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74 Terms

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Operating System (OS)

An interface between applications and hardware.

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Process Utilization =

\frac{P}{\text{Sum of processing times}}

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Evolution of Operating Systems:

Serial Processing
Simple Batch Systems
Multiprogrammed Batch Systems
Time Sharing Systems

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Serial Processing

Programs interface directly with the hardware.

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Simple Batch Systems

Uses a monitor to control a program’s access to the hardware.

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Monitor

Controls a program’s access to the hardware. Software will always branch back to the monitor.

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Resident Monitor

Software that is always in memory.

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Job Control Language (JCL)

Used to provide instructions to the monitor.

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User Mode

No access to certain areas of memory.

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Kernel Mode

Complete access to hardware.

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Monitor Drawbacks:

Monitor takes time to exectute
Monitor takes up space in memory.

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Multiprogramming / Multitasking

Swapping control to a different program when the current one needs to wait for I/O.

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Time-Sharing Systems

Handles multiple programs that require I/O.
Can be accessed by multiple users at once.

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Time-Sharing Systems use ___ to directly interface with the program.

Terminals

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Compatible Time-Sharing Systems (CTSS)

One of the first OS’s.

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CTSS Time Slicing

Would “interrupt” the system at set intervals to swap to another program.
Incomplete programs would be written to a disk while another job was finishing.

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Process

A program being executed.

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Improper Synchronization

A program is overwritten before it could be saved.

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Nondeterminate Program Operation

Programs overwrite an area of memory that is already occupied by another program.

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Failed Mutual Exclusion

More than one user or program tries to make use of a shared resource at the same time.

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Deadlocks

Two programs can be hung up waiting on each other.

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3 Components of a Process:

Program
Associated Memory
Execution Context / State

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Execution Context

The “state” the a process is currently in.

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OS Storage Responsibilities:

Process Isolation
Automatic Memory Management
Modular Programming Support
Access Control
Long-Term Storage

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Virtual Memory

Allows programs to access memory as symbols instead of directly accessing memory.
This allows the OS to handle memory allocation.

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Paging

Allows processes to be comprised of a number of fixed-size blocks called pages.
Provides a dynamic mapping between a virtual and physical address.

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Main Issues with Access Control:

Availability
Confidentiality
Data Integrity
Autheticity

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Resources Allocation policies that must be considered:

Efficiency
Fairness
Differential Responsiveness

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Process Control Block

A data block created and maintained by the os.
Contains all the elements that make up a process.

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Notable process elements:

ID
State
Priority
Counter
Pointers
Context
I/O
Accounting

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Trace

The behavior of a process as defined by the list of executed instructions.

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Dispatcher

A small program that switches the processer from one process to another.

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Process Counter

The address of the next instruction to be executed.

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Process Pointers

Pointers to any data associated with the process + any memory shared with other processes.

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Process Context

The data present in the context registers while the process is executing.

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Process I/O

Includes any I/O requests, devices, files, etc.

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Process Accounting

May include things such as the amount of time a process has taken, time limits, etc.

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Reasons for process creation:

New batch job
Login
Service-by-OS
From existing process

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Process Spawning

The act of a process requesting another process be made.
The requester is the parent and the resulting process is the child.

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Reasons for process termination:

Completion
Timeout
Error
OS Intervention
Parent Termination
Parent Request

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Process Blocked State

Occurs when the process is waiting for an event to occur, typically I/O.

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Process Swapping

Moving parts of a process from main memory to disk, putting it into a suspended state.

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7-State Process Model

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Reasons for process suspension:

Swapping
OS-Suspicion
User Request
Timing
Parent Request

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Memory Tables

Used to keep track of real and virtual memory.

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Memory tables must include:

Allocation of real memory.
Allocation of virtual memory.
Protection attributes for real and virtual memory.
Info needed to manage virtual memory.

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I/O Tables

Used by the OS to manage the I/O devices and channels.

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File Tables keep track of:

Existence of files.
File location on virtual memory.
Current status.
File-specific attributes.

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Process Tables

Used and maintained by the OS to manage processes.

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Process tables help the OS keep track of:

Process Location
Process Attributes

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Process Location

Where a process’s programs and data is located.
A process will always have sufficient enough memory to hold said programs and data.

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Process Attributes

Any unique attributes a process may possess.

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Process Image

The collection of a process’s program, data, stack, and attributes.

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A process image contains:

User Data
User Program
Stack
Process Control Block

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Process Control Block

Contains all the information about a process that is needed by the OS.

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A process control block contains:

ID
State Information
Control Information

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Process ID

A unique numeric ID given to a process.

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Processor State Information contains the:

User-Visible Registers
Control & Status Registers
Stack Pointers

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Program Status Word (PSW)

Contains condition codes and other status information.

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Process Control Information

The additional information needed by the OS to coordinate the various active processes.

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Process Modes of Execution:

User Mode
System Mode

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Typical Functions of a OS Kernel:

Process Management
Memory Management
I/O Management

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Process Creation Steps:

Assign ID
Allocate space
Initialize control block
Set appropriate links
Create/expand other data structures

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System Interrupt

Stops a process from running to either handle I/O or switch processes.

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System Trap

Stops a process from running to handle an error or exception.

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Scheduling and Process State Transitions

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Long-term Scheduler

Determines which programs are admitted to the system for processing.

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Medium-term Scheduler

Determines which parts of any process is moved into main memory.
Part of the swapping function.

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Short-term Scheduler / Dispatcher

Determines which process gets executed next.

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Priority Queueing

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Selection Function

Determines which process, among ready processes, is selected for execution.

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Decision Mode

Specifies the instants in time at which the selection function is exercised.

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Nonpreemptive Decision Mode

Once a process is in the running state, it will continue until it terminates or requires I/O.

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Preemptive Decision Mode

Interrupts running processes and moves them to the “ready” state.