1. Operating - System Structures
Chapter 1& 2
Operating - System Structures
Dr. Amjad Ali & Yella Mehrooz

2. Computer-System Architecture
Most systems contain a single general-purpose processor
Few systems contain special-purpose processors as well
Multiprocessors systems growing in use and importance
Also known as parallel systems, tightly-coupled systems
Advantages include:
Increased throughput- more work done in less time
Speed up ratio with N processor is not N
Overhead of keep all component working correctly
Contention for shared resources
Economy of scale
Increased reliability – graceful degradation or fault tolerance
The ability of a system to maintain limited functionality even when a large portion of it has been destroyed or rendered inoperative

3. Computer-System Architecture
Multiprocessor systems are of two types:
Asymmetric Multiprocessing– each processor assigned a specific task
Master-slave relationship
The tasks are strictly divided by their types on processors
Typically, each processor has its own memory address space
These features make asymmetric multiprocessing difficult to implement.
Symmetric Multiprocessing (SMP)– each processor performs all tasks within the OS or they can do anything the others can. 
All processor are peers
Contain own set of registers and private or local caches
Shared physical memory
Asymmetric multiprocessing; one processor doing all I/O
Symmetric multiprocessing

4. A Dual-Core Design Cores- Multiprocessor chips
on-chip communication is faster
well suited for server (i.e., database, Web servers)
Multicore CPUs appear to the operating system as N standard processors
A dual-core design with two cores places on the same chip

5. Blade Servers
Blade server is a stripped down  server computer with a modular design optimized to minimize the use of physical space and energy 
Multiple processor boards, I/O boards, and networking boards are placed in the same chassis
HP blade system c7000 enclosure (populated with 16 blades), with two 3U UPS units below.

6. Clustered Systems
Another type of multiple-CPU system is clustered system
Usually sharing storage via a storage-area network (SAN)
Provides a high-availability service which survives failures
Asymmetric clustering; One machine in hot-standby mode while the other is running the applications
Symmetric clustering; multiple machines are running the applications as well as monitoring each other
Some clusters are for high-performance computing (HPC)
Applications must be written to use parallelization
General structure of a clustered system

7. Operating System Structure
Multiprogramming required for better efficiency
Single user cannot keep CPU and I/O devices busy all the time
Multiprogramming organizes jobs (code and data) so CPU always has one to execute
A subset of total jobs in system is kept in memory
One job selected and run via job scheduling
When it has to wait (for I/O for example), OS switches to another job
Timesharing (multitasking) is logical extension, in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing
Response time should be < 1 second
Process; A program loaded into memory and executing
Job Scheduling; Decision to load some jobs from all jobs residing in job pool
CPU scheduling; Decision to execute a job among many jobs waiting for execution
Swapping; processes are swapped in and out of main memory to the disk
Virtual Memory; a technique that allows the execution of a process that is not completely in memory

8. Comparison Multiprogramming There is only one processor
More than one user
It is the function of OS to share CPU time among all the users equally
Example: A computer running Excel and Firefox simultaneously
Multiprocessing
Multiprocessing is the use of two or more CPUs within a single computer system
Symmetric and asymmetric multiprocessing
Multitasking
There is only one processor and one user.
User can activate more than one program/task at a time.
OS schedule multiple jobs for their fair share processing.
Multithreading
Execute different parts of a program called threads at the same time
Threads: The light wait processes which are independent part of a process or program.

9. Comparison Memory layout for Multiprogramming System
Multi-processing System
Multi-tasking System
Multithreading System

10. Operating-System Operations
Interrupt driven by hardware
Software error or request creates exception or trap
Division by zero, request for operating system service
Other process problems include infinite loop, processes modifying each other or the operating system
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

11. 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) rather than direct system call use
Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based systems for UNIX, Linux, and Mac OS, and Java API for the Java virtual machine (JVM)
Why use APIs rather than system calls?
System calls Mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use
System calls differ from platform to platform. By using a stable API, it is easier to migrate your software to different platforms.

