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CS 345
Stalling’s Chapter
#
Project
1: Computer System Overview
2: Operating System Overview
4
P1: Shell
3: Process Description and Control
4: Threads
4
P2: Tasking
5: Concurrency: ME and Synchronization
6: Concurrency: Deadlock and Starvation
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P3: Jurassic Park
7: Memory Management
8: Virtual memory
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P4: Virtual Memory
9: Uniprocessor Scheduling
10: Multiprocessor and Real-Time Scheduling
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P5: Scheduling
11: I/O Management and Disk Scheduling
12: File Management
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P6: FAT
Student Presentations
6
BYU CS 345
OS Overview (Chapter 1)
1
1. Compile and Validate
A task is a unit of execution (also referred to as a process).
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A shell (Command Language Interpreter) is a task that functions as an
interface between the user and an Operating System.
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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
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
Operating Systems
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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
What Does a Modern OS Do?
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Provides Abstractions:
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Hardware has low-level physical resources with
complicated, idiosyncratic interfaces.
OS provides abstractions that present clean interfaces.
Goal: make computer easier to use.
Examples: Processes, Unbounded Memory, Files,
Synchronization and Communication Mechanisms.
Provides Standard Interface:
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Goal: portability.
Unix runs on many very different computer systems.
BYU CS 345
OS Overview (Chapter 1)
5
What Does a Modern OS Do?
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Mediates Resource Usage:
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Goal: allow multiple users to share resources fairly,
efficiently, safely and securely.
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
The Future…
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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.
Operating systems will have to adjust to deliver
acceptable performance for these new forms of
data.
BYU CS 345
OS Overview (Chapter 1)
7
Finally
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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
Chapter 1 – Computer Systems
Let’s figure out
what’s inside
this thing...
Learning Objectives
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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
Computer Systems
What is a computer system?
What are the basic elements of a
computer system?
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Registers
Interrupts
Caching
Input/Output
Protection
BYU CS 345
OS Overview (Chapter 1)
12
Objectives
Computer Systems
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Registers
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Interrupts
Caching
Input/Output
Protection
Summary
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BYU CS 345
OS Overview (Chapter 1)
13
Registers
CPU
BYU CS 345
OS Overview (Chapter 1)
14
Registers
Processor 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
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
Lots of Registers…
BYU CS 345
OS Overview (Chapter 1)
17
Computer Systems
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Registers
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Interrupts
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Caching
Input/Output
Protection
Summary
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BYU CS 345
OS Overview (Chapter 1)
18
Interrupts
Interrupts
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The interrupt was the principle tool available to
system programmers in developing multitasking systems!
Improves processing efficiency by allowing the
processor to execute other instructions while an
I/O operation is in progress
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
Processing of 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
An interrupt handler determines nature of the
interrupt and performs whatever actions are
needed
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Control is transferred to this program
Generally part of the operating system
BYU CS 345
OS Overview (Chapter 1)
20
Interrupts
Interrupt Cycle
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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
Fetch Cycle
Execute Cycle
Interrupt Cycle
Interrupts
Disabled
START
Fetch Next
Instruction
Execute
Instruction
Interrupts
Enabled
Check for
Interrupt:
Process Interrupt
HALT
BYU CS 345
OS Overview (Chapter 1)
21
Interrupts
Multiple Interrupts
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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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BYU CS 345
Allow an interrupt
handler to be
interrupted
Priorities?
OS Overview (Chapter 1)
23
Computer Systems
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Registers
Interrupts
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Caching
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Input/Output
Protection
Summary
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BYU CS 345
OS Overview (Chapter 1)
26
Caching
Memory Hierarchy
More Expensive
Faster & Smaller
Registers
Cache
Main Memory
Disk Cache
Magnetic Disk
Magnetic Tape
BYU CS 345
Bigger
Slower
Optical Disk
OS Overview (Chapter 1)
27
Computer Systems
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Registers
Interrupts
Caching
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Input/Output
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Protection
Summary
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BYU CS 345
OS Overview (Chapter 1)
31
Input / Output
Programmed I/O
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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)
32
Input / Output
Interrupt-Driven I/O
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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)
33
Input / Output
Direct Memory Access
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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)
34
Computer Systems
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Registers
Interrupts
Caching
Input/Output
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Protection
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Summary
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BYU CS 345
OS Overview (Chapter 1)
35
Protection
Hardware Protection
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Why protect hardware?
From what?
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Shared hardware resources – memory, disk, …
Errant programs
How?
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CPU provides 2 modes of operation
 User Mode (non-privileged)
 Supervisor mode (privileged)
Privileged instructions can only be executed in monitor
mode
 All I/O instructions are privileged
BYU CS 345
OS Overview (Chapter 1)
36
Summary…
Summary
What is an O.S.?
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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
Hardware Review
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Elements of a system:
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Processor
 Registers (address, data, control)
 Instruction cycle (fetch, decode, execute)
 Interrupts
 Usually includes hardware and special instructions to help
the O.S. manage memory, devices, etc.
Memory
 Different levels (cache, main memory, disk)
 Operating system will generally manage memory (both
RAM and disk), and move data back and forth as required
I/O
 Usually use Interrupts, DMA
 Operating system usually controls use of I/O devices
BYU CS 345
OS Overview (Chapter 1)
42
Summary
Registers
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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
Interrupts
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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)
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
Interrupts and I/O
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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
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BYU CS 345
OS Overview (Chapter 1)
45
Summary
Memory
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Varying types of memory
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Registers, Cache, RAM, Disk, CD
Vary in speed, size, cost
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
Topics to Cover…
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OS Objectives
OS Services
Resource Manager
Evolution
Achievements
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Processes
Memory management
Information protection and security
Scheduling and resource management
System architecture
BYU CS 345
OS Overview (Chapter 1)
48