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CS 346 – Chapter 2

CS 346 – Chapter 2. OS services OS user interface System calls System programs How to make an OS Implementation Structure Virtual machines Commitment For next day, please finish chapter 2. OS services. 2 types For the user’s convenience Shell Running user programs Doing I/O

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CS 346 – Chapter 2

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  1. CS 346 – Chapter 2 • OS services • OS user interface • System calls • System programs • How to make an OS • Implementation • Structure • Virtual machines • Commitment • For next day, please finish chapter 2.

  2. OS services 2 types • For the user’s convenience • Shell • Running user programs • Doing I/O • File system • Detecting problems  • Internal/support • Allocating resources • System security • Accounting • Infamous KGB spy ring uncovered due to discrepancy in billing of computer time at Berkeley lab

  3. User interface • Command line = shell program • Parses commands from user • Supports redirection of I/O (stdin, stdout, stderr) • GUI • Pioneered by Xerox PARC, made famous by Mac • Utilizes additional input devices such as mouse • Icons or hotspots on screen • Hybrid approach • GUI allowing several terminal windows • Window manager

  4. System calls • “an interface for accessing an OS service within a computer program” • A little lower level than an API, but similar • Looks like a function call • Examples • Performing any I/O request, because these are not defined by the programming language itself e.g. read(file_ptr, str_buf_ptr, 80); • assembly languages typically have “syscall” instruction. When is it used? How? • If many parameters, they may be put on runtime stack

  5. Types of system calls • Controlling a process • File management • Device management • Information • Communication between processes • What are some specific examples you’d expect to find?

  6. System programs • Also called system utilities • Distinction between “system call” and “system program” • Examples • Shell commands like ls, lp, ps, top • Text editors, compilers • Communication: e-mail, talk, ftp • Miscellaneous: cal, fortune • What are your favorites? • Higher level software includes: • Spreadsheets, text formatters, etc. • But, boundary between “application” and “utility” software is blurry. A text formatter is a type of compiler!

  7. OS design ideas • An OS is a big program, so we should consider principles of systems analysis and software engineering • In design phase, need to consider policies and mechanisms • Policy = What should we do; should we do X • Mechanism = how to do X • Example:  a way to schedule jobs (policy) versus: what input needed to produce schedule, how schedule decision is specified (mechanism)

  8. Implementation • Originally in assembly • Now usually in C (C++ if object-oriented) • Still, some code needs to be in assembly • Some specific device driver routines • Saving/restoring registers • We’d like to use HLL as much as possible – why? • Today’s compilers produce very efficient code – what does this tell us? • How to improve performance of OS: • More efficient data structure, algorithm • Exploit HW and memory hierarchy • Pay attention to CPU scheduling and memory management

  9. Kernel structure • Possible to implement minimal OS with a few thousand lines of code  monolithic kernel • Modularize like any other large program • After about 10k loc, difficult to prove correctness • Layered approach to managing the complexity • Layer 0 is the HW • Layer n is the user interface • Each layer makes use of routines and d.s. defined at lower levels • # layers difficult to predict: many subtle dependencies • Many layers  lots of internal system call overhead 

  10. Kernel structure (2) • kernel • Kernel = minimal support for processes and memory management • (The rest of the OS is at user level) • Adding OS services doesn’t require changing kernel, so easier to modify OS • The kernel must manage communication between user program and appropriate OS services (e.g. file system) • Microsoft gave up on kernel idea for Windows XP • OO Module approach • Components isolated (OO information hiding) • Used by Linux, Solaris • Like a layered approach with just 2 layers, a core and everything else

  11. Virtual machine • How to make 1 machine behave like many • Give users the illusion they have access to real HW, distinct from other users • Figure 2.17 levels of abstraction: • Processes / kernels / VM’s / VM implementations / host HW As opposed to: • Processes / kernels / different machines • Why do it? • To test multiple OS’s on the same HW platform • Host machine’s real HW protected from virus in a VM bubble

  12. VM implementation • It’s hard! • Need to painstakingly replicate every HW detail, to avoid giving away the illusion • Need to keep track of what each guest OS is doing (whether it’s in kernel or user mode) • Each VM must interpret its assembly code – why? Is this a problem? • Very similar concept: simulation • Often, all we are interested in is changing the HW, not the OS; for example, adding/eliminating the data cache • Write a program that simulates every HW feature, providing the OS with the expected behavior

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