CMSC Operating Systems I An Introduction to Operating System Concepts

CMSC What’s an Operating System? l “The Boss” l “The Big Cheese” l “The Godfather” l “The Power Behind the Scenes” l A control program the manages all the resources of the computer on which it runs.

CMSC OS Introduction l Most computers multitask (do many things “at once”) oread/write disks and tapes osend/receive network traffic orun several programs. l Resources must be shared and programs coordinated

CMSC OS Introduction l The operating system manages multi- tasking systems. oWhat resources are available. oWho gets to use them. --- “Not now Bobby, it’s Sally’s turn.” --- l Also provides: o standard interfaces (ways to use) to computer hardware ostandard utility programs supporting other software.

CMSC “McDonalds Model” of an Operating System Counter Drinks Grill Fries Customers (Applications) Crew (Operating System) Crew Serves Customers. Customers Don’t Cross Counter

CMSC Operating System Evolution “Batch” Operating Systems l “One-at-a-time” in a “batch” oUsually single-task oExecution required significant setup of equipment and software oEach program was a job (“batch job”) oOS designed to simplify setup and transition between jobs

CMSC Batch Operating Systems (cont’d) l Operators served as “High Priests.” oUsers didn’t touch computers User’s “submitted” jobs Picked up program output Written/punched input and instructions Printed/punched output and error reports oTurn around: minutes to days “Error 410: Unterminated Do Loop, line 66”

CMSC Operating System Evolution Interactive Processing l OS for Interactive Processing oStarted with “operator-like” single user oUsers interact directly via remote terminals (workstations) --- “On-Line processing” oUsers demand timely response oMachines too expensive for single users N.B. “real-time” means “must respond in limited time or fail” --- not on-line system

CMSC Operating System Evolution Time-Sharing Systems l Really “time-slicing” systems l Operating system: oputs jobs in a “circular queue” ogives each job a predetermined “time slice” oswaps old job out and new one in at the end of each slice orepeats fast enough to give illusion of simultaneously execution

CMSC Operating System Evolution Time-Sharing Systems (continued) l “Radar O’Reilly” model of operating systems oRead the mail oMake the coffee oCheck on Hawkeye oAnswer the phone l Only one job at a time, but all progress “on the average.”

CMSC Operating System Evolution Time Sharing (cont’d) l Other Benefits: obest allocation of user time --- no “dead time” at the keyboard ono waiting for peripherals --- other processes can execute while one waits. l Costs: ocomplexity

CMSC Operating System Evolution Examples l Batch Systems: oIBM 1620, early MITS l Interactive systems: oDOS, early MacOS, VIC20, Commodore 64 l Time-Sharing systems: oUnix, Linux, Multics, IBM MVS, DEC VMS oWindows 95/98/NT, later MacOS

CMSC Different Operating Systems on the Same Machine ? l Only one at a time oVAX: VMS or Ultrix oIBM PCs: Linux or Windows or OS/2 l Emulators let one OS act like another o“Soft-PC”: Mac Windows o“Wine” and “Wabi”: Linux Windows oCygnus Windows: Win95/98/NT Unix oNot always complete

CMSC Inside the Operating System The OS Shell l Defines interface between OS and users oWindows GUI oUNIX command line oUNIX users can choose among a variety of shells csh is the “C shell” tcsh is an enhanced “C shell” oShell programming

CMSC Inside the Operating System OS Shell interface O / S shell Users

CMSC Inside the Operating System Different Shells/Same Results Operating System BASH TCSH CSH SH Different Command Sets Same Results

CMSC Inside the Operating System The OS Kernel l Central part (“hard nugget”) of Operating System l Kernel Components oFile Manager oDevice Drivers oMemory Manager oScheduler oDispatcher

CMSC Inside the Opeating System File Manager l Maintains information about the files that are available on the system olocation osize, type, and protections owhat storage is still available l Files usually allowed to be grouped in directories or folders. Allows hierarchical organization.

CMSC Inside the Operating System Device Drivers l Software to communicate with peripheral devices or controllers l Each driver is unique l Translates general requests into specific steps for that device “read poker.exe” “read device 0xA3, cylinder 3, track 13, sector 43” Device Driver

CMSC Inside the Operating System Memory Manager l Controls use of main memory l Allocates memory for each program and its data l Reclaims memory when programs release it (or finish). l Is supposed to know what areas are free. “Memory leak” manager loses track

CMSC Inside the Operating System Memory Manager (cont’d) l Virtual memory (“Imaginary memory”) oPrograms only need their current data in memory. (“What’s your VIN?”) oStore remainder on disk & keep location. oBring other data back (and put this data out) when other data is needed. oCreates the illusion of additional memory by rotating (swapping) programs and data between main memory and mass storage

CMSC Inside the Operating System Virtual Memory for People You Calendar Sticky notes Notebooks

CMSC Inside the Operating System Scheduler l Maintains a record of processes that are present, adds new processes, removes completed processes l Tracks omemory area(s) assigned opriority ostate of readiness to execute (ready/wait)

CMSC Inside the Operating System Dispatcher l Executes processes when scheduled and ready l Gives each process a small (50 ms) time slice in which to execute l Stops process when time is up. l Waits for scheduler to update process record l Starts next scheduled process.

CMSC What’s a Process l The dynamic activity of executing a program l The process state is a “snapshot” of the machine, including values of the CPU registers, program counter and other memory cells l A single program can be associated with multiple processes simultaneously

CMSC Inside the Operating System Types of software l Applications software l Do something “for the outside world.” l Often transportable l System Software l Performs tasks supporting operation of computer systems (“not outside world”) e.g. operating systems l Usually tightly coupled to hardware

CMSC Inside the Operating System Types of software (cont’d) l Utility software oprovides fundamental not included OS (check spelling, count words, calculator) o“extend” the OS (or “support” applications) oDistinction between applications and utilities is often vague oDistinction between OS and utilities is also vague

CMSC Inside the Operating System Utilities l Operating Systems usually come with some associated utility programs l UNIX usually has the text editors emacs and vi (and sometimes pico) l UNIX has its own sort utility l UNIX has its own mail utility (The contraction “it’s” means “it is,” not “belonging to it.”)

CMSC Inside the Operating System Summary l Shell -- interface to user l File Manager -- manages mass memory l Device Drivers -- communicate with peripherals l Memory manager -- manages main memory l Scheduler & Dispatcher -- manage processes