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

13. Example of Standard API

14. 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)

15. API – System Call – OS Relationship

16. Types of System Calls Process control end, abort load, execute
create process, terminate process
get process attributes, set process attributes
wait for time
wait event, signal event
allocate and free memory
Dump memory if error
Debugger for determining bugs, single step execution
Locks for managing access to shared data between processes

17. 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
get device attributes, set device attributes
logically attach or detach devices

18. 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
attach and detach remote devices

19. Types of System Calls (Cont.)
Protection
Control access to resources
Get and set permissions
Allow and deny user access

20. Examples of Windows and Unix System Calls

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

22. Operating System Kernel
A kernel is a central component of an operating system.
It acts as an interface between the user applications and the hardware.
The sole aim of the kernel is to manage the communication between the software (user level applications) and the hardware (CPU, disk memory etc.).
The main tasks of the kernel are :
Process management
Device management
Memory management
Interrupt handling, I/O communication, and File system.

23. Types of kernels Classified mainly in two categories
Monolithic (Mono means single, Lithic means layer)
Micro Kernel.
Monolithic
Older approach
Basic system services like process and memory management, interrupt handling etc. were packed into a single module in kernel space.
Some serious problems like,
Size was huge,
Poor maintainability (bug fixing and addition of new features resulted in recompilation of the whole kernel code which could consume hours.
E.g. MS-DOS

24. Types of kernels Micro Kernel
Modern Kernel approach (Modular Approach)
Kernel consists of different modules which can be dynamically loaded and un-loaded.
Easy extension of OS’s capabilities.
Maintainability became so easy as only the concerned module needs to be loaded and unloaded every time if there is change or bug fix in a particular module.

25. Micro Kernel
This solve the problem of ever growing size of kernel code which we could not control in monolithic approach.
This architecture allows some basic services like,
Device driver management
Protocol stack
File system etc. to run in user space.
It reduces the kernel size and increases the security and stability of OS e.g., if network service crashes due to buffer overflow, then only the networking services memory would be corrupted, leaving the rest of the system still functional.

26. Instruction Execution
Once a program is in memory it has to be executed.
To do this, each instruction must be looked at, decoded and acted upon in turn until the program is completed.
This is achieved by the use of what is termed the “instruction execution cycle”, which is the cycle by which each instruction in turn is processed.

27. Instruction Execution(IE)
The process in which CPU reads instruction from memory and execute each instruction
The processing required for single instruction called instruction cycle.
At the beginning of each instruction cycle, CPU fetches instruction from memory.
PC holds the address of the next instruction to be executed.
The fetched instruction is loaded into IR.

28. Jobs types There are two types of jobs I/O limited jobs
If in an application most of the tasks are I/O oriented then it is called I/O jobs.
For (k=0;k<=5;k++)
cout<<k;
CPU limited jobs
If a program only needs computation/processing then this is called CPU limited jobs.
Sum=0;
sum=sum+k;

29. Interrupt processing Interrupt
An Interrupt is an event that alters the sequence in which the processor executes instructions.
When an I/O device completes an I/O operation some events occurs,
The device issue an interrupt signal to the CPU.
The processor finishes the execution of the current instruction before responding to the interrupt and save the state of the interrupted process.
The OS analyses the interrupt and passes the control to the appropriate interrupt handling routine.
The IHR handles the interrupt.
The state of the interrupted process restored.
The interrupted process executes.

30. Interrupt Handling While executing a process, an interrupt triggers.
It is handled by the IHR.
The process starts its execution again.
However what will happen if while handling an interrupt, another interrupt triggers?
It can be handled using two approaches
Sequential approach
Nested approach

31. Sequential Interrupt Handling

32. Nested Interrupt Handling

33. Context switching
A context switch is the computing process of storing and restoring the state (context) of a CPU so that execution can be resumed from the same point at a later time.
This enables multiple processes to share a single CPU.
The context switch is an essential feature of a multitasking operating system.
Context switches are usually computationally intensive and much of the design of operating systems is to optimize the use of context switches