COSC 4740

Chapter 2

Operating System Structures

O/S Components

• Process management• I/O management• Main Memory management• File & Storage Management• Networking/Communications• Protection and Error Detection• User interface

– Varies between Command-Line (CLI), Graphics User Interface (GUI), Batch Command Interpreter

A View of Operating System Services

Process Management

• Process (or job): A program or a fraction of a program that is loaded in main memory and executing. – We do not need the entire program code at once.

To process an instruction, CPU fetches and executes one instruction of a process after another in the main memory.

Tasks of Process Managemento Create, load, execute, suspend, resume, and

terminate processeso Switch system among multiple processes in the main

memory (process scheduling)o Provides communication mechanisms so that

processes can send (or receive) data to (or from) each other (process communication).

o Control concurrent* access to shared data to keep shared data consistent (process synchronization).

o Allocate/de-allocate resources properly to prevent or avoid deadlock situation**

I/O Management

• Motivations: o Provide the abstract level of H/W devices and keep the

details from applications to ensure proper use of devices, to prevent errors, and to provide users with convenient and efficient programming environment.

• Tasks of I/O Management of OS: o Hide the details of H/W deviceso Manage main memory for the devices using cache, buffer,

and spoolingo Maintain and provide device driver interfaces

Main Memory management

Process must be mapped to physical addresses and loaded into main memory to be executed.

• Motivations:o Increase system performance by increasing “hit” ratio

(e.g., optimum: when CPU read data or instruction, it is in the main memory always)

o Maximize memory utilization• Tasks of Main Memory Management of OS:

o Keep track of which memory area is used by whom.o Allocate/de-allocated memory as need

File & Storage Management

• Motivation: o Almost everything is stored in secondary storage.

Therefore, secondary storage access must be efficient (i.e., performance) and convenient (i.e., easy to program I/O function in application level)

o Important data are duplicated and/or stored in ternary storage.

• Tasks of File Managemento Create, manipulate, delete files and directories

• Tasks of Storage Managemento Allocate, de-allocate, and defrag blocks[1]

o Bad block markingo Scheduling for multiple I/O request to optimize

the performance

Networking/Communications

• Allow communications between computers (more important for Client/Server OS and Distributed OS).

• Communications may be via shared memory or through message passing (packets moved by the OS)

Protection and Error Detection

• Protection– Hardware resources, Kernel code, processes, files, and

data from erroneous programs and malicious programs. • Error detection

– May occur in the CPU and memory hardware, in I/O devices, and in user program

– For each type of error, OS should take the appropriate action to ensure correct and consistent computing

– Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system

Command Interpreter

• Command Interpreter is one of the most important system programs[1]. Because almost every OS provide system programs, some people argue that command interpreter is a part of OS.

• Motivation:o Allow human users to interact with OSo Provide convenient programming environment to users

• Tasks:o Execute a user command by calling one or more number

of underlying system programs or system calls

• Examples:o Windows DOS command windowo Bash of Unix/Linuxo CSHELL of Unix/Linux

System Calls• Programming interface to the services provided by the OS

– Typically written in a high-level language (C or C++)– Mostly accessed by programs via a high-level Application Program

Interface (API) rather than direct system call use• Three most common APIs are Win32 API for Windows, POSIX

API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM)

• Why use APIs rather than system calls?

Example of System Calls

• System call sequence to copy the contents of one file to another file

Example of Standard API• Consider the ReadFile() function in the• Win32 API—a function for reading from a file

• A description of the parameters passed to ReadFile()– HANDLE file—the file to be read– LPVOID buffer—a buffer where the data will be read into and written from– DWORD bytesToRead—the number of bytes to be read into the buffer– LPDWORD bytesRead—the number of bytes read during the last read– LPOVERLAPPED ovl—indicates if overlapped I/O is being used

Types of System Calls

• Process control• File management• Device management• Information maintenance• Communications

System Call Implementation• Typically, a number associated with each system call

– System-call interface maintains a table indexed according to these numbers

• The system call interface invokes intended system call in OS kernel and returns status of the system call and any return values

• The caller need know nothing about how the system call is implemented– Just needs to obey API and understand what OS will do as a

result call– Most details of OS interface hidden from programmer by API

• Managed by run-time support library (set of functions built into libraries included with compiler)

API – System Call – OS Relationship

Standard C Library Example

• C program invoking printf() library call, which calls write() system call

API vs. System Call summary

• An API is application programming interface– This are a set of standard functions that the

program calls. – This API then call the specific System call for that

specific O/S

• A system call is specific to an O/S– The system call for an O/S may be different

between versions• Different parameters, even different names.

Operating System Design and Implementation

• Design and Implementation of OS not “solvable”, but some approaches have proven successful

• Internal structure of different Operating Systems can vary widely

• Start by defining goals and specifications • User goals and System goals

– User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast

– System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

Operating System Design and Implementation (Cont.)

