Stalling’s Chapter
1: Computer System Overview
2: Operating System Overview
3: Process Description and Control
4: Threads
5: Concurrency: ME and Synchronization
6: Concurrency: Deadlock and Starvation
7: Memory Management
8: Virtual memory
9: Uniprocessor Scheduling
10: Multiprocessor and Real-Time Scheduling
11: I/O Management and Disk Scheduling
12: File Management
Student Presentations
BYU CS 345
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OS Overview (Chapter 1)
Project
P1: Shell
P2: Tasking
P3: Jurassic Park
P4: Virtual Memory
P5: Scheduling
P6: FAT
1

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A task is a unit of execution (also referred to as a process).
A shell (Command Language Interpreter) is a task that functions as an interface between the user and an Operating System.
A shell interprets textual commands coming either from the user’s keyboard or from a script file and executes the commands either directly or creates a new child process to execute the command.
For Project 1:
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Download all the project files from class website.
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 os345.c, os345interrupts.c, os345signals.c os345tasks.c, os345semaphores.c
os345.h, os345config.h, os345signals.h
os345p1.c, os345p2.c, os345p3.c, os345p4.c, os345p5.c, os345p6.c
os345park.c, os345park.h, os345lc3.c, os345lc3.h, os345mmu.c, os345fat.c, os345fat.h
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Edit os345config.h (if necessary) to select host OS/IDE/ISA. (Only enable one of the following defines: DOS, GCC, MAC, or NET.)
Compile and execute your OS.
BYU CS 345 Project 1 - Shell 2

Why CS 345?
BYU CS 345 OS Overview (Chapter 1) 3

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What is an operating system?
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Hard to define precisely, because operating systems arose historically as people needed to solve problems associated with using computers.
How about…
“Software that makes computing power available to users by controlling the hardware.”
“Software executes when nothing else is happening.”
“A collection of software modules including device drivers, libraries, and access routines.”
BYU CS 345 OS Overview (Chapter 1) 4

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Provides Abstractions:
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Hardware has low-level physical resources with complicated, idiosyncratic interfaces.
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OS provides abstractions that present clean interfaces.
Goal: make computer easier to use.
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Examples: Processes, Unbounded Memory, Files,
Synchronization and Communication Mechanisms.
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Provides Standard Interface:
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Goal: portability.
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Unix runs on many very different computer systems.
BYU CS 345 OS Overview (Chapter 1) 5

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Mediates Resource Usage:
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Goal: allow multiple users to share resources fairly, efficiently, safely and securely.
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Examples:
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Multiple processes share one processor. (preemptable resource)
Multiple programs share one physical memory (preemptable resource).
Multiple users and files share one disk. (non-preemptable resource)
Multiple programs share a given amount of disk and network bandwidth (preemptable resource).
Consumes Resources:
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Solaris takes up about 8 Mbytes physical memory (or about
$400).
BYU CS 345 OS Overview (Chapter 1) 6

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In the future, computers will continue to become physically smaller and more portable.
Operating systems have to deal with issues like disconnected operation and mobility.
Media rich information within the grasp of common people - information with psuedo-real time components like voice and video.
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Operating systems will have to adjust to deliver acceptable performance for these new forms of data.
BYU CS 345 OS Overview (Chapter 1) 7

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Operating systems are so large no one person understands whole system. Outlives any of its original builders.
The major problem facing computer science today is how to build large, reliable software systems.
Operating systems are one of very few examples of existing large software systems, and by studying operating systems we may learn lessons applicable to the construction of larger systems.
BYU CS 345 OS Overview (Chapter 1) 8

Let’s figure out what’s inside this thing...

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Describe the basic elements of a computer system and their interrelationship.
Explain the steps taken by a processor to execute an instruction.
Understand the concept of interrupts and how and why a processor uses interrupts
List and describe the levels of a typical computer memory hierarchy.
Explain the basic characteristics of multiprocessor and multicore organization.
Discuss the concept of locality and analyze the performance of a multilevel memory hierarchy.
Understand the operation of a stack and its use to support procedure call and return.
BYU CS 345 OS Overview (Chapter 1) 10

Objectives
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Registers
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Interrupts
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Caching
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Input/Output
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Protection
BYU CS 345 OS Overview (Chapter 1) 12

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Interrupts
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Caching
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Input/Output
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Protection
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Summary
BYU CS 345 OS Overview (Chapter 1)
Objectives
13

Registers
BYU CS 345 OS Overview (Chapter 1) 14

Registers
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User-visible registers
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May be referenced by machine language
Available to all programs - application programs and system programs
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Data Registers – can be changed by user
Address Registers – could be separate from data register
Stack Registers – user / supervisor stacks
Condition Codes – results of operations
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Control and status registers
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May or may not be visible
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BYU CS 345
Program Counter (PC) – address of next instruction
Instruction Register (IR) – most recently fetched instruction
MAR/MBR – memory reference registers
Program Status Word (PSW) – condition codes, interrupts, mode
OS Overview (Chapter 1) 15

Registers
BYU CS 345 OS Overview (Chapter 1) 17

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Registers
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Caching
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Input/Output
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Protection
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Summary
BYU CS 345 OS Overview (Chapter 1) 18

Interrupts
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The interrupt was the principle tool available to system programmers in developing multitasking systems!
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Improves processing efficiency by allowing the processor to execute other instructions while an
I/O operation is in progress
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A suspension of a process caused by an event external to that process and performed in such a way that the process can be resumed
BYU CS 345 OS Overview (Chapter 1) 19

