12/2/09 1 What is an Operating System Andy Konwinski CS61CL Dec 2, 2009 Lecture 13 UCB CS61CL F09 Lec 13!Today • What is an operating system • Dual mode operation: kernel vs. user mode • Current trends and issues 12/2/09 UCB CS61CL F09 Lec 13!2What is an Operating System • Resources – Textbooks I like: 12/2/09 UCB CS61CL F09 Lec 13!3What is an Operating System • No single all encompassing definition • Used to be an actual person, an “operator” – You were operator for your CAL16 processor • General definition: – A layer of software that provides user programs with a simpler, cleaner, model of the computer and handles the messy job of managing the resources. 12/2/09 UCB CS61CL F09 Lec 13!4What is an Operating System • Where the OS fits in 12/2/09 UCB CS61CL F09 Lec 13!5 (modified version of fig 1-1, tanenbaum, pg2) software hardware Operating System Kernel mode User mode Music player Email reader Web browserPart 1: Clean abstractions of HW • Hardware is messy (e.g. assembly lang., interacting with a device) • Application developers want useful high level abstractions 12/2/09 UCB CS61CL F09 Lec 13!6Example: File System • Application level: files • Hardware level: controller, blocks • Operating system to manage the mapping between files and blocks, also protection. 12/2/09 UCB CS61CL F09 Lec 13!7 MS Word Foo.txt File System Operating System Sector #, disk# Disk controller Disk Cylinder, sector, head Network controller Frames, MAC address NICPart 2: Resource manager • Multiplex one set of hardware resources between apps/users – CPU/Memory/cache – I/O devices » Communication (network cards, hard drive) » Human I/O (mouse, keyboard, monitor, printer) 12/2/09 UCB CS61CL F09 Lec 13!8 CPU Hard Drive network … Andy: MS Word Andy: Day of Defeat Jamie: WinAmpPart 2b: fault/performance isolation • Resources should be shared fairly • Isolation between users (processes) – Fault isolation: when one program crashes, it should not cause others to crash – Performance isolation: e.g. if spotlight runs, my Quicktime movie shouldn’t skip. 12/2/09 UCB CS61CL F09 Lec 13!9Example: virtual memory • Each application sees continuous, nearly infinite, mem. address namespace • Hardware provides: finite memory, page table base register, TLB, disk • Operating system: orchestrates. – manages page table entries (e.g. updating Valid bit when swapping), flushes the TLB when necessary, pages to disk, etc. 12/2/09 UCB CS61CL F09 Lec 13!10Administrative • Midterm 2 back last week, regrades done • This is final class • Optional lab on threads • Regrade requests for Midterm 2 due by end of day Friday • Review lecture next week 12/2/09 UCB CS61CL F09 Lec 13!11Dual mode operation • Hardware knows that an operating system will be used, and that it needs more privileges than application software. • Hardware bit for user/kernel mode. • Need kernel mode access for: – Accessing kernel data structures, e.g. list open files – Mapping device mem to main mem, e.g. graphics card – Kernel registers, e.g. page table offset – Privileged instructions, e.g. switch to kernel mode 12/2/09 UCB CS61CL F09 Lec 13!12Switching between User/Kernel mode 12/2/09 UCB CS61CL F09 Lec 13!13 Application code: OS code: Kernel User Mode bit: Instruction 1 Instruction 2 … Syscall Instruction N … … Bootup instructions … Dispatch application … Handle syscall … Finish syscallInterrupts • Hardware interrupts • Software-generated interrupts (called Traps) – System calls: user has OS do something on its behalf, trap or syscall instruction. – Exceptions: if privileged instruction called when in user-level, handled similar to a system call 12/2/09 UCB CS61CL F09 Lec 13!14Trends • OS used to handle concurrency for us (time sharing), now applications are making smarter use of concurrency (threading) – ParLab • Cloud computing – RAD Lab 12/2/09 UCB CS61CL F09 Lec 13!15Summary • OS multiplexes hardware resources & provides clean abstraction for applications. • Dual mode operation, interrupts, exceptions • Parallel and cloud computing • Take CS162 to pick up where we’re leaving off and actually build an OS (and see more of me)! 12/2/09 UCB CS61CL F09 Lec
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