• Important principle to separatePolicy: What will be done? Mechanism: How to do it?

• Mechanisms determine how to do something, policies decide what will be done– The separation of policy from mechanism is a very

important principle, it allows maximum flexibility if policy decisions are to be changed later

Simple Structure

• MS-DOS – written to provide the most functionality in the least space– Not divided into modules– Although MS-DOS has some structure, its

interfaces and levels of functionality are not well separated

MS-DOS Layer Structure

Monolithic (The big mass) O/S• OS is simply a collection of functions, which can call

any other ones when needed.• One very big OS including everything (system calls, system

programs, every managers, device drivers, etc)• Entire OS resides in main memory

• Pros:• High performance• Easy to implement

• Cons:• Difficult to debug, modify, and upgrade• Poor security, protection, and stability of OS

Layered Approach • still large single program

but internally broken up into layers providing different functionalities.

• Information hiding between layers Increased security and protection

• Easy to debug, test, and modify OS

• If one layer stops working, entire system will stop

• Example:System CallsMemory ManagementProcess SchedulingI/O ManagementDevice Drivers

• Mapping overhead between layers

• Difficult to categorize into layers

Layered Operating System

UNIX

• UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts– Systems programs– The kernel

• Consists of everything below the system-call interface and above the physical hardware

• Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level

UNIX System Structure

Microkernel System Structure • Moves as much from the kernel into “user” space• OS is made up of several separated/independent modules

– kernel, FS, MM, etc and only kernel resides in main memory. • Kernel calls other modules as needed to perform certain

tasks.– Benefits:

• Easier to extend a microkernel• Easier to port the operating system to new architectures• More reliable (less code is running in kernel mode)• More secure

– Detriments:• Performance overhead of user space to kernel space communication

Mac OS X Structure

Modules

• Most modern operating systems implement kernel modules– Uses object-oriented approach– Each core component is separate– Each talks to the others over known interfaces– Each is loadable as needed within the kernel

• Overall, similar to layers but with more flexible

Solaris Modular Approach

Virtual Machines

• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware

• A virtual machine provides an interface identical to the underlying bare hardware

• The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory

Virtual Machines Cont. • Allow multiple different OS to run on a single

machine. VM gives an illusion that the machine is at entire disposal of OS by providing each OS an abstract machine.

o Pros:o Solve machine-OS-software compatibility issues.o Stable and Good for OS development and testing (If one

OS fails, the other OS can continue to run)o Cons:

o Complex to develop/implement– Poor performance (abstract layer, mapping overhead

between layers, …)

Virtual Machines History and Benefits

• First appeared commercially in IBM mainframes in 1972• Fundamentally, multiple execution environments (different

operating systems) can share the same hardware• Protect from each other• Some sharing of file can be permitted, controlled• Commutate with each other, other physical systems via

networking• Useful for development, testing• Consolidation of many low-resource use systems onto fewer

busier systems• “Open Virtual Machine Format”, standard format of virtual

machines, allows a VM to run within many different virtual machine (host) platforms

Virtual Machines (Cont.)

(a) Nonvirtual machine (b) virtual machine

Non-virtual Machine Virtual Machine

VMware Architecture

The Java Virtual Machine

Client-Server

• OS consists of servers, clients, and kernel. Kernel is very tiny and handles only the communication between the clients and servers. Both clients and servers run in user mode.o Pros:

o Stable (If a server fails, the OS can still operate)o Small kernel

o Cons:o Complex to develop/implement (servers run in user mode and can

access resources through kernel)o Security problem because servers reside in user space.

Back to Design Goals• Efficiency and Convenience• Keep kernel adequately small

o Too much functionality in kernel low flexibility at higher level

o Too little functionality in kernel low functional support at higher level

• Maximize the utilization, performance, and functionality of the underlying H/W resourceso OS design is affected by H/W propertieso OS design is affected by the type of system to design (e.g.,

Multiprogramming, Multi-tasking, Real-time, Distributed, .etc)

System Generation

• We should configure OS for the underlying hardware when we install it (i.e., installing OS)

• Approaches:• Modify the source code of OS and recompile it on

the target machine• Pros: small OS size, We can tune the details for the H/W• Cons: Very difficult, less flexible for H/W upgrade

System Generation Cont.

• Provide many pre-compiled modules (e.g., five different memory manager object files). When we install OS, select certain object modules and link them together accordingly.• Pros: easy to configure, flexible for H/W upgrade• Cons: big OS size, We cannot tune the very details

for the H/W

System Boot• Operating system must be made available

to hardware so hardware can start it– Small piece of code – bootstrap loader, locates

the kernel, loads it into memory, and starts it– Sometimes two-step process where boot block

at fixed location loads bootstrap loader– When power initialized on system, execution

starts at a fixed memory location• Firmware used to hold initial boot code

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