Interrupts
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Classes of Interrupts
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Program
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 arithmetic overflow division by zero execute illegal instruction reference outside user’s memory space
Timer
I/O
Hardware failure
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An interrupt handler determines nature of the interrupt and performs whatever actions are needed
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Control is transferred to this program
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Generally part of the operating system
BYU CS 345 OS Overview (Chapter 1) 20

Interrupts
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Processor checks for interrupts
If no interrupts fetch the next instruction for the current program
If an interrupt is pending, suspend execution of the current program, and execute the interrupt handler
START
BYU CS 345
Fetch Cycle
Fetch Next
Instruction
Execute Cycle Interrupt Cycle
Interrupts
Disabled
Execute
Instruction
Interrupts
Enabled
Check for
Interrupt:
Process Interrupt
HALT
OS Overview (Chapter 1) 21

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2 Choices
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Disable Interrupts
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Disable upon entering an interrupt handler
Enable upon exiting
Allow Interrupts
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Allow an interrupt handler to be interrupted
Priorities?
BYU CS 345 OS Overview (Chapter 1)
Interrupts
23

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Registers
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Interrupts
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Input/Output
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Protection
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Summary
BYU CS 345 OS Overview (Chapter 1) 26

Caching
More Expensive
Faster & Smaller
Registers
Cache
Main Memory
Disk Cache
Magnetic Disk
Magnetic Tape Optical Disk
BYU CS 345 OS Overview (Chapter 1)
Bigger
Slower
27

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Registers
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Interrupts
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Caching
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Protection
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Summary
BYU CS 345 OS Overview (Chapter 1) 31

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I/O module performs the action, not the processor
Sets appropriate bits in the I/O status register
No interrupts occur
Processor is kept busy checking status
BYU CS 345 OS Overview (Chapter 1)
Input / Output
32

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Processor is interrupted when
I/O module ready to exchange data
Processor is free to do other work
No needless waiting
Consumes a lot of processor time because every word read or written passes through the processor
BYU CS 345 OS Overview (Chapter 1)
Input / Output
33

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Transfers a block of data directly to or from memory
An interrupt is sent when the task is complete
The processor is only involved at the beginning and end of the transfer
What does this mean with respect to a paged system?
BYU CS 345 OS Overview (Chapter 1)
Input / Output
34

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Registers
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Interrupts
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Caching
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Input/Output
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Summary
BYU CS 345 OS Overview (Chapter 1) 35

Protection
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Why protect hardware?
From what?
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Shared hardware resources – memory, disk, …
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Errant programs
How?
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CPU provides 2 modes of operation
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User Mode (non-privileged)
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Supervisor mode (privileged)
Privileged instructions can only be executed in monitor mode
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All I/O instructions are privileged
BYU CS 345 OS Overview (Chapter 1) 36

Summary
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Not always a clear definition as to what constitutes an
O.S. and what is an application
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CD-Rom Driver
Scandisk
Internet Explorer
Intermediary between the hardware and the users
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Allocate resources (CPU, Memory, disk space, etc.) between programs and users efficiently
Allow the user to conveniently access data and programs
Protect the system from incorrect or malicious programs and users
BYU CS 345 OS Overview (Chapter 1) 41

Summary
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Elements of a system:
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Processor
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Registers (address, data, control)
Instruction cycle (fetch, decode, execute)
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Interrupts
Usually includes hardware and special instructions to help the O.S. manage memory, devices, etc.
Memory
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Different levels (cache, main memory, disk)
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Operating system will generally manage memory (both
RAM and disk), and move data back and forth as required
I/O
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Usually use Interrupts, DMA
Operating system usually controls use of I/O devices
BYU CS 345 OS Overview (Chapter 1) 42

Summary
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Used for frequently accessed items
User-Visible registers – Available to the programmer and compiler
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Data Registers
Address Registers (Index, Segment, Stack Pointer)
Condition code/flags
Control and Status registers – Used to control the processor
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Program Counter/Instruction Pointer
Memory address/data
Processor Status Word
Debugging registers
Temp registers
Memory Management registers
BYU CS 345 OS Overview (Chapter 1) 43

Summary
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Interrupts
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Allow I/O devices to get the CPUs attention at regular intervals (Program, Timer, I/O, Hardware failure)
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Helps the O.S. by reducing the time spent monitoring
I/O devices
CPU checks for interrupts after each instruction, starts the handler if needed
May allow nested interrupts
I/O techniques
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Programmed I/O
Interrupt-Driven I/O
Direct Memory Access
BYU CS 345 OS Overview (Chapter 1) 44

Summary
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Handling and Interrupts:
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Figure 1.10 (pg 23)
Device sends interrupt request to CPU
CPU finishes current instruction
CPU acknowledges request
CPU saves PC and PSW
CPU loads PC with the address of the first instruction in the interrupt handler (may get help from interrupt request)
Interrupt handler starts, often saves other CPU registers and key values
Interrupt handler responds to the device
Interrupt handler restores CPU registers and key values
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CPU restores PC and PSW and resumes previous program
BYU CS 345 OS Overview (Chapter 1) 45

Summary
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Varying types of memory
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Registers, Cache, RAM, Disk, CD
Vary in speed, size, cost
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CPU and O.S. try to keep frequently used data in faster memory
Cache – Use a small high-speed memory to improve the apparent speed of a larger low-speed memory
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Keep track of what is currently being used, load into high-speed memory
Replacement Algorithm – What do we get rid of when we run out of memory?
Write Policy – How do we respond to modifications?
BYU CS 345 OS Overview (Chapter 1) 46

BYU CS 345 OS Overview (Chapter 1) 47

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OS Objectives
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OS Services
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Resource Manager
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Evolution
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Achievements
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Processes
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Memory management
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Information protection and security
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Scheduling and resource management
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System architecture
BYU CS 345 OS Overview (Chapter 1) 48