Lecture 2:
Operating System Operations
System Calls
Operating System Structure
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Operating-System Operations
 Dual-mode operation allows OS to protect itself and other system components

User mode and kernel mode
 Mode bit provided by hardware
Provides ability to distinguish when system is running user code or
kernel code
 Some instructions designated as privileged, only executable in kernel
mode
 System call changes mode to kernel, return from call resets it to user

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Operating-System Operations (cont.)
 Benefits of Dual Mode:

MSDOS for 8088 architecture had no mode bit—no dual mode

An errant user program could wipe out the OS data

Multiple programs are able to write to a device at the same time

Current Intel CPUs have all mode bits

Most contemporary operating systems, Windows, Unix, Linux, Solaris
take advantage of this feature and provide greater protection of the OS.
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Process Management
 A process is a program in execution. It is a unit of work within the
system. Program is a passive entity, process is an active entity.
 Process needs resources to accomplish its task

CPU, memory, I/O, files

Process termination requires reclaim of any reusable
resources
 Single-threaded process has one program counter specifying
location of next instruction to execute

Process executes instructions sequentially, one at a time,
until completion
 Typically system has many processes, some user, some
operating system running concurrently on one or more CPUs

Concurrency by multiplexing the CPUs among the processes
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Process Management Activities
The operating system is responsible for the following activities in
connection with process management:
 Creating and deleting both user and system processes
 Suspending and resuming processes
 Providing mechanisms for process synchronization
 Providing mechanisms for process communication
 Providing mechanisms for deadlock handling
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Memory Management
 To execute a program all (or part) of the instructions must be in
memory
 All (or part) of the data that is needed by the program must be in
memory.
 Memory management determines what is in memory and when

Optimizing CPU utilization and computer response to users
 Memory management activities

Keeping track of which parts of memory are currently being
used and by whom

Deciding which processes (or parts thereof) and data to
move into and out of memory

Allocating and deallocating memory space as needed
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Storage Management
 OS provides uniform, logical view of information storage

Abstracts physical properties to logical storage unit - file

OS maps files onto physical media and accesses these files via
the storage devices (i.e., disk drive, tape drive)
 File-System management

Files usually organized into directories

Access control on most systems to determine who can access
what (e.g. read/write/execute access)

OS activities include

Creating and deleting files and directories

Primitives to manipulate files and directories

Mapping files onto secondary storage

Backup files onto stable (non-volatile) storage media
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I/O Subsystem
 One purpose of OS is to hide peculiarities of hardware devices
from the user
 I/O subsystem responsible for

Memory management of I/O including buffering (storing data
temporarily while it is being transferred), caching (storing parts
of data in faster storage for performance), spooling (the
overlapping of output of one job with input of other jobs)

General device-driver interface

Drivers for specific hardware devices
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Protection and Security
 Protection – any mechanism for controlling access of processes or
users to resources defined by the OS
 Security – defense of the system against internal and external attacks
 Huge range of attacks: denial-of-service (using all resources and
keeping legitimate users out of the system), worms, viruses,
identity theft, theft of service (unauthorized use of a system)
 On some systems an OS function, while some leave it to
additional software.
 Systems generally first distinguish among users, to determine who
can do what
 User identities (user IDs--security IDs in Windows) include name
and associated number, one per user
 User ID then associated with all files, processes of that user to
determine access control

Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file
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Open-Source Operating Systems
 Operating systems made available in source-code (like Linux)
format rather than just compiled binary code (like Windows)

Mac OS X and iOS: hybrid, open-source kernel called
Darwin, but also closed-source components.
 Interested programmers can freely debug, analyze, support and
provide changes.
 Learning tool for students: modify, compile, run.
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Open-Source Operating Systems-History
 To counter the move to limit software use and redistribution
 In 1983, Richard Stallman started the GNU project to create a free, open-
source Unix compatible OS.



