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Chapter 2, Operating System Structures

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2.1 Operating System Services

• Overview List:– User interface– Program execution– File system– Communications– Error detection– Efficiency functionality– Security

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• User interface:– Command line– Batch– Graphical user interface

• Program execution– Load– Run– End successfully or not

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• I/O operations– Access to physical devices (usually protected—a

system service)• File system manipulation, files, directories– Read, write– Make, delete– Find, navigate

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• Communications– Inter-process communication– Same computer or different systems– Message passing– Or shared memory

• Error detection– In CPU– In memory– In user program– Detect and handle gracefully, allowing continued use of the

system.

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• O/S functions to promote efficiency, especially on multi-user systems– Resource allocation

• Scheduling CPU cycles• memory access• I/O• file system, etc.

– Accounting• How much of each resource used by each user• In what pattern• At what times• Billling and system configuration

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• Protection and security– Logins and rwx type permissions– Protecting one process from another (in memory)– Protecting from outside intrusions

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2.2 User Operating System Interface

• Overview List:• Command interpreter function• Command interpreter architecture• GUI• Choice of interface

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• Command interpreter function– Setup for a command line interface– Gives the prompt– Takes in the command and takes action

• Command interpreter architecture– May be part of the kernel– May be run as a separate program– May include the code to be run as subroutines– May call separate system programs based on the

command entered

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• GUI—no important new information here• Choice of interface– User preference– Note power of programming in a command line

interface vs. a GUI

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2.3 System Calls

• Overview list:– Explicit system calls– Embedded system calls– Implicit system calls– Passing parameters in the system– System calls in Java

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• Explicit system call– Example: copying a file– Entered from command line or through GUI– One user system call may result in a sequence of

internal system calls• Embedded system call– In ASM, C, C++, etc., it is possible to include lines

of code containing system calls

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• Implicit system calls– The compiler translates user instructions to

machine instructions– At run time, machine instructions which are

protected trigger system calls– The classic example is file I/O

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• Passing parameters in the system– Stored in registers– Stored in memory, memory address stored in

register– Pushed onto the stack

• Note that this heading is relevant to the topic of system calls

• Various system calls require that parameters be passed when (before) the call is made

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• System calls in Java– Write a method in Java declared “native”– Let the native method be a holder for C or C++

(system language) code– The C/C++ code can call system programs– Note that the Java code is now platform

dependent, not portable

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2.4 Types of System Calls

• Overview List:– Process control– File management– Device management– Information management– Communications

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• Process control– End, abort– Load, execute– Create, terminate– Get attributes, set attributes– Wait for time– Wait for event, signal event– Allocate and free memory

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• File management– Create, delete– Open, close– Read, write, reposition– Get attributes, set attributes

• Device management– Request, release– Read, write, reposition– Get attributes, set attributes– Logically attach or detach

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• Information management– Get time or date, set time or date– Get system data, set system data– Get process, file, or device attributes– Set process, file, or device attributes

• Communications– Create, delete communication connection– Send, receive messages– Transfer status information– Attach or detach remote devices

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2.5 System Programs

• Representative of O/S code that’s not in the kernel

• Some items are interfaces to call system services

• Some are distinct system programs or collections of programs

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• Six categories (overlapping with previously presented lists)– File management– Status information– File modification– Programming language support (compilers,

assemblers, interpreters, debuggers)– Program loading and execution (absolute loaders,

relocatable loaders, linkers, etc.)– Communications

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2.6 Operating System Design and Implementation

• Overview List:– Parameter 1: What kind of hardware?– Parameter 2: What kind of system?– Parameter 3: User requirements– Parameter 4: Developer requirements– Mechanisms and policies– Examples– O/S implementation

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• Parameter one: What kind of hardware?• Parameter two: What kind of system?– Single user– Multi-user– Distributed– Real-time, etc.

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• Parameter three: User requirements– Easy to learn, use, etc.– Fast, safe, reliable, etc.– Note that there is no obvious connection between

these requirements and how you code the system in order to meet them

• Parameter four: Developer requirements– Easy to design, implement, maintain, etc.

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• Mechanisms and policies• Software design principle, separate the two– Policy = what to do– Mechanism = how to do it

• In other words, don’t hardcode policy• Implement mechanisms that can enforce

different policies depending on input parameters

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• Examples• Unix– The micro-kernel can support batch, time-sharing, real-time,

or any combination– What is supported depends on values in tables loaded at

start-up• Windows takes the opposite approach– Mechanism and policy are tightly coupled– The system is relatively inflexible– There is a reason for this: It enforces the same look and feel

across all installations

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O/S implementation

• Originally: assembly language• Modern systems• ~10% assembly language– Context switching– Possibly process scheduling– Possibly memory management

• ~90% C—all remaining system functionality

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2.7 Operating System Structure

• Overview List:– Simple structure/no structure/monolithic– The layered approach– Modular design (object-oriented)

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• Simple structure = not well structured– Simple systems that grew without a plan– Systems where immediate performance needs

were paramount– Monolithic systems in general

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MS DOS example

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• The left hand set of arrows represents a layered path through the software

• The right-hand, non-layered path enabled performance, but made the system error-prone

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Original Unix example

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• The original Unix was no better• Superficially layered• The layers contained lots of stuff• Internally the layers were undisciplined in

design

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The Layered Approach

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• Design functionality from the top down (from the outside in)

• Code and debug from the bottom up (from the inside out)

• Each layer has data structures, code, and interface• The implementation of the layers can change as long as

the interface discipline is maintained• Somewhat object-oriented• Less general because each layer interacts with two

neighbors only

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• The fundamental challenge of layered systems is breaking it into layers and getting the dependencies straight

• Each upper layer can only be implemented with calls to its lower neighbor

• Sorting this out in the kernel is not trivial. Which is lowest, process scheduling, memory management, or some other function?

