OS 1

Operating System (OS)

An interface between applications and hardware.

Process Utilization =

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

Evolution of Operating Systems:

Serial Processing
Simple Batch Systems
Multiprogrammed Batch Systems
Time Sharing Systems

Serial Processing

Programs interface directly with the hardware.

Simple Batch Systems

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

Monitor

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

Resident Monitor

Software that is always in memory.

Job Control Language (JCL)

Used to provide instructions to the monitor.

User Mode

No access to certain areas of memory.

Kernel Mode

Complete access to hardware.

Monitor Drawbacks:

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

Multiprogramming / Multitasking

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

Time-Sharing Systems

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

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

Terminals

Compatible Time-Sharing Systems (CTSS)

One of the first OS’s.

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.

Process

A program being executed.

Improper Synchronization

A program is overwritten before it could be saved.

Nondeterminate Program Operation

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

Failed Mutual Exclusion

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

Deadlocks

Two programs can be hung up waiting on each other.

3 Components of a Process:

Program
Associated Memory
Execution Context / State

Execution Context

The “state” the a process is currently in.

OS Storage Responsibilities:

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

Virtual Memory

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

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.

Main Issues with Access Control:

Availability
Confidentiality
Data Integrity
Autheticity

Resources Allocation policies that must be considered:

Efficiency
Fairness
Differential Responsiveness

Process Control Block

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

Notable process elements:

ID
State
Priority
Counter
Pointers
Context
I/O
Accounting

Trace

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

Dispatcher

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

Process Counter

The address of the next instruction to be executed.

Process Pointers

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

Process Context

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

Process I/O

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

Process Accounting

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

Reasons for process creation:

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

Process Spawning

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

Reasons for process termination:

Completion
Timeout
Error
OS Intervention
Parent Termination
Parent Request

Process Blocked State

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

Process Swapping

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

7-State Process Model

Reasons for process suspension:

Swapping
OS-Suspicion
User Request
Timing
Parent Request

Memory Tables

Used to keep track of real and virtual memory.

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.

I/O Tables

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

File Tables keep track of:

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

Process Tables

Used and maintained by the OS to manage processes.

Process tables help the OS keep track of:

Process Location
Process Attributes

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.

Process Attributes

Any unique attributes a process may possess.

Process Image

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

A process image contains:

User Data
User Program
Stack
Process Control Block

Process Control Block

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

A process control block contains:

ID
State Information
Control Information

Process ID

A unique numeric ID given to a process.

Processor State Information contains the:

User-Visible Registers
Control & Status Registers
Stack Pointers

Program Status Word (PSW)

Contains condition codes and other status information.

Process Control Information

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

Process Modes of Execution:

User Mode
System Mode

Typical Functions of a OS Kernel:

Process Management
Memory Management
I/O Management

Process Creation Steps:

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

System Interrupt

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

System Trap

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

Scheduling and Process State Transitions

Long-term Scheduler

Determines which programs are admitted to the system for processing.

Medium-term Scheduler

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

Short-term Scheduler / Dispatcher

Determines which process gets executed next.

Priority Queueing

Selection Function

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

Decision Mode

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

Nonpreemptive Decision Mode

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

Preemptive Decision Mode

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