CSE 380 Computer Operating Systems Instructor: Insup Lee University of

CSE 380 Computer Operating Systems Instructor: Insup Lee University of Pennsylvania Fall 2003 Lecture Note 1: Introduction 1

What is an Operating System? Operating systems provides an interface between hardware and user programs, and makes hardware usable 2

Resource Abstraction and Sharing It is an extended machine providing abstraction of the hardware Hides the messy details which must be performed Presents user with a virtual machine, easier to use It is a resource manager Time on CPU is shared among multiple users/programs Space in memory and on disks is shared among multiple users/programs 3

Pentium Architecture 4

Abstractions in OS Hardware OS abstraction Disks Files Memory Programs Processors Threads / Processes Network Communication Monitor Windows and GUI Keyboard Input Mouse Locator 5

Sharing of Memory Issues Program 1 Free space Program 3 Allocation schemes Protection from each other Protecting OS code Translating logical addresses to physical Swapping programs Program 2 OS What if physical memory is small: Virtual memory 6

Timesharing P1 OS P2 OS P1 OS P3 OS At any point, only one program can run on CPU Context switch: changing the program that has CPU When to switch (goal: to optimize the CPU usage) When a program terminates When a program has run “long enough” When a program executes a system call or waits for I/O When an external interrupt arrives (e.g. mouse click) OS must do all the book-keeping necessary for context switch, with minimum number of instructions 7

Challenges in OS Why can’t Microsoft still get rid of all bugs in Windows ? Performance is critical How to reduce the memory and time overhead due to OS Synchronization and deadlocks due to shared resources Scheduling of multiple programs Fairness, response time, real-time applications Memory management Virtual memory, paging, segmentation Security and Protection Authorization, authentication, viruses Interrupt management and error handling Marketability and backward compatibility 8

How does OS work? OS gets control of the CPU repeatedly Let’s look at two typical scenarios to get a glimpse of how things work (we will get a more accurate and detailed understanding as the course progresses) Basic knowledge about computer architecture is essential ! (Read Sec 1.4 to review CSE 240) 9

Inside a CPU State of a running program Registers Program counter (PC) Stack pointer Program status word (PSW) Key distinction in PSW: user mode vs kernel (OS) mode Key instruction for OS calls: TRAP (switch to kernel mode) Many operations (such as accessing I/O devices) are possible only in the kernel mode 10

Different types of Memory Use of disks unavoidable (permanence and size) Access time is significantly slower for disks 11

Sample Scenario 1 Consider a statement to read from a file in a user program P User program stores parameters such as file-id, memory-address, number-of-bytes, and system-call number of read, and executes TRAP instruction to invoke OS Hardware saves the state of current program, sets the mode-bit in PSW register in CPU to 1, and transfers control to a fixed location in OS code OS maintains an internal file table that stores relevant information about all open files 12

Sample Scenario 1 (continued) OS read routine examines the parameters, checks for errors (e.g. file must be open), consults its file table, and determines the disk address from where data is to be retrieved then it sets up registers to initiate transfer by the disk controller While disk controller is transferring data from disk to memory, OS can suspend current program, and switch to a different program When OS routine finishes the job, it stores the status code, and returns control to the user program P (hardware resets mode-bit) Note: Disk controller is accessed only by OS code (this is ensured by hardware protection) 13

Sample Scenario 2 Consider an assignment x: y in a program P Compiler assigns logical addresses, say Add1 and Add2, for program variables in P’s data space When P is loaded in memory, OS assigns a physical base address to store P and its data Compiled code looks like Load (R, Add1); Store (R, Add2) While executing Load instruction the hardware translates the logical address Add1 to a physical memory location (this is done by Memory Management Unit MMU) 14

Sample Scenario 2 (continued) However, OS may not keep all of P in memory all the time OS maintains an internal table, called page table, that keeps track of which blocks of P are in memory If Add1 is not in memory, MMU generates a page fault, and transfers control to OS OS examines the cause, and initiates a disk transfer to load in the relevant block of P OS needs to decide memory allocation for the block to be fetched (page replacement algorithms) While this block is being fetched, P may be suspended using a context switch 15

Brief History of Operating Systems 1940's -- First Computers 1950's -- Batch Processing 1960's -- Multiprogramming (timesharing) 1970's -- Minicomputers & Microprocessors 1980's -- Networking, Distributed Systems, Parallel (multiprocessor) Systems 1990's and Beyond -- PCs, WWW, Mobile Systems, embedded systems 16

1940's -- First Computers Computer dedicated to one user/programmer at a time. Program loaded manually by programmer, using console switches. Debugging using console lights. Advantages: Interactive (user gets immediate response) Disadvantages: Expensive machine idle most of time, because people are slow. Programming & debugging are tedious. Each program must include code to operate peripherals -- error prone, device dependencies. Libraries of subroutines to drive peripherals are example of typical OS service. 17

1950's -- Batch Processing User/programmer submits a deck of cards that describes a job to be executed. Jobs submitted by various users are sequenced automatically by a resident monitor. Tape drives available for batching of input and spooling of output. Advantages: Computer system is kept busier. Disadvantages: No longer interactive; longer turnaround time. CPU is still idle for I/O-bound jobs. OS issues -- command processor (JCL), protection of resident monitor from user programs, loading of user programs after monitor. 18

Typical Batch System Early batch system bring cards to 1401 read cards to tape put tape on 7094 which does computing put tape on 1401 which prints output 19

1960's -- Multiprogramming (timesharing) The advent of the I/O processor made simultaneous I/O and CPU processing possible. CPU is multiplexed (shared) among a number of jobs -- while one job waiting for I/O, another can use CPU. Advantages: Interactiveness is restored. CPU is kept busy. Disadvantages: Hardware and O.S. required become significantly more complex. Timesharing - switch CPU among jobs for pre-defined time interval Most O.S. issues arise from trying to support multiprogramming -- CPU scheduling, deadlock, protection, memory management, virtual memory, etc. CTSS (Compatible Time Sharing System), Multics 20

1970's - Minicomputers & Microprocessors Trend towards many small to mid-range personal computers, rather than a single mainframe. Early minicomputers and microprocessors were small, so there was some regression to earlier OS ideas. e.g. DOS on PC is still essentially a batch system similar to those used in 1960, with some modern OS ideas thrown in (e.g., hierarchical file system). This trend changing rapidly because of powerful new microprocessors. Also, the user interface (GUI) became more important. UNIX, DOS 21

1980's - Networking Powerful workstations (e.g., PDP, VAX, Sunstations, etc.) Local area networks (e.g., Ethernet, Token ring) and longdistance network (Arpanet) Networks organized with clients and servers Decentralization of computing requires more communication (e.g., resource sharing) O.S. issues -- network communication protocols, data encryption, security, reliability, consistency of distributed data Real-Time Systems – timing constraints, deadlines, QoS (quality of service) 22

1990's and Beyond Parallel Computing (tera-flops) Powerful PCs, Multimedia computers High-speed, long-distance communication links to send large amounts of data, including graphical, audio and video World Wide Web Electronic notebooks and PDAs using wireless communication technologies Embedded computers: medical devices, cars, smartcards O.S. issues -- Large heterogeneous systems, mobile computing, utilization of power, security, etc. 23

Operating System Structure Monolithic Systems Layered Systems Virtual Machines Client-Server Model 24

Operating System Structure (1) Simple structuring model for a monolithic system 25

Operating System Structure (2) Structure of the THE operating system 26

Operating System Structure (3) Structure of VM/370 with CMS 27

Operating System Structure (4) The client-server model 28

Operating System Structure (5) The client-server model in a distributed system 29