COS 318: Operating Systems Overview Kai Li Computer Science Department Princeton University (http://www.cs.princeton.edu/courses/cos318/)2 Logistics  Precepts:  Tue, Wed: TBD, 105 CS building  Design review:  9/28 during 6-10pm, 010 Friends center  Project 1 due:  10/5 at 11:59pm  Reminder:  Subscribe to the cos318 mailing list today!3 Today  Overview of OS structure  Overview of OS components4 Hardware of A Typical Computer CPU Chipset Memory I/O bus CPU . . . Network ROMComputing machinery Analytical Engine (~1850) Charles Babbage ENIAC (~1946) Eckert & Mauchly, UPenn Johnniac (~1953) von Neumann, IAS6 A Typical Computer System Memory CPU CPU . . . OS Apps Data Network Application Operating System ROM BIOS7 Interrupts  Raised by external events  Interrupt handler is in the kernel  Switch to another process  Overlap I/O with CPU  …  Eventually resume the interrupted process 0: 1: … i: i+1: … N: Interrupt handler8 Typical Unix OS Structure Application Libraries Machine-dependent layer User level Kernel level Portable OS Layer9 Typical Unix OS Structure Application Libraries Machine-dependent layer Portable OS Layer User function calls written by programmers and compiled by programmers.10 Typical Unix OS Structure Application Libraries Machine-dependent layer Portable OS Layer • Written by elves • Objects pre-compiled • Defined in headers • Input to linker • Invoked like functions • May be “resolved” when program is loaded11 Pipeline of Creating An Executable File  gcc can compile, assemble, and link together  Compiler (part of gcc) compiles a program into assembly  Assembler compiles assembly code into relocatable object file  Linker links object files into an executable  For more information:  Read man page of a.out, elf, ld, and nm  Read the document of ELF foo.c gcc as foo.s foo.o ld bar.c gcc as bar.s bar.o libc.a … a.out12 Execution (Run An Application)  On Unix, “loader” does the job  Read an executable file  Layout the code, data, heap and stack  Dynamically link to shared libraries  Prepare for the OS kernel to run the application a.out loader *.o, *.a ld Application Shared library13 What’s An Application?  Four segments  Code/Text – instructions  Data – initialized global variables  Stack  Heap  Why?  Separate code and data  Stack and heap go towards each other Stack Heap Initialized data Code 2n -1 014 Responsibilities  Stack  Layout by compiler  Allocate/deallocate by process creation (fork) and termination  Names are relative off of stack pointer and entirely local  Heap  Linker and loader say the starting address  Allocate/deallocate by library calls such as malloc() and free()  Application program use the library calls to manage  Global data/code  Compiler allocate statically  Compiler emit names and symbolic references  Linker translate references and relocate addresses  Loader finally lay them out in memory15 Typical Unix OS Structure Application Libraries Machine-dependent layer Portable OS Layer “Guts” of system calls16 OS Service Examples  Examples that are not provided at user level  System calls: file open, close, read and write  Control the CPU so that users won’t stuck by running • while ( 1 ) ;  Protection: • Keep user programs from crashing OS • Keep user programs from crashing each other  System calls are typically traps or exceptions  System calls are implemented in the kernel  When finishing the service, a system returns to the user code17 Typical Unix OS Structure Application Libraries Machine-dependent layer Portable OS Layer • Bootstrap • System initialization • Interrupt and exception • I/O device driver • Memory management • Mode switching • Processor management18 Applications Software “Onion” Layers Libraries OS Services Device Driver Kernel User and Kernel boundary HW19 Processor Management  Goals  Overlap between I/O and computation  Time sharing  Multiple CPU allocations  Issues  Do not waste CPU resources  Synchronization and mutual exclusion  Fairness and deadlock free CPU I/O CPU CPU CPU CPU I/O CPU CPU CPU I/O20 Memory Management  Goals  Support programs to run  Allocation and management  Transfers from and to secondary storage  Issues  Efficiency & convenience  Fairness  Protection Register: 1x L1 cache: 2-4x L2 cache: ~10x L3 cache: ~50x DRAM: ~200-500x Disks: ~30M x Archive storage: >1000M x21 I/O Device Management  Goals  Interactions between devices and applications  Ability to plug in new devices  Issues  Efficiency  Fairness  Protection and sharing User 1 User n . . . Library support I/O device I/O device . . . Driver Driver22 File System  Goals:  Manage disk blocks  Map between files and disk blocks  A typical file system  Open a file with authentication  Read/write data in files  Close a file  Issues  Reliability  Safety  Efficiency  Manageability User 1 User n . . . File system services File File . . .23 Window Systems  Goals  Interacting with a user  Interfaces to examine and manage apps and the system  Issues  Direct inputs from keyboard and mouse  Display output from applications and systems  Labor of division • All in the kernel (Windows) • All at user level • Split between user and kernel (Unix)24 Bootstrap  Power up a computer  Processor reset  Set to known state  Jump to ROM code (BIOS is in ROM)  Load in the boot loader from stable storage  Jump to the boot loader  Load the rest of the operating system  Initialize and run  Question: Can BIOS be on disk? Boot loader OS sector 1 OS sector 2 OS sector n . . . Boot loader25 Ways to Develop An Operating System  A hardware simulator  A virtual machine  A good kernel debugger  When OS crashes, always goes to the debugger  Debugging over the network  Hire some