1. Assoc. Prof. Dr. Ahmet Turan ÖZCERİT

2.  What Operating Systems Do  Computer-System Organization  Computer-System Architecture  Operating-System Structure  Operating-System Operations  Process Management  Memory Management You will learn: 2

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5. 5 In isolation, the microprocessor, the memory and the input/output ports are interesting components, but they cannot do anything useful. In combination, they can form a complete system if they can communicate with each other. This communication is accomplished over bundles of signal wires (known as buses) that connect the parts of the system together. There are normally three types of bus in any processor system: An address bus: this determines the location in memory that the processor will read data from or write data to. A data bus: this contains the contents that have been read from the memory location or are to be written into the memory location. A control bus: this manages the information flow between components indicating whether the operation is a read or a write and ensuring that the operation happens at the right time. Processor Schematic Architecture

6. 6 Computer-system operation One or more CPUs, device controllers connect through common bus providing access to shared memory Concurrent execution of CPUs and devices competing for memory cycles

7. Computer system can be divided into four components  Hardware – provides basic computing resources CPU, memory, I/O devices  Operating system Controls and coordinates use of hardware among various applications and users  Application programs – define the ways in which the system resources are used to solve the computing problems of the users Word processors, compilers, web browsers, database systems, video games  Users People, machines, other computers 7

8. A program that acts as an intermediary between a user of a computer and the computer hardware  Operating system goals: Execute user programs and make solving user problems easier Make the computer system convenient to use Use the computer hardware in an efficient manner OS is a resource allocator Manages all resources Decides between conflicting requests for efficient and fair resource use OS is a control program Controls execution of programs to prevent errors and improper use of the computer 8

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11. 11 Main memory – only large storage media that the CPU can access directly Secondary storage – extension of main memory that provides large nonvolatile storage capacity Magnetic disks – rigid metal or glass platters covered with magnetic recording material Disk surface is logically divided into tracks, which are subdivided into sectors The disk controller determines the logical interaction between the device and the computer

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14. 14 Important principle, performed at many levels in a computer (in hardware, operating system, software) Information in use copied from slower to faster storage temporarily Faster storage (cache) checked first to determine if information is there If it is, information used directly from the cache (fast) If not, data copied to cache and used there Cache smaller than storage being cached Cache management important design problem Cache size and replacement policy

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16. 16 The terms task and process are often used interchangeably. A process is a program in execution. The execution of a process must progress in a sequential fashion. A single-tasking system can only run one program at a time, while a multi- tasking operating system allows more than one program to be running in concurrency. This is achieved by time-sharing, dividing the available processor time between multiple processes which are each interrupted repeatedly in time-slices by a task scheduling subsystem of the operating system.multi- taskingtime-sharing A thread is the smallest unit of processing that can be performed in an OS. In most modern operating systems, a thread exists within a process - that is, a single process may contain multiple threads.

17. 17  A process is an executing program, including the current values of the program counter, registers, and variables.  The subtle difference between a process and a program is that the program is a group of instructions whereas the process is the activity.  In multiprogramming systems, processes are performed in a pseudo parallelism as if each process has its own processor.  In fact, there is only one processor but it switches back and forth from one process to another.

18. 18  According to these definitions, we classify programs as  Processor-bound program: A program having long processor bursts (execution instants).  I/O-bound program: A program having short processor bursts.

19. 19 Assume we have two processes A and B. Both execute for 1 second and do some I/O work for 1 second. This pattern is repeated 3 times for process A and 2 times for process B.

20. 20 If we have no multiprogramming, the processes are executed sequentially as below. So, the processor executes these two processes in a total time of 10 seconds. However, it is idle at I/O instants of processes. So, it is idle for 5 seconds and utilized for 5 seconds. Then the processor utilization is 50%

21. 21 Start : The process has just arrived. Ready : The process is waiting to grab the processor. Running : The process has been allocated by the processor. Waiting : The process is doing I/O work or blocked. Halted : The process has finished and is about to leave the system

22. 22  In the OS, each process is represented by its PCB (Process Control Block). The PCB, generally contains the following information: Process State Process ID, Program Counter (PC) value, Register values Memory Management Information (page tables, base/bound registers,…) Processor Scheduling Information( priority, last processor burst time etc.) I/O Status Info (outstanding I/O requests, I/O devices held, etc.) List of Open Files Accounting Information

23. 23 Scheduler: Adds new processes to the process table and removes completed processes from the process table Dispatcher: Controls the allocation of time slices to the processes in the process table –The end of a time slice is signaled by an interrupt.

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25. 25 Logical versus Physical Address Space An address generated by the CPU is a logical address whereas address actually available on memory unit is a physical address. Logical address is also known a Virtual address. Virtual and physical addresses are the same in compile-time and load- time address-binding schemes. Virtual and physical addresses differ in execution-time address-binding scheme. The set of all logical addresses generated by a program is referred to as a logical address space. The set of all physical addresses corresponding to these logical addresses is referred to as a physical address space. The run-time mapping from virtual to physical address is done by the memory management unit (MMU) which is a hardware device. MMU uses following mechanism to convert virtual address to physical address. The value in the base register is added to every address generated by a user process which is treated as offset at the time it is sent to memory. For example, if the base register value is 10000, then an attempt by the user to use address location 100 will be dynamically reallocated to location 10100. The user program deals with virtual addresses; it never sees the real physical addresses.

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