Operation System

OS

An operating system (OS) is a collection of software that manages computer hardware resources and provides common services for computer programs. The operating system is an essential component of the system software in a computer system. Application programs usually require an operating system to function.The operating system acts as an interface between the hardware and the programs requesting I/O. It is the most fundamental of all system software programs.
Responsibilities of the OS include:

• Hiding the complexities of hardware from the user

• Managing between the hardware's resources which include the processors, memory, data storage and I/O devices

• Handling "interrupts" generated by the I/O controllers

• Sharing of I/O between many programs using the CPU

The most well known Operating Systems include:

• System Software - programs that manage the operation of a computer

• Application Software - programs that help the user perform a particular task

User InterfacesThe User Interface is the interaction between the User and the Machine, letting the user send commands with the expected results. Two forms of the Interface User are the Command Line Interface and the Graphical User Interface.


Memory management is the act of managing computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when no longer needed. This is critical to any advanced computer system where more than a single process might be underway at any time.

Process management is the ensemble of activities of planning and monitoring the performance of a process. The term usually refers to the management of business processes and manufacturing processes. Business process management (BPM) and business process reengineering are interrelated, but not identical.

In computer science, scheduling is the method by which threads, processes or data flows are given access to system resources (e.g. processor time, communications bandwidth). This is usually done to load balance a system effectively or achieve a target quality of service. The need for a scheduling algorithm arises from the requirement for most modern systems to perform multitasking (execute more than one process at a time) and multiplexing (transmit multiple flows simultaneously).
--The scheduler is concerned mainly with:
--Throughput - The total number of processes that complete their execution per time unit.
--Latency, specifically:
--Turnaround time - total time between submission of a process and its completion.--
--Response time - amount of time it takes from when a request was submitted until the first response is produced.

--Fairness / Waiting Time - Equal CPU time to each process (or more generally appropriate times according to each process' priority). It is the time for which the process remains in the ready queue.

An instruction cycle (sometimes called fetch-and-execute cycle, fetch-decode-execute cycle, or FDX) is the basic operation cycle of a computer. It is the process by which a computer retrieves a program instruction from its memory, determines what actions the instruction requires, and carries out those actions. This cycle is repeated continuously by the central processing unit (CPU), from bootup to when the computer is shut down.

Each computer's CPU can have different cycles based on different instruction sets, but will be similar to the following cycle:

1. Fetching the instruction

The next instruction is fetched from the memory address that is currently stored in the program counter (PC), and stored in the instruction register (IR). At the end of the fetch operation, the PC points to the next instruction that will be read at the next cycle.

2. Decode the instruction

The decoder interprets the instruction. During this cycle the instruction inside the IR (instruction register) gets decoded.

3.In case of a memory instruction (direct or indirect) the execution phase will be in the next clock pulse.

If the instruction has an indirect address, the effective address is read from main memory, and any required data is fetched from main memory to be processed and then placed into data registers(Clock Pulse: T3). If the instruction is direct, nothing is done at this clock pulse. If this is an I/O instruction or a Register instruction, the operation is performed (executed) at clock Pulse.

4. Execute the instruction

The control unit of the CPU passes the decoded information as a sequence of control signals to the relevant function units of the CPU to perform the actions required by the instruction such as reading values from registers, passing them to the ALU to perform mathematical or logic functions on them, and writing the result back to a register. If the ALU is involved, it sends a condition signal back to the CU.