Wide Area Network Structure

Local Area Network Structure

Network Structure

• Local Area Network (LAN)
• Wide Area Network (WAN)

CPU Protection

• Timer-interrupts computer after specified period to ensure operating system maintains control.
• Timer is decremented every clock tick.
• When timer reaches the value 0, an interrupt occurs.
• Timer commonly used to implement time sharing.
• Time also used to compute the current time.
• Load-timer is a privileged instruction.

Hardware Protection

• When executing in monitor mode,the operating system has unrestricted access to both monitor and user's memory.
• The load instructions for the base and limit registers are privileged instructions.

Hardware Address Protection

Use of A Base and Limit Register

Memory Protection

• Must provide memory protection atleast for the interrupt vector and the interrupt services routines.
• In order to have memory protection,add two registers that determines about the range of legal addresses a program may access:
• Base Register-holds the smallest legal physical memory address.
• Limit Register-contains the size of the range.
• Memory outside the defined range is protected.

Use of A System Call to Perform I/O

I/O Protection

• All I/O instructions are privileged instructions.
• Must ensure that a user program could never gain control of the computer in monitor mode (I.e., a user program that, as part of its execution, stores a new address in the interrupt vector). 

Dual-Mode Operation

• Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.
• Provide hardware support to differentiate between at least two modes of operation.
• User mode- execution done on behalf of a user.
• Monitor mode- execution done on behalf of operating system.
• Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1).
• When an interrupt or fault occurs hardware switches to monitor mode.
• Privileged instructions can be issued only in monitor mode.

Hardware Protection

• Dual-Mode Operation
• I/O Protection
• Memory Protection
• CPU Protection

Migration of A From Disk to Register

Caching

• Use of high-speed memory to hold recently-accessed data.
• Requires a cache management policy.
• Caching introduces another level in storage hierarchy.This requires data that is simultaneously stored in more than one level to be consistent.

Storage-Device Hierarchy

Storage Hierarchy

• Storage systems organized in hierarchy.
• Speed
• Cost
• Volatility
• Caching- copying information into faster storage system; main memory can be viewed as a last cache for secondary storage.

Moving-Head Disk Mechanism

Storage Structure

• Main memory-only large storage media that the CPU can access directly.
• Secondary storage-extension of main memory that provides large non-volatile 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.

Direct Access Memory Structure

• Used for high-speed I/O devices able to transmit information at close to memory speeds.
• Device controllers transfers blocks of data from buffer storage directly to main memory without CPU intervention.
• Only on interrupt is generated per block, rather than the one interrupt per byte.

Device-Status Table

Two I/O Methods

I/O Structure

• After I/O starts, control returns to user program only upon I/O completion.
• Wait instruction idles the CPU until the next interrupt.
• Wait loop (contention for memory access).
• At most one I/O request is outstanding at a time, no simultaneous I/O processing.
• After I/O starts, control returns to user program without waiting for I/O completion.
• System call-request to the operating system to allow user to wait for I/O completion.
• Device Status Table contains entry for each I/O device indicating its type, address and state.
• Operating System indexes into I/O device table to determine device status and to modify table entry to include interrupt.

Interrupt Time Line For a Single Process Doing Output

Interrupt Handling

• The Operating System preserves the state of the CPU by storing registers and the program counter.
• Determines which types of interrupt has occured:
• polling
• vectored interrupt system
• Separate segments of code determine what action should be taken for each type of interrupt.

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 interrupt 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.

Computer System Operation

• I/O devices and the CPU can execute concurrently.
• Each device controller is in charge of a particular device type.
• Each device controller has a local buffer.
• CPU moves data from/to main memory to/from local buffers.
• I/O is from the device to local buffer to controller.
• Device controller informs CPU that it has finished its operation by causing an interrupt.

Computer System Architecture

Computing Environments

• Traditional Computing
• Web-Based Computing
• Embedded Computing

Handheld Systems

• Personal Digital Assistants (PDAs)
• Cellular Telephones
• Issues:
• Limited Memory
• Slow Processors
• Small Display Screens

Real-Time Systems

1. Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.

2. Well-defined fixed-time constraints.

3. Real-Time systems may be either hard or soft real-time.

Clustered Systems

1. Clustering allows two or more systems to share storage.

2. Provides high reliability.

3. Asymmetric clustering: one server runs the application while other servers standby.

4. Symmetric clustering: all N hosts are running the application.

General Structure of Client-Server

Distributed Systems

1. Distribute the computation among several physical processors.

2. Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines.

3. Advantages of distributed systems.

a. Resources Sharing

b. Computation speed up – load sharing

c. Reliability

d. Communications

Symmetric Multiprocessing Architecture

Parallel Systems

1. Multiprocessor systems with more than on CPU in close communication.

2. Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.

3. Advantages of parallel system:

a. Increased throughput

b. Economical

c. Increased reliability

* graceful degradation

                        * fail-soft systems

 4. Symmetric multiprocessing (SMP)

a. Each processor runs and identical copy of the operating system.

b. Many processes can run at once without performance deterioration.

c. Most modern operating systems support SMP

5. Asymmetric multiprocessing

a. Each processor is assigned a specific task; master processor schedules and allocated work to slave processors.

b. More common in extremely large systems

Desktop Systems

1. Personal computers – computer system dedicated to a single user.

2. I/O devices – keyboards, mice, display screens, small printers.

3. User convenience and responsiveness.

4. Can adopt technology developed for larger operating system’ often individuals have sole use of computer and do not need advanced CPU utilization of protection features.

5. May run several different types of operating systems (Windows, MacOS, UNIX, Linux)

Time-Sharing Systems–Interactive Computing

1. The CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory).

2. A job swapped in and out of memory to the disk.

3. On-line communication between the user and the system is provided; when the operating system finishes the execution of one command, it seeks the next “control statement” from the user’s keyboard.

4. On-line system must be available for users to access data and code.

OS Features Needed for Multiprogramming

1. I/O routine supplied by the system.

2. Memory management – the system must allocate the memory to several jobs.

3. CPU scheduling – the system must choose among several jobs ready to run.

4. Allocation of devices.

Multiprogrammed Batch Systems

Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them.

Memory Layout for a Simple Batch System

Mainframe Systems

1. Reduce setup time by batching similar jobs

2. Automatic job sequencing – automatically transfers control from one job to another.  First rudimentary operating system.

3. Resident monitor

a.initial control in monitor

b.control transfers to job

c.when job completes control transfers pack to monitor

Operating System Definitions

1. Resource allocator – manages and allocates resources.

2. Control program – controls the execution of user programs and operations of I/O devices .

3. Kernel – the one program running at all times (all else being application programs).

Abstract View of System Components

Computer System Components

1.  Hardware – provides basic computing resources (CPU, memory, I/O devices).

2.  Operating system – controls and coordinates the use of the hardware among the various application programs for the various users.

3.  Applications programs – define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs).

4.  Users (people, machines, other computers). 

What is an Operating System?

1.A program that acts as an intermediary between a user of a computer and the computer hardware.

2.Operating system goals:

a.Execute user programs and make solving user problems easier.

b.Make the computer system convenient to use.

3.Use the computer hardware in an efficient manner.