Transcript chapter 1
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