Os_module - I.pdf

Operating Systems (MCA-2007)

Dr. B. B. Sagar Department of Computer Science and Engineering Birla Institute of Technology, Ranchi (Noida Campus)

Overview of Operating Systems: OS and the Computer System, Efficiency, System Performance and User Convenience, Classes of Operating Systems, Batch Processing Systems, Multiprogramming

Systems, Time Sharing Systems, Real Time Operating Systems, Distributed Operating Systems, Modern Operating Systems.

Introduction • Fundamental Principles of OS Operation • The Computer • OS Interaction with the Computer and User Programs

Fundamental Principles of OS Operation

• The kernel of the OS is the collection of routines that form the core of the operating system – Implements control functions – Set of services to user programs – Exists in memory during operation of the OS

• An interrupt diverts the CPU to execution of the kernel code • A software interrupt is used by programs to communicate their requests to the kernel

Fundamental Principles of OS Operation (continued)

• CPU has two modes of operation: – Kernel mode • CPU can execute all instructions • Kernel operates with CPU in this mode so that it can control computer operations

– User mode • CPU cannot execute instructions that could interfere with other programs or with the OS if used indiscriminately • CPU is put in this mode to execute user programs

The Computer

The Computer (continued) • • • • •

The CPU Memory Management Unit (MMU) Memory Hierarchy Input/Output Interrupts

The CPU • Two features of the CPU are visible to user programs or the OS: – General-purpose registers (GPRs) • Also called program-accessible registers • Hold data, addresses, index values, or the stack pointer during execution of a program

– Control registers • Contain information that controls or influences operation of the CPU • Set of control registers is called the program status word (PSW)

The CPU (continued) • CPU can operate in two modes: – Kernel mode • Can execute privileged instructions • OS puts CPU in kernel mode when it is executing instructions in the kernel

– User mode • Cannot execute privileged instructions • OS puts CPU in user mode while executing user programs

• Mode (M) field of PSW contains 0 if CPU is in privileged mode and 1 if it is in user mode

State of the CPU • GPRs and PSW contain the information needed to know what the CPU is doing – State of the CPU

• Kernel saves state of CPU when it takes away the CPU from program – When program is to be resumed, it reloads the saved CPU state into GPRs and PSW

Example 2.1: State of the CPU (Contd)

Memory Management Unit (MMU) • Virtual memory is an illusion of memory that may be larger than the real memory of a computer – Implemented using noncontiguous memory allocation and the MMU • CPU passes the address of data or instruction used in an instruction to MMU – It is called the logical addresses

• MMU translates logical address to physical address

Memory hierarchy • The memory hierarchy provides a large and fast memory, at a low cost – It is an arrangement of several memories with different access speeds and sizes • The CPU accesses only the fastest memory; i.e., the cache • If a required byte is not present in the memory being accessed, it is loaded there from a slower memory

Operation of a memory hierarchy

Memory Hierarchy (continued) • When CPU performs a cache lookup, a cache hit or miss may occur – Hit ratio (h) of the cache is the fraction of bytes accessed by the CPU that score a hit in the cache tema = h × tcache + (1 – h) × (ttra + tcache) = tcache + (1 – h) × ttra where tema = effective memory access time, tcache = access time of cache, and ttra = time taken to transfer a cache block from memory to cache.

Memory Hierarchy (continued) • Operation of memory is analogous to operation of a cache – Blocks of bytes (pages) are transferred from disk to memory or from memory to disk – But, • Memory management and transfer of blocks between memory and disk are performed by SW • In the cache, the transfer is performed by HW

• Memory hierarchy comprising MMU, memory, and the disk is called the virtual memory

Memory Hierarchy (continued) • Memory protection is implemented by checking whether a memory address used by a program lies outside the memory area allocated to it – Control registers used: • base and size (also called limit) – Address of first byte = – Address of last byte = +<size> – 1

Example 2.2: Basics of Memory Protection • Execution of •interrupt Load instruction causes protection violation

A memory protection violation interrupt would be generated if an address used during execution of P1 lies outside the range 20000 – 25000.

