduling Lec 1 Operating Systems

Process Scheduling: Concerns Lec 1 Operating Systems

Process Scheduling Concerns Fairness Give equal and fair access to resources Differential responsiveness Discriminate among different classes of jobs Efficiency Maximize throughput, minimize response time, and accommodate as many users as possible Lec 1 Operating Systems

Memory Management Memory management is key to multiprogramming Memory is a space sharing resource Memory management is necessary to protect co-residing programs from each other Depending on the occupancy pattern, a program would not know the location until load time Memory management should provide: Protection Relocation Lec 1 Operating Systems

How Does an OS Manage Memory? Idea: Divide the memory into partitions and allocate them to different processes Partitions can be fixed size or variable size A process Cannot access memory not allocated to it Can request more memory from OS Can release already held memory to OS May only use a small portion of the allocated memory Lec 1 Operating Systems

How Does an OS Manage Memory? We can load more processes than that fitting the memory Where do the rest go? Answer: Virtual Memory Lec 1 Operating Systems

Virtual Memory in Action Some memory chunks (pages) are in memory Rest in disk OS is responsible for retrieving the pages from and flushing the pages to disk Lec 1 Operating Systems

To Use Storage, We Need a Filesystem Lec 1 Operating Systems

Storage Management: Component View Lec 1 Operating Systems

Topics Covered Fundamentals (Operating Systems Structures) Process Control and Threads Synchronization and scheduling Protection, Address translation, Caching Demand Paging File Systems Networking and Distributed Systems Protection and Security Lec 1 Operating Systems