founded the Free Software Foundation (FSF),
“copyleft” GNU Public License (GPL)
requires source code to be distributed with binaries.
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Open-Source Operating Systems-History
 Examples include



GNU/Linux

Linus Torvalds originally

Enhanced by several thousand programmers

Spawned many distributions: RedHat, Fedora, Debian, Ubuntu
BSD UNIX

Distributions: FreeBSD, OpenBSD

Darwin: core of Mac OS X
Solaris:

Commercial UNIX-based OS of Sun Microsystems

partially open-sourced as some of the code is still owned by At&T.
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User Operating System Interface - CLI
Command-line interface (CLI) or command interpreter allows direct
command entry

Sometimes implemented in kernel, sometimes by systems
program

Sometimes multiple flavors implemented – shells


Unix/Linux: Bourne Shell, C Shell, Korn Shell, etc.
Primarily fetches a command from user and executes it
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Bourne Shell Command Interpreter
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User Operating System Interface - GUI
 User-friendly desktop metaphor interface

Usually mouse, keyboard, and monitor

Icons represent files, programs, actions, etc

Various mouse buttons over objects in the interface cause
various actions (provide information, options, execute function,
open directory (known as a folder)

Invented at Xerox PARC
 Many systems now include both CLI and GUI interfaces

Microsoft Windows is GUI with CLI “command” shell

Apple Mac OS X is “Aqua” GUI interface with UNIX kernel
underneath and shells available

Unix and Linux have CLI with optional GUI interfaces (CDE,
KDE, GNOME)
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The Mac OS X GUI
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Touchscreen Interfaces
 Touchscreen devices require new
interfaces


Mouse not possible or not desired

Actions and selection based on
gestures

Virtual keyboard for text entry
Voice commands.
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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
Programming Interface (API)

Set of functions that are available to an application programmer,
including their parameters, and return values
 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)
 A programmer accesses an API via a library of code

E.g. libc in UNIX/Linux for C programmers
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Example of System Calls
 System call sequence to copy the contents of one file to another file
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Example of Standard API
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System Call Interface
 Managed by run-time support library (set of functions built into
libraries included with compiler)
 Intercepts API function calls
 Typically, a number associated with each system call

System-call interface maintains a table indexed according to
these numbers
 The system call interface invokes the 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
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API – System Call – OS Relationship
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System Call Parameter Passing
 Often, more information is required than simply identity of desired
system call
 Exact type and amount of information vary according to OS
and call
 Three general methods used to pass parameters to the OS

Simplest: pass the parameters in registers
 In some cases, may be more parameters than registers
 Parameters stored in a block, or table, in memory, and
address of block passed as a parameter in a register
This approach taken by Linux and Solaris
 Parameters placed, or pushed, onto the stack by the program
and popped off the stack by the operating system
 Block and stack methods do not limit the number or length of
parameters being passed

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Parameter Passing via Table
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Types of System Calls
 Process control

create process, terminate process

get process attributes, set process attributes

wait for time

wait event, signal event

allocate and free memory

Locks for managing access to shared data between processes
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Types of System Calls
 File management

create file, delete file

open, close file

read, write, reposition

get and set file attributes
 Device management

request device, release device

read, write, reposition

get device attributes, set device attributes

logically attach or detach devices
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Types of System Calls (Cont.)
 Information maintenance

get time or date, set time or date

get system data, set system data

get and set process, file, or device attributes
 Communications

create, delete communication connection

send, receive messages if message passing model to host
name or process name

From client to server

Shared-memory model create and gain access to memory
regions

transfer status information
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Types of System Calls (Cont.)
 Protection

Control access to resources

Get and set permissions

Allow and deny user access
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Examples of Windows and Unix System Calls
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Standard C Library Example
 C program invoking printf() library call, which calls write() system call
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Example: FreeBSD
 Unix variant
 Multitasking
 User login -> invoke user’s choice of
shell
 Shell executes fork() system call to create
process

Executes exec() to load program into
process

Shell waits for process to terminate or
continues with user commands
 Process exits with:

code = 0 – no error

code > 0 – error code
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Operating System Structure
 General-purpose OS is very large program
 Various ways to structure ones

Monolithic

Layered

Microkernel
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Monolithic -- UNIX
UNIX – limited by hardware functionality, the original UNIX operating
system had limited structuring. The UNIX OS consists of two
separable parts

Systems programs (compilers, editors, system libraries, etc.)

