Copyright © Curt Hill Operating Systems An Introductory Overview

Copyright © Curt Hill Application or System? What is the difference? Can be fuzzy at times System –Runs the machine –Necessary evil –Without system software we cannot run the machine Application –Why we bought system –Performs the work that we want done –Spreadsheets, Word processors, Databases etc.

Copyright © Curt Hill What does an OS do? Execute application programs Provide services to the program Establish and manage a file system Utility functions Many functions that are used by most programs will eventually be incorporated into the OS

Copyright © Curt Hill Execute application programs Interpret commands –Interpret typed commands or mouse actions Load programs –The loader is an OS program Relocation Memory allocation Set as a task –Provide services to the running programs

Copyright © Curt Hill Provide services to the program Memory management –Memory allocation –Virtual or fixed memory I/O management –Manage file activity Task Scheduler –Inter-process communication –Managing priorities

Copyright © Curt Hill Establish and manage a file system Determine how things are stored on disk or tape Do we have a directory or just search? Are things stored in a contiguous fashion or scattered? How are things named? How are things accessed? Interrupt processing Utility functions –Prepare a disk for use –Copy/move/delete files –Show contents of a file or disk

Copyright © Curt Hill How are files accessed? Access to files is a program service Program says that it wants access to file: fn System finds it Gets it ready Gives it to the program one chunk at a time Several things should be transparent to program Buffering Devices

Copyright © Curt Hill Utility functions Protection –Files from deletion by someone other than owner –Memory of one program from another Hardware manipulation –Mount disks and tapes Miscellaneous –Error detection –Multitasking –Accounting

Copyright © Curt Hill Types of OS Tasking types –How are processes handled Number of Users –Single or multiple Program Interaction Type –Console or GUI or both

Copyright © Curt Hill Single tasking vs multitasking Tasking –How many things are running at once DOS is really a single tasking OS –The print command attempts simple multitasking, but it only gets control when DOS gets control –Not time-slice or interrupt driven Windows 3 tries harder it did simple multitasking UNIX, NT and most mainframe OS do real multitasking

Copyright © Curt Hill Single user vs. multiuser Windows 3, Win 95 and DOS are really single user –There was no protection of one user from another Windows NT and descendents claim to be multiuser –The system does not protect users from one another as well as it could UNIX is multiuser –Most of the bugs have been fixed

Copyright © Curt Hill Batch and timesharing Old mainframes were batch oriented and not interactive –Maximizes utilization of CPU Timesharing –Timesharing implies multi-tasking –However, DOS was modeled after a time sharing interface Most OSs are time sharing these days, but that was not the case in the old days

Copyright © Curt Hill Pieces of the operating system CPU management Memory management Process management I/O management User management File management Others

Copyright © Curt Hill CPU management - Interrupt handling All the external hardware will use interrupts How is a device interrupt handled? Device drivers –Low level programs that oversee the actions of various devices The interrupt must be processed in a timely fashion without disturbing other processes

Copyright © Curt Hill CPU management - Task scheduling Each task must be known to OS When the task is suspended another should be started Task management is the handling of these details The task dispatcher determines which is next –This is one part of OS that must be serialized

Copyright © Curt Hill Memory management Virtual memory systems will be discussed later Allocation requests –Loader for creating a process –Program for new space, such as new –File system for buffers Deallocation Memory leaks are the sign of a poorly constructed OS –Win 95/98/ME

Copyright © Curt Hill Process management Starting or loading –Usually initiated by OS in response to program start request Creation and deletion –One program starting another Interprocess communication –Two independent programs communicating through the OS

Copyright © Curt Hill I/O management I/O services –These are services provided to a program –Open, close, read, write a file –Rename, move, delete a file without processing it Device management –These are usually done through OS directly –Mount or demount a device –Format or partition a disk

Copyright © Curt Hill File management Some overlap with I/O management –Usually the OS perspective rather than program perspective Device maintenance Volume maintenance Creation/Deletion/Accessing of file system Access controls Backup and recovery

Copyright © Curt Hill User management Logging in –Includes authentification –Logging Accounting –Gives the ability to charge a user for services –CPU time, disk storage, connect time Access controls –Which user may access which files?

Copyright © Curt Hill OS Layers Most OSs are onion shaped –For historical and maintenance reasons –Historically the innermost was all that was available Each layer then communicates with adjacent layers through well defined pathes –Well defined in terms of this particular OS Layers: Kernel, Service, Command

Copyright © Curt Hill Kernel is the innermost layer Low level activities only –Load and/or execute a program –Device allocation and access –Process management –Low level file access This is the lowest level interface to the hardware and the only one that touches the hardware directly In DOS there is the BIOS which amounts to a kernel The kernel is usually always resident in the memory Basic process management is usually here, since we need to make each program we load a process

Copyright © Curt Hill Service layer Set of routines that provide services –Use the kernel –This is where the traditional file manipulation commands are executed, such as opening and closing files, reading and writing records –Entire file copying is usually done at the next higher level The service layer represents the services that are available to a program The service layer is also designed to generalize the interface between devices –A CRT, disk and tape are wildly different, yet we would like their interface to look similar

Copyright © Curt Hill Service Layer In the assembly language interface each of the service calls becomes a macro or subroutine call In a HLL it is usually built into the command part of the language or is in a library of some sort Some process management is here as well –Inter-process communication

Copyright © Curt Hill Command layer The interface that allows the user to interface with the system directly This includes command interpretation This is Command.com in DOS or program manager in Windows Many utility functions are placed in this layer –The DOS Format is a command in this layer It performs an essential function, but is its own program

Copyright © Curt Hill Next Before going any further we should consider the history of operating systems