1. Slide 1
Chapter 3: Operating-System Structures
(18M)
System Components
Operating System Services
System Calls
System Structure

2. Slide 2
Services of O.S.
Program Execution
I/O Management
File system manipulation
Communication
Error detection
Accounting
Protection
Resource Allocation

3. Slide 3
Common System Components
A system as large and complex as an OS can be
created only by partitioning it into smaller pieces
Process Management
Main Memory Management
File Management
I/O System Management
Secondary Management

4. Slide 4
Process Management
A process is a program in execution. A process needs
certain resources, including CPU time, memory, files,
and I/O devices, to accomplish its task.
 A process is the unit of work in a system
The operating system is responsible for the following
activities in connection with process management.
 Process creation and deletion.
 Creation and deletion of user and system process
 Process suspension and resumption.
 Provision of mechanisms for:
 process synchronization
 process communication

5. Slide 5
Main-Memory Management
Memory is a large array of words or bytes, each with its own address. It is a
repository of quickly accessible data shared by the CPU and I/O devices.
 The CPU reads instructions from main memory (MM) during the
instruction-fetch cycle and both reads and writes data from MM during
the data-fetch cycle
Main memory is a volatile storage device. It loses its contents in the case of
system failure.
To improve both CPU utilization and response speed, several programs are
kept in memory
The following are some of the activities in connections with memory
management that are handled by the operating system :
 Keep track of which parts of memory are currently being used and by
whom.
 Decide which processes to load when memory space becomes available.
 Allocate and deallocate memory space as needed.

6. Slide 6
File Management
A file is a collection of related information defined by its
creator. Commonly, files represent programs (both source
and object forms) and data.
The operating system is responsible for the following
activities in connections with file management:
 File creation and deletion.
 Directory creation and deletion.
 Support of primitives for manipulating files and directories.
 Mapping files onto secondary storage.
 File backup on stable (nonvolatile) storage media.

7. Slide 7
I/O System Management
Hide the peculiarities of specific hardware devices from the
user
The I/O system consists of:
 A memory management component that includes buffering,
caching and spooling
 A general device-driver interface
 Drivers for specific hardware devices

8. Slide 8
Secondary-Storage Management
Since main memory (primary storage) is volatile and too
small to accommodate all data and programs permanently,
the computer system must provide secondary storage to
back up main memory.
Most modern computer systems use disks as the principle
on-line storage medium, for both programs and data.
Because secondary storage is used frequently, it must be
used efficiently or else it will become a processing bottleneck
The operating system is responsible for the following
activities in connection with disk management:
 Free space management
 Storage allocation
 Disk scheduling

9. Slide 9
System Calls
System calls provide the interface between a running program and the
operating system.
 For example – open input file, create output file, print message to
console, terminate with error or normally
 Generally available as routines written in C and C++
 Certain low-level tasks (direct hardware access) may be written in
assembly-language
Mostly accessed by programs via a high-level Application Program
Interface (API) rather than direct system call use
 Provides portability (underlying hardware handled by OS)
 Hides the detail from the programmer
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)

10. Slide 10
Example of System Calls
System call sequence to copy the contents of one file to
another file

11. Slide 11
API – System Call – OS Relationship

12. Slide 12
Standard C Library Example
C program invoking printf() library call, which calls
write() system call

13. Slide 13
Passing Parameters as a Table

14. Slide 14
Types of System Calls
Process control
 end, abort, load, execute, allocate/free memory
File management
 Create/delete file, open, close, read, write
Device management
 Request/release device, read, write
Information maintenance
 Get/set date or time, get process, get/set system data
Communications
 Send/receive message, create/delete comm link

15. Slide 15
UNIX System Calls

16. Slide 16
SYSTEM CALLS
Process control
o end, abort
o load, execute
o create process, terminate process
o get process attributes, set process
attributes
o wait for time
o wait event, signal event
o allocate and free memory
File management
o create file, delete file
o open, close
o read, write, reposition
o get file attributes, set file attributes
Device management
o request device, release device
o read, write, reposition
o get device attributes, set device attributes
o logically attach or detach devices
Information maintenance
o get time or date, set time or date
o get system data, set system data
o get process, file, or device attributes
o set process, file, or device attributes
Communications
o create, delete communication connection
o send, receive messages
o transfer status information
o attach or detach remote devices
o get host id
o open,close connection

17. Slide 18
SYSTEM STRUCTURE
Simple Structure
Layered
Monolithic
Microkernel

18. Slide 19
Operating System Structure
MS-DOS – written to provide the most
functionality in the least space
 Not well divided into modules
 Designed without realizing it was
going to become so popular
 Although MS-DOS has some
structure, its interfaces and levels of
functionality are not well separated
 Vulnerable to system crashes when
a user program fails
 Written for Intel 8088 which had
no dual mode or hardware
protection
MS-DOS layer structure

19. Slide 20
Operating System Structure
UNIX – the original UNIX operating system had limited
structuring due to limited hardware functionality.
The UNIX OS consists of two separable parts.
 Systems programs
 The kernel – series of interfaces and device drivers
 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.
 All this functionality in one level makes UNIX difficult to
enhance – difficult to determine impact of change in one part of
the kernel on other parts

20. Slide 21
UNIX System Structure
kernel

21. Slide 22
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.
 Simplifies debugging and system
verification – each layer only uses
those below it, so by testing from
bottom up, you isolate errors to the
layer being tested
 Requires careful definition of layers
 Less efficient as each layer adds
overhead to the system increasing
overall time for a system call to
execute
An operating system layer

22. Slide 23
Layered Approach
These limitations have led to the design of OSs with fewer layers each
with more functionality
Provides most of the advantages of modularized code while avoiding the
difficulties of layer definition and interaction
OS/2, a descendent of MS-DOS, adds multitasking and dual-mode
operation
 Designed in a more layered approach than MS-DOS
 Direct access to low-level facilities is not allowed; this gives the OS more
control over the hardware and more knowledge of which resources each
user program is using
The first release of Windows NT had a highly layered approach but it delivered
low performance compared to Windows 95
 NT 4.0 addressed that problem in part by moving layers from user space to
kernel space and more closely integrating them

23. Slide 24
OS/2 Layer Structure

24. Slide 25
Microkernel System Structure
In the mid 1980s, researchers at Carnegie Mellon university developed the
Mach microkernel OS
Moves nonessential components from the kernel into “user” space resulting
in a smaller kernel
Main function of the microkernel is to provide a communication facility
between the client program and various services that are also running in user
space
 Communication takes place between user modules using message passing.
Benefits:
 easier to extend a microkernel
 all new services are added to user space
 easier to port the operating system to new architectures
 more reliable and secure
 less code is running in kernel mode
Apple MacOS X Server OS is based on the Mach kernel
 maps system calls into messages to the appropriate user-level services
 Tru 64 Unix (formerly Digital UNIX)

25. Slide 26
MONOLITHIC