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;

Disadv: a large number of functions for one level
–

Difficult to implement and maintain
Performance adv: minimum overhead for communication
within the kernel
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Traditional UNIX System Structure
Beyond simple but not fully layered
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Layered Approach
 The operating system is divided into
a number of layers (levels), each
built on top of lower layers. The
bottom layer (layer 0), is the
hardware; the highest (layer N) is
the user interface.
 With modularity, layers are selected
such that each uses functions
(operations) and services of only
lower-level layers
 Adv: Simplicity of construction and
debugging
 Difficulty: appropriately defining
various layers
 Disadv: added overhead by each
layer
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Microkernel System Structure
 Nonessential components removed from the kernel and
implemented as system or user programs.
 Kernel contains minimal process and memory management
and communication facility.
 Mach example of microkernel

Mac OS X kernel (Darwin) partly based on Mach
 Main function: provide communication between client programs
and various services also running in user-space

Communication takes place between user modules using
message passing
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Microkernel System Structure
Application
Program
File
System
messages
Interprocess
Communication
Device
Driver
user
mode
messages
memory
managment
CPU
scheduling
kernel
mode
microkernel
hardware
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Microkernel System Structure
 Benefits:

Easier to extend a microkernel, new services added to user
space, no or minimal modification to the kernel

More secure (less code is running in kernel mode)
 Detriments:

Performance overhead of user space to kernel space
communication
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Modules
 Many modern operating systems implement loadable kernel
modules

Has a set of core components

Links in additional modules during boot time or runtime

Dynamic linking as the kernel is running

Each talks to the others over known interfaces

Overall, similar to layered approach but with more flexible
since each module can call each module

Linux, Solaris, etc
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Hybrid Systems
 Most modern operating systems are actually not one pure model

Hybrid combines multiple approaches to address
performance, security, usability needs

Linux and Solaris kernels in kernel address space, so
monolithic, plus modular for dynamic loading of functionality

Windows mostly monolithic, plus retains some microkernel
behaviour providing support for user-space different
subsystems
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Hybrid Systems
 Apple Mac OS X

Somewhat layered and microkernel behaviour
 At the top: Aqua UI plus a set of application environments
(such as Cocoa programming environment)

Below is kernel consisting of Mach microkernel
 Memory management
 IPC (interprocess communication), message passing
 and BSD Unix kernel parts,
Command-line interface
 Networking
 File system
 plus I/O kit and dynamically loadable modules (called kernel
extensions)

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Mac OS X Structure
graphical user interface
Aqua
application environments and services
Java
Cocoa
Quicktime
BSD
kernel environment
BSD
Mach
I/O kit
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Virtual Machines (VM)
 Allows operating systems to run as applications within other
operating systems.
 Vast and growing industry
 Virtualization – OS natively compiled for CPU, running guest
OSes also natively compiled
 Several guest operating systems (execution environment)
 Giving the illusion that each guest execution environment
is running its own private computer
 VMware application: ran one or more guest copies of
Windows or other OSes, each running applications, all on
native Windows host OS
 VMM (Virtual Machine Manager) runs the guest OSes,
manages their resources and protects each guest from
others.
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Computing Environments - Virtualization
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Computing Environments - Virtualization
 Use cases involve laptops and desktops running multiple OSes
for exploration or compatibility

Apple laptop running Mac OS X host, Windows as a guest

Developing apps for multiple OSes without having multiple
systems

QA testing applications without having multiple systems
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