• The practical disadvantage of layering is the performance cost of going down through the layers

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• Microkernels contain the bare minimum of system code– Process management– Memory management– Message passing

• The kernel remains small and fast and manageable by developers

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• All other system code written into modules– Modules run like user applications– But they are privileged to make system calls– Communication between them is managed by the

kernel• All other system programs can be changed

without affecting the kernel

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Modular Design (Solaris)

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Object-oriented design

• Kernel and other modules can be dynamically loaded and linked to provide needed services

• Different objects/modules can communicate with each other

• Modern Unix systems take this approach

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Mac OS X

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• BSD = Berkeley Standard Distribution (kernel)• Mach = Carnegie Mellon (micro-kernel)• Different O/S functions are handled by the

two• Mac OS X is a hybrid system

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2.8 Virtual Machines

• Logical conclusion of multi-user, multi-tasking• Logical conclusion of layering idea• Each user has access to a simulated hardware

interface• Each user can load a different O/S, access

different virtual peripherals, etc.

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• The challenge in implementation is keeping track of real system mode vs. virtual user mode and real user mode vs. virtual user mode

• With only one underlying machine there is only one real system mode, but there are many virtual system modes, one for each virtual machine

• Sharing and communication between virtual machines is also an important challenge in implementation

• A diagram follows

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Benefits of virtual machines

• Users can run different O/S’s• Users are completely isolated from each other• System developers can work on O/S and other

system code on one virtual machine while users run on the others

• The original VM system—IBM mainframe—now configured to run Linux as well as CMS

• VMware runs on Intel systems, making it possible to load >1 O/S on a system at a time

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2.9 Java

• Java has three parts– Programming language specification– Application programming interface (API)– Virtual machine specification

• Note that Java is another example of a virtual machine architecture

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• Programming language characteristics– Object-oriented– Architecture neutral– Distributed– Multi-threaded– Secure– With automatic garbage collection

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• Java API editions– JSE = standard edition• Desktop applications and network applets• Graphics, I/O, security, db connectivity, networking

– EE = enterprise edition• Server applications with db support

– ME = micro edition• Applications on phones, pagers, handheld devices…

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• The Java Virtual Machine = JVM• The Java platform =– An implementation of the JVM specification for

any given hardware environment– Plus the Java API

• The Java platform can be implemented– On top of a host O/S– In a browser– Or the JVM can be implemented in hardware

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About the JVM

• In a software environment, when main() is called in an application, this causes an instance of the JVM to be created

• Each separate application or applet in execution has its own JVM

• The JVM consists of a loader and an interpreter. The loader loads a .class file containing Java byte code

• The interpreter, or just in time compiler, translates Java code into the commands and calls necessary in a given environment

• Implementations of the JVM differ across environments, but not the specification

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Running a Java Program

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Random additional comments

• For development purposes a Java Development Environment would consist of a compiler plus a Java Runtime Environment (JRE)

• In theory you might write an O/S with a microkernel in C/ASM and the rest in Java, using Java’s type safety mechanism for security. JX is an example

• The MS .NET framework is similar to the Java approach

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2.10 Operating System Debugging

• This section can be summarized as, “If you ever had to debug an operating system, it might be the hardest thing you ever had to do.”

• Failure analysis and performance tuning are worthwhile concepts, but the details are unimportant

• DTrace is of no particular consequence to us

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• Kernighan’s Law appears here, and it’s memorable

• “Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.”

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• There are several worthwhile conclusions to draw from this:

• 1. Strive for clarity and simplicity when code writing

• Let an optimizing compiler do the cleverness part for you

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• 2. Sometimes code is so incomprehensible that it’s easier to start from scratch than it is to debug it

• 3. My take on commenting code:• You will start commenting code in earnest

when the day comes that you have to fix code that you previously wrote and you no longer understand how it works

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2.11 Operating System Generation

• System generation = installing and setting up an O/S on a machine

• Most O/S’s can function on multiple hardware platforms

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• Installation requires setting parameters:– What CPU, how many, which instruction set(s)?– How much main memory?– What attached devices, type and model,

numerical designation, interrupt number– O/S parameters: scheduling algorithm, number of

I/O buffers, number of processes allowed, etc.

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• Approaches to system generation fall into a range:– Set parameters in source, compile O/S, install– Select precompiled modules from libraries– Table driven—select options after O/S is up and

running

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• O/S performance parameters– Ease and speed of installation and set-up– Performance speed and memory size at run time– Ease of modification

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• Ease of modification– Most systems probably have elements of both:• Major modifications: Change settings, recompile,

reinstall• Minor modifications: Change settings from table

options on the fly

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2.12 System Boot

• Already covered while going over chapter 1

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The End