Chapter 1: Introduction and History

1. Chapter 1: Introduction and History • Where does the operating system fit in a computing system? • What does the operating system achieve? • What are the general OS functions? • Who needs an operating system? • What is operating system? • History of operating system

2. Where does the OS fit in? • The layer between the hardware and the user program (application programs). • OS is a software system that directly interacts with the hardware. User programs Operating system interface Operating system Hardware interface Hardware

3. Hardware • CPU, registers, disks, monitors, etc • Hardware interface • the instruction set • other things like interrupt – anything that a programmer needs to know in order to write programs that use the hardware. • Operating system • Implements the OS interface + resource management • OS interface • The enhanced instruction set: hardware instruction set + special instructions called “traps” or “system calls”. • User programs: • Instructions in the enhanced instruction set + data

4. Question: How many of you have dealt with the OS (enhanced) interface before? • One more layer between a typical user program and the OS interface, the programming environment (compiler + run time library). • cout << “hello world.\n”  write(1, “hello world.\n”, 13); User programs programming environment Operating system interface Operating system Hardware interface Hardware

5. What does the OS achieve? • Make it easy to write programs • Add more powerful instructions to the hardware instruction set. • What kind of instructions are added? • Common functions used by many different applications. • E.g. write (fileno, buf, len); • Resource virtualization • E.g. the illusion of Infinite memory.

6. What does the OS achieve? • Make it easy to run programs • How does a program run on the raw machine? • The program is in memory, starting from program counter (pc), run one instruction, goto the next instruction, run until the halt instruction. • How do you run a program? • g++ helloworld.cpp and then a.out.

7. The OS must fill the gap between how you run a program and how the hardware runs a program. The OS must be able to: • Take the user command (g++, a.out, etc) – shell • Find the executable file (g++ or a.out) – File system • Load the executable into memory – memory management • Set the registers properly (e.g. pc = starting address of the executable) • When there are multiple programs running, the OS must make each program feel like it solely owns the whole machine (CPU, memory, registers) – virtual machine for each process • Manage processes. • OS functionality:Implements the OS interface + resource management

8. What does an OS achieve? • It hides the complexity and limitations of hardware (hardware interface) and creates a simpler, more powerful abstraction (OS interface).

9. Hardware reality vs. OS abstraction

10. What are the general OS functions? • Standard services • Screen display, disk accesses, etc • Coordinationamong applications • Protection, correctness, efficiency, and fairness

11. Coordination • Example: Protection • Applications should not crash one another • Address space: all memory addresses that an application can touch. • Address space for one process is separated from address space for another process and from the OS. • Applications should not crash the OS • Dual-mode operations • Kernel mode: some instructions can only be executed by the OS (must be executed with a kernel mode). • User mode: an application can only access its own address space • Examples of kernel mode instructions?

12. What is the operating system? • OS is the software layer between the hardware and user programs. • OS is the ultimate API. • OS is the first program that runs when the computer boots up. • OS is the program that is always running. • OS is the resource manager. • OS is the creator of the virtual machine.

13. Resources managed by OS • CPU: process management • Memory: memory management • Storage: storage and file management • I/O devices: I/O management

14. Who needs OS? • OS makes a computer easier to use • All general purpose computers need OS. • A better question: Who does not need OS? • Some very specialized systems that usually do one thing (OS can be embedded in the application). • Microwave oven control • MP3 players, etc.

15. History of OS: Change!

16. History Phase I: Hardware Expensive, Humans Cheap • Hardware: mainframes • OS: human operators • Handle one job (a unit of processing) at a time • Computer time wasted while operators walk around the machine room

17. OS Design Goal • Efficient use of the hardware • Batch system: collects a batch of jobs before processing them and printing out results • Job collection, job processing, and printing out results can occur concurrently • Multiprogramming: multiple programs can run concurrently • Example: I/O-bound jobs and CPU-bound jobs

18. History Phase II: Hardware Cheap, Humans Expensive • Hardware: terminals • OS design goal: more efficient use of human resources • Timesharing systems: each user can afford to own terminals to interact with machines

19. History Phase III: Hardware Very Cheap, Humans Very Expensive • Hardware: personal computers • OS design goal: allowing a user to perform many tasks at the same time • Multitasking: the ability to run multiple programs on the same machine at the same time • Multiprocessing: the ability to use multiple processors on the same machine

20. History Phase IV: Distributed Systems • Hardware: computers with networks • OS design goal: ease of resource sharing among machines

21. The Bottom Line OS designs need to adapt to changing technology