CS 346 – Chapter 1 Operating system – definition Responsibilities What we find in computer systems Review of –Instruction execution –Compile – link – load – execute Kernel versus user mode

Questions What is the purpose of a computer? What if all computers became fried or infected? How did Furman function before 1967 (the year we bought our first computer)? Why do people not like computers?

Definition How do you define something? Possible approaches: –What it consists of –What is does (a functional definition) – purpose –What if we didn’t have it –What else it’s similar to OS = set of software between user and HW –Provides “environment” for user to work –Convenience and efficiency –Manage the HW / resources –Ensure correct and appropriate operation of machine 2 Kinds of software: application and system –Distinction is blurry; no universal definition for “system”

Some responsibilities Can glean from table of contents –Book compares an OS to a government –Don’t worry about details for now Security: logins Manage resources –Correct and efficient use of CPU –Disk: “memory management” –network access File management I/O, terminal, devices Kernel vs. shell

Big picture Computer system has: CPU, main memory, disk, I/O devices Turn on computer: –Bootstrap program already in ROM comes to life –Tells where to find the OS on disk. Load the OS. –Transfer control to OS once loaded. From time to time, control is “interrupted” –Examples? Memory hierarchy –Several levels of memory in use from registers to tape –Closer to CPU: smaller, faster, more expensive –OS must decide who belongs where

Big picture (2) von Neumann program execution –Fetch, decode, execute, data access, write result –OS usually not involved unless problem Compiling –1 source file  1 object file –1 entire program  1 executable file –“link” object files to produce executable –Code may be optimized to please the OS –When you invoke a program, OS calls a “loader” program that precedes execution I/O –Each device has a controller, a circuit containing registers and a memory buffer –Each controller is managed by a device driver (software)

2 modes When CPU executing instructions, nice to know if the instruction is on behalf of the OS OS should have the highest privileges  kernel mode –Some operations only available to OS –Examples? Users should have some restriction  user mode A hardware bit can be set if program is running in kernel mode Sometimes, the user needs OS to help out, so we perform a system call

Management topics What did we ask the OS to do during lab? File system Program vs. process –“job” and “task” are synonyms of process –Starting, destroying processes –Process communication –Make sure 2 processes don’t interfere with each other Multiprogramming –CPU should never be idle –Multitasking: give each job a short quantum of time to take turns –If a job needs I/O, give CPU to another job

More topics Scheduling: deciding the order to do the jobs –Detect system “load” –In a real-time system, jobs have deadlines. OS should know worst-case execution time of jobs Memory hierarchy –Higher levels “bank” the lower levels –OS manages RAM/disk decision –Virtual memory: actual size of RAM is invisible to user. Allow programmer to think memory is huge –Allocate and deallocate heap objects –Schedule disk ops and backups of data