Operating Systems

Overview What is an Operating System (OS) What is an Operating System (OS) What Operating Systems do. What Operating Systems do. Operating system issues Operating system issues Kinds of Operating Systems Kinds of Operating Systems

What is an OS? “An operating system is a layer of software which takes care of technical aspects of a computer's operation. It shields the user of the machine from the low-level details of the machine's operation and provides frequently needed facilities” Taken from

What is an OS? An Operating System makes the computing power available to users by controlling the hardware An Operating System makes the computing power available to users by controlling the hardware Without an OS, computers would only be able to run one program at a time Without an OS, computers would only be able to run one program at a time Programs must run according to the rules of the OS Programs must run according to the rules of the OS

What Operating Systems do: 1. Provide software for driving the hardware of the computer 2. Implement a file system which provides a way of organizing files logically 3. Distribute computer resources between applications

1. Software that Runs Hardware A driver is software that designed to run a piece of hardware A driver is software that designed to run a piece of hardware Ex: Printer, fancy keyboard, scanner Ex: Printer, fancy keyboard, scanner New drivers are built into the OS. This is why it is often necessary to restart your computer when a driver is added New drivers are built into the OS. This is why it is often necessary to restart your computer when a driver is added

2. File System The OS must save files in a logical efficient way The OS must save files in a logical efficient way Hard drive, floppy disk Hard drive, floppy disk Through FAT (File Allocation Table) Through FAT (File Allocation Table) Recall, FAT32 vs FAT16 Recall, FAT32 vs FAT16 32 bit addressing means each section of memory is broken up into 2^32 individual addressing locations 32 bit addressing means each section of memory is broken up into 2^32 individual addressing locations Much faster to find a file! Much faster to find a file!

File Systems: NTFS NTFS is the “NT File System” developed by Microsoft NTFS is the “NT File System” developed by Microsoft Faster and more efficient. Faster and more efficient. XP and Vista would use NTFS XP and Vista would use NTFS

RAM OS is also responsible for memory allocation in RAM OS is also responsible for memory allocation in RAM The OS itself stays in RAM while the computer is on The OS itself stays in RAM while the computer is on OS controls what is moved into and out of RAM OS controls what is moved into and out of RAM

Virtual Memory Several programs can’t fit in main memory so what happens? Several programs can’t fit in main memory so what happens? Virtual memory is parts of active programs stored on the hard drive. Virtual memory is parts of active programs stored on the hard drive. EX: I’ve got Windows XP running, MS Word, 3 Explorer Windows one being active EX: I’ve got Windows XP running, MS Word, 3 Explorer Windows one being active Rather then not letting you open more programs the OS will run them using Virtual Memory Rather then not letting you open more programs the OS will run them using Virtual Memory

3. Distributing Computer Resources The CPU can only do one thing at a time. Fortunately it can do billions of things per second… The CPU can only do one thing at a time. Fortunately it can do billions of things per second… The OS is responsible for giving CPU time to currently running applications. The OS is responsible for giving CPU time to currently running applications. The OS also makes sure two applications don’t try accessing the same piece of hardware at the same time. The OS also makes sure two applications don’t try accessing the same piece of hardware at the same time.

2 OS Issues Concurrency control (multi-tasking) Concurrency control (multi-tasking) Deadlock Deadlock

Concurrency Control Several programs running at a time Several programs running at a time Means Sharing CPU time Means Sharing CPU time While a program waits for an I/O operation another program can use the CPU While a program waits for an I/O operation another program can use the CPU

Deadlock Program A wants to copy from disk1 to disk2 and takes control of disk1 Program A wants to copy from disk1 to disk2 and takes control of disk1 Program B wants to copy from disk2 to disk1 and takes control of disk2 Program B wants to copy from disk2 to disk1 and takes control of disk2 Program A must wait until program B releases disk2 and program B must wait until program A releases disk1 Program A must wait until program B releases disk2 and program B must wait until program A releases disk1 Programs A and B will wait forever! Programs A and B will wait forever!

Deadlock Prevention OS must have a systematic way of controlling program execution OS must have a systematic way of controlling program execution This is a very complex problem This is a very complex problem One simple not very efficient solution is to have programs “timeout”, in our example program A would timeout first and release disk 1 and try again shortly One simple not very efficient solution is to have programs “timeout”, in our example program A would timeout first and release disk 1 and try again shortly

Kinds of Operating Systems What’s the best? What’s the best? Windows=Microsoft =monopoly!!! Windows=Microsoft =monopoly!!! Success due to??? Success due to??? Other alternatives: Other alternatives: Mac- I’ve never met a sad Mac user Mac- I’ve never met a sad Mac user Linux/Unix- Small very efficient, maybe less intuitive Linux/Unix- Small very efficient, maybe less intuitive

Work on OS Assignment