Input/Output

Interrupts • An event is a situation that requires OS’s attention – Designer associates an interrupt with each event • Purpose is to report occurrence of the event to OS and enable it to perform event handling actions

• Interrupt action saves CPU state and loads new contents into the PSW and GPRs – CPU starts executing instructions of an interrupt servicing routine (ISR) in the kernel

Interrupts (continued)

OS Interaction with the Computer and User Programs • OS interacts with the computer to – know information about events, so that it can service them – restore CPU state to resume a program after servicing an interrupt

• Programs need to use services of the OS for purposes such as initiating an I/O operation – The method to cause an interrupt and pass requirement to the OS is known as a System call

• Will learn about: – Controlling Execution of Programs – Interrupt Servicing – System Calls

Controlling Execution of Programs 1. When user program starts, PSW should contain: a. Program counter (PC) field b. Mode (M) field, set to user mode (1) c. Memory protection information (MPI) field contains start address in memory and size of program d. Interrupt mask (IM) field, set to enable interrupts

2. When program is interrupted, CPU state (PSW and GPRs) are saved •

Program table or process control block (PCB)

3. When program is resumed, its CPU state is restored

Interrupt Servicing

• Context save saves CPU state of the program • The scheduler selects a a program for execution

Interrupt Servicing (continued)

Figure 2.8(a) shows the arrangement of interrupt vectors and interrupt servicing routines in memory, while Figure 2.8(b) shows contents of the PSW at various times during servicing of an I/O interrupt. The interrupt vectors are formed by the OS boot procedure. Each interrupt vector contains the address of an interrupt servicing routine, an interrupt mask and a 0 in the mode field. A user program is about to execute the instruction that exists at the address ddd in memory when an interrupt occurs signaling the end of an I/O operation on device d1. The leftmost part of Figure 2.8(b) shows the PSW contents at this time. Step 1 of the interrupt action puts d1 in the IC field of the PSW and saves the PSW in the saved PSW information area. The saved PSW contains a 1 in the mode field, ddd in the PC field, and d1 in the IC field. The contents of the interrupt vector for the I/O completion interrupt are loaded into the PSW. Effectively, the CPU is put in the kernel mode of operation, and control is transferred to the routine that has the start address bbb, which is the I/O interrupt servicing routine (see the arrow marked A in Figure 2.8(a), and the PSW contents shown in Figure 2.8(b)). The I/O interrupt servicing routine saves the PSW and contents of the GPRs in the program table. It now examines the IC field of the saved PSW, finds that device d1 has completed its I/O operation, and notes that the program that had initiated the I/O operation can be considered for scheduling. It now transfers control to the scheduler (see the arrow marked B in Figure 2.8(a)). The scheduler happens to select the interrupted program itself for execution, so the kernel switches the CPU to execution of the program by loading back the saved contents of the PSW and GPRs (see arrow marked C in Figure 2.8(a)). The Program would 2/1/2017 resume execution at the instruction with the address ddd (see the PSW contents in the rightmost part of Figure 2.8(b)).

Interrupt Servicing (continued)

• Two approaches for nested interrupt servicing: – Disable nested interrupts through masking – Handle nested interrupts─preemptible kernel

System Calls

Example 2.4: System Call in a Hypothetical OS • CPU provides the SI instruction to cause a software interrupt • OS provides system call for obtaining current time – Code is 78: Instruction SI 78 causes a SW interrupt

• 78 is entered in IC field of PSW before it is saved • Interrupt vector for program contains aaa in PC – CPU is switched to routine with start address aaa – It finds that IC is 78 and determines that program needs time of day • Time is returned to program in a standard location, typically a data register

System Calls (continued)

Computing Environments and Nature of Computations

• A computing environment consists of a computer system, its interfaces with other systems, and the services provided by its operating system to its users and their programs • Evolution: – Noninteractive Computing Environments – Interactive Computing Environments – Real-Time, Distributed, and Embedded Environments – Modern Computing Environments

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Computing Environments and Nature of

Computations (continued)

• Noninteractive Computing Environments – OS focuses on efficient use of resources – Computations in form of program or job

• Interactive Computing Environments – OS focuses on reducing average amount of time required to implement an interaction between a user and his computation – Execution of a program is called a process

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Computing Environments and Nature of Computations (continued)

