CS 162 Spring 2003 Lecture 1 1/9 CS162 Operating Systems and Systems Programming Professor: Anthony D. Joseph Spring 2003 Lecture 1: Introduction 0.0 Administrivia: 1.0 Main points: What is an operating system --- and what isn’t it? Principles of operating system design Why study operating systems? 1.1 What is an operating system? (Figure is from Silberschatz and Galvin, Chapter 1) Definition: An operating system implements a virtual machine that is (hopefully) easier and safer to program and use than the raw hardware. API: Application Programming Interface Hardware Interface CS 162 Spring 2003 Lecture 1 2/9 In some sense, OS is just a software engineering problem: how do you convert what the hardware gives you into something that the application programmers want? For any OS area (file systems, virtual memory, networking, CPU scheduling), you begin by asking two questions: • What’s the hardware interface? (the physical reality) • What’s the application interface? (the nicer abstraction) Of course, should also ask why the interfaces look the way they do, and whether it might be better to push more responsibilities into applications or into hardware, or vice versa. (examples, RISC architectures, VBasic libraries, etc.) 1.1.1 Virtual Machines • Virtual machine model provides software emulation of an abstract machine • Also used to allow programs for one hardware & OS platform to run on another one (e.g., Windows programs on a Macintosh), perhaps even running several VMs concurrently. • Useful for OS research and development (much easier to debug) • Protection and portability (e.g., Java VM) • The project in this course is to build some of the portable components of an OS on top of Nachos, which provides a simulation environment. That is, it simulates the hardware and machine-dependent layer (interrupts, I/O, etc.) and the execution of user programs running on top of it. Note that Nachos runs on many different hardware/OS platforms. 1.1.2 Operating systems have two general functions: Silberschatz and Galvin: “An OS is similar to a government”. This becomes political --- do you think a government does anything useful by itself? Hardware ApplicationOperating System Virtual Machine Interface Physical Machine InterfaceCS 162 Spring 2003 Lecture 1 3/9 1. Coordinator & traffic cop: allow multiple applications/users to work together in efficient and fair ways (examples: concurrency, memory protection, file systems, networking). This is Resource Allocation and Control. Goals are fair management of shared resources, protection, and efficiency. 2. Standard services: provide standard facilities that everyone needs (examples: standard libraries, windowing systems). View OS as a facilitator. Goal is to make application programming easier, faster, and less error-prone. 1.1.3 So, what is in an OS and what isn’t? Of course, there is no universal agreement on this but: Most likely: • Memory management • i/o management • CPU scheduling • communications? • multitasking/multiprogramming What about?: • File System • Multimedia support • User Interface (window manager) • Internet Browser? ☺ Bonus question: is this just of interest to academics? If not then why might it be important? 1.1.4 What if you didn’t have an operating system? source code -> compiler -> object code -> hardware How do you get object code onto the hardware? How do you print out the answer? Before OS’s, used to have to toggle in program in binary, and then read out answers from LED’s! CS 162 Spring 2003 Lecture 1 4/9 1.1.5 Simple OS: What if only one application at a time? Examples: very early computers, early PC’s, embedded controllers (elevators, cars, Nintendos, ...), PDAs, intelligent light switches,… Then OS is just a library of standard services. Examples: standard device drivers, interrupt handlers, math libraries, etc. 1.1.6 More complex OS: what if you want to share the machine among multiple applications? Then OS must manage interactions between different applications and different users, for all hardware resources: CPU, physical memory, I/O devices like disks and printers, interrupts, etc. Of course, the OS can still provide libraries of standard services. 1.1.7 Example of OS coordination: protection Problem: How do different applications run on the same machine at the same time, without stomping on each other? Goals of protection: • Keep user programs from crashing OS • Keep user programs from crashing each other 1.1.7.1 Hardware support for protection Hardware provides two things to help isolate a program’s effects to within just that program: • Address translation • Dual mode operationCS 162 Spring 2003 Lecture 1 5/9 1.1.7.2 Address translation Address space: literally, all the memory addresses a program can touch. All the state that a program can affect or be affected by. Achieve protection by restricting what a program can touch! Hardware translates every memory reference from virtual addresses to physical addresses; software sets up and manages the mapping in the translation box. CPUTranslation Box (MMU)Physical MemoryVirtual AddressPhysical AddressData read or write (untranslated) Address Translation in Modern Architectures Two views of memory: • View from the CPU – what program sees, virtual memory • View from memory – physical memory Translation box (also called a memory management unit) converts between the two views. CS 162 Spring 2003 Lecture 1 6/9 codedataheapstackcodedataheapstackprog 1prog2code1code2data1data2stack1stack2heap1heap2virtualaddressspacevirtualaddressspacephysicalmemoryOS codeOS dataOS heap& stacks Example of Address Translation Translation helps implement protection because there is no way for a program to even talk about other program’s addresses; no way for it to touch operating system code or data. Translation also helps with the issue of how to stuff multiple programs into memory. Translation is implemented using some form of table lookup (we’ll discuss various options for implementing the translation box later). Separate table for each user address space.CS 162 Spring 2003 Lecture 1 7/9 1.2.4.3 Dual mode operation Can an application modify its own translation tables? If it could, then it could get access to all of physical memory. Has to be restricted somehow. Dual-mode operation •
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