1. OPERATING SYSTEM CONCEPT AND PRACTISE
CSC633

2. INTRODUCTION
A computer is an electronic machine which consists of hardware and software resources.
Hardware components refers to physical parts of computer.
Software means computer instructions or data that is stored electronically on computer.

3. A system refers to a complete working computer.
Input devices are what is used to put data into computer example mouse, keyboard microphone etc.
Output devices are through which the computer gives information to user example printers, monitors, loudspeakers etc

4. There are two types of memory in Computer:
Primary Memory
Secondary Memory
Primary memory is internal memory of computer and provides main working space to computer.
Terms under Primary memory
Random Access Memory
Read Only Memory
Programmable Read Only Memory(PROM)

5. Cache Memory
Erasable Programmable Read Only Memory(EPROM)
Random Access Memory (RAM) is that can be accessed randomly ,a common type of memory found in computer and devices like printers.
Read Only Memory(ROM) refers to special memory used to store programs.

6. CPU is made up of 3 main parts :
In a programmable read only memory (PROM) programs or instructions written cannot be changed.
In Erasable programmable read only memory(EPROM) chip can be written again by erasing the information stored earlier in it.
CPU is made up of 3 main parts :
Arithmetic Logic Unit(ALU)
Registers
Control Unit

7. Arithmetic Logic Unit (ALU) is a part of computer in CPU used to perform all arithmetic computations.
A Register is a high speed storage area in the CPU
Control Unit is a component of CPU, it controls and coordinates between input and output devices.

8. The Motherboard serves to connect all of the parts of a computer together.
A Motherboard is the piece of computer hardware that can be thought of as the “backbone” of pc.
Main memory is where anything that the computer is working with is kept.

9. A Buffer is used for temporary storage of data that is waiting to be sent to a device.
A Central Processing Unit (CPU) is the most important element of computer system where most calculations takes place.
The number of registers that a CPU has and the size of each help determine the power and speed of Central Processing Unit(CPU)

10. A Register is a high-speed storage area within the Central Processing Unit(CPU). All data must be represented in a register before it is processed.
The Register contains address of memory location where data is stored.
Sometimes the speed of CPU is high compared to access time of main memory, therefore the performance of CPU decreases. To decrease this mismatch a small memory chip is attached between CPU and main memory known as cache.

11. What is an Operating System?
A program that acts as an intermediary between a user of a computer and the computer hardware.
The operating system is a chief piece of software that manages all the hardware and software resources.
Use the computer hardware in an efficient manner.

12. Tasks of Operating system:
Memory Management
Human computer Interface
Error Reporting
File management
System software is a term given to set of computer programs that make the hardware work properly.

13. Four Components of a Computer System

14. Computer system can be divided into 4 components:
Users: People, machines, other computers
Compiler: It is a computer program that translates source code from a high level programming language to a lower level language
An assembler is a program that takes basic computer instructions and converts them into a pattern of bits.
A text editor is a type of program used for editing plain text files

15. A Database is an organized collection of data.
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
Hardware provides basic computing devices like CPU, memory, I/O devices.
Operating System controls the use of hardware among various applications and users.

16. User View of Computer Most systems are designed for one user.
Main goal is to maximize the work the user performs.
In some cases user is connected through a minicomputer and other users access same computer to exchange information.
Some cases users sit at workstation connected to a network of other stations

17. System View
From computer's point of view, the operating system is the program most intimately involved with the hardware.
Operating system is a resource allocator.
A operating system is mainly to control program.

18. COMPUTER SYSTEM ORGANIZATION

19. One or more CPUs, device controllers connect through common bus providing access to shared memory.
Each device controller is in charge of a specific type of device (for example, disk drives, audio devices, and video displays)
Central Processing Unit known as CPU is the hardware within the computer.

20. For a computer to start running , when it is rebooted it needs initial program named bootstrap program.
Bootstrap must know how to load the operating system and start executing.
It locates and load into memory the operating system kernel.
The operating system executes the first process such as Init.

21. The occurrence of event is signaled by an interrupt.
When CPU is interrupted it stops and immediately and transfers execution to fixed location.
The location contains starting address where service routine is located.
The fixed location usually contains the starting address where the service routine for the interrupt is located.

22. Common Functions of Interrupts
Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines.
Interrupt architecture must save the address of the interrupted instruction.
Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.
A trap is a software-generated interrupt caused either by an error or a user request.
An operating system is interrupt driven.

23. I/O Structure
A general computer system consists of CPU and multiple devices connected through a common bus.
Each device controller is in charge of a specific type of device
A device controller maintains some local buffer storage and a set of special-purpose registers.
Operating systems have a device driver for each device controller.

24. To start a I/O operation the following steps takes place:
Device driver loads appropriate registers within device controller.
Device controller examines the contents of registers
The controller transfers data from device to local buffer
Once transfer is complete the device controller informs device driver via interrupt.
Device driver returns control to OS.

25. Direct Memory Access (DMA)used for high-speed I/O devices able to transmit information at close to memory speeds.
Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention.
Only one interrupt is generated per block, rather than the one interrupt per byte.

26. COMPUTER SYSTEM OPERATION
A trap (or an exception) is a software-generated interrupt caused either by an error or by a specific request from a user program that an operating-system service be performed.
For each type of interrupt, separate segments of code in the operating system determine what action should be taken.
A properly designed operating system must ensure that an incorrect (or malicious) program cannot cause other programs to execute incorrectly.

27. Dual-Mode Operation we need two separate modes of operation: user mode
kernel mode

28. A bit, called the mode bit, is added to the hardware of the computer to indicate the current mode.
When the computer system is executing on behalf of a user application, the system is in user mode.
When a user application requests a service from the operating system it must transition from user to kernel mode to fulfill the request.

29. At system boot time, the hardware starts in kernel mode
At system boot time, the hardware starts in kernel mode. The operating system is then loaded and starts user applications in user mode.
Whenever a trap or interrupt occurs, the hardware switches from user mode to kernel mode.
The system always switches to user mode before passing control to user program

30. COMPUTER SYSTEM ARCHIECTECTURE
It can be categorized with respect to number of processors used.
Single-Processor System: There is one main CPU capable of executing a general-purpose instruction set.
Multiprocessor Systems :Systems have two or more processors in close communication sharing the computer bus.

31. Clustered Systems: Clustered computers share storage and are closely linked via a local-area network (LAN).

32. OPERATING SYSTEM STRUCTURE

33. Important aspect of operating system is multi-programming.
The operating system keeps several jobs in memory.
The operating system picks and begins to execute one of the jobs in memory.
Eventually, the job may have to wait for some task, such as an I/O operation, to complete.