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Computing Environments and Nature of Computations (continued) • Real-Time, Distributed, and Embedded Environments – A real-time computation has specific time constraints • OS ensures computations complete within constraints

– Distributed computing environment: enables a computation to use resources located in several computer systems through a network – Embedded computing environment: computer system is a part of a specific hardware system • OS has to meet the time constraints arising from the nature of the system being controlled

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Computing Environments and Nature of Computations (continued)

• Modern Computing Environments – Has features of several of the computing environments described earlier • OS uses complex strategies to manage user computations and resources

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Efficiency, System Performance, and User Service • Two of the fundamental goals of an OS: – Efficiency of use • Present utilization of a resource

– User convenience • Measurable aspect: User service – Turnaround time – Response time

• To a system administrator, performance of a system in its environment is more important – Typically measured as throughput

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Efficiency, System Performance, and User Service (continued)

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Classes of Operating Systems

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Batch Processing Systems • Batch: sequence of user jobs formed for processing by the OS • Batching kernel initiates processing of jobs without requiring computer operator’s intervention • Card readers and printers were a performance bottleneck in the 1960s – Virtual card readers and printers implemented through magnetic tapes were used to solve this problem

• Control statements used to protect against interference between jobs • Command interpreter read a card when currently executing program in job wanted the next card 42

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Multiprogramming Systems • Provide efficient resource utilization in a noninteractive environment • Uses DMA mode of I/O – Can perform I/O operations of some program(s) while using the CPU to execute some other program • Makes efficient use of both the CPU and I/O devices

• Turnaround time of a program is the appropriate measure of user service in these systems

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Multiprogramming Systems (continued)

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Multiprogramming Systems (continued)

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Multiprogramming Systems (conti…) • An appropriate measure of performance of a multiprogramming OS is throughput – Ratio of the number of programs processed and the total time taken to process them

• OS keeps enough programs in memory at all times, so that CPU and I/O devices are not idle – Degree of multiprogramming: number of programs – Uses an appropriate program mix of CPU-bound programs and I/O-bound programs – Assigns appropriate priorities to CPU-bound and I/Obound programs 47

Priority of Programs

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Priority of Programs (continued) In multiprogramming environments, an I/O-bound program should have a higher priority than a CPU-bound program.

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Performance of Multiprogramming systems

• How to improve performance?

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Performance of Multiprogramming systems (cont..)

When an appropriate program mix is maintained, an increase in the degree of multiprogramming would result in an increase in throughput. 51

Time-Sharing Systems • Provide a quick response to user subrequests – Round-robin scheduling with time-slicing • Kernel maintains a scheduling queue • If time slice (δ) elapses before process completes servicing of a subrequest, kernel preempts it, moves it to end of queue, and schedules another process – Implemented through a timer interrupt

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Time-Sharing Systems (continued)

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Time-Sharing Systems (cont….) • Response time (rt): measure of user service – If processing of a subrequest requires δ CPU seconds rt = n × (δ + σ) η = δ / (δ + σ) where η: CPU efficiency, σ: scheduling overhead, n: number of users using system, δ: time required to complete a subrequest

• Actual response time would be different

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Time-Sharing Systems (cont…)

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Swapping of Programs

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Real-Time Operating Systems • In real-time applications, users need computer to perform some actions in a timely manner – To control activities in an external system, or to participate in them – Timeliness depends on time constraints

• If application takes too long to respond to an activity, a failure can occur in the external system – Response requirement – Deadline: time by which action should be performed

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Hard and Soft Real-Time Systems • A hard real-time system meets response requirements under all conditions – It is typically dedicated to processing real-time applications

• A soft real-time system makes best effort to meet response requirement of a realtime application – Cannot guarantee that it will be able to meet it • Meets requirements in a probabilistic manner

– E.g., multimedia applications 58

Features of a Real-Time Operating System

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Distributed Operating Systems • A distributed computer system consists of several individual computer systems connected through a network – Each computer system could be a PC, a multiprocessor system, or a cluster – Many resources of a kind exist in system • This feature is used to provide the benefits summarized in Table 3.8

– Handling network or individual computers’ failure requires special techniques – Users must use special techniques to access 60 resources over the network

Distributed Operating Systems (continued)

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Special Techniques of Distributed Operating Systems

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Modern Operating Systems

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