1. Module 1: Introduction What is an operating system?
Multiprogramming Systems
Time-Sharing Systems
Parallel Systems
Distributed Systems
Real -Time Systems
System Calls and APIs
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2. What is an Operating System?
A program that acts as an intermediary between a user of a computer and the computer hardware.
Operating system goals:
Execute user programs and make solving user problems easier.
Make the computer system convenient to use.
Use the computer hardware in an efficient manner.
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3. Computer System Components
1. Hardware – provides basic computing resources (CPU, memory, I/O devices).
2. Operating system – controls and coordinates the use of the hardware among the various application programs for the various users.
3. Applications programs – define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs).
4. Users (people, machines, other computers).
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4. Abstract View of System Components
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5. Multiprogramming and Timesharing
Multiprogramming: CPU is multiplexed (shared) among a number of jobs -- while one job waiting for I/O, another can use CPU.
Advantages:
CPU is kept busy.
Disadvantages:
Hardware and O.S. became significantly more complex for handling and scheduling multiple jobs.
Timesharing: switch CPU among jobs for pre-defined time interval.
Most O.S. issues arise from trying to support multiprogramming -- CPU scheduling, deadlock, protection, memory management, virtual memory, etc.
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6. Why need multiprogramming? I/O Times vs. CPU times
400 MHz Pentium II = 400 million cycles/second
10 cycles/instruction = 40 million instructions/second
Read 1 disk block = 20 msec
CPU can do (40 x 106) / 103= 40,000 instructions/msec
Thus:
In time to read one disk block, CPU can do 20 * 40,000 = 800,000 instructions !!
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7. Parallel Systems
Multiprocessor systems with more than one CPU in close communication.
Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.
Advantages of parallel system:
Increased throughput
Economical
Increased reliability
graceful degradation or fault-tolerant
The ability to continue providing service proportional to the level of surviving hardware.
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8. Parallel Systems (Cont.)
Symmetric multiprocessing (SMP)
Each processor runs an identical copy of the operating system.
Many processes can run at once without performance deterioration.
Most modern operating systems support SMP
Asymmetric multiprocessing
Each processor is assigned a specific task; master processor schedules and allocates work to slave processors.
More common in extremely large systems
Also used in PS/3 and Sbox
PS/3 has 8 CPUs with one CPU acting as a master
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9. Symmetric Multiprocessing Architecture
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10. Distributed Systems
Distribute the computation among several physical processors.
Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines.
Advantages of distributed systems.
Resource sharing
sharing and printing files at remote sites
processing information in a distributed database
using remote specialized hardware devices
Computation speedup – load sharing
Reliability – detect and recover from site failure, function transfer, reintegrate failed site
Communication – message passing
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11. Network vs. Distributed OS
Network OS
A configuration in which there is a network of application machines, typically a workstations with multiple server machines.
Server machines can be file servers, printer servers, mail, etc.
Each computer has its own OS. The user must be aware that there are multiple independent machine and must deal with them explicitly.
NW OS allows machines to interact with each other by having a common communication architecture.
Distributed OS
A common OS shared by a network of computers
Offers the illusion of a unified system image, i.e. single system image
i.e, a pool of interconnected computers appears as a single unified computing resource
can say that these machines have a Single System Image (SSI) [Buyya vol.1, 1999].
It provides the user with transparent access to the resources of multiple machines
Therefore:
less autonomy between computers
gives the impression there is a single operating system controlling the network.
Research vehicle
Examples: Bell Labs Inferno & Plan 9, Mach, Amobea by Tanenbaum, Chorus by CMU
SSI Single System Image
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12. Computing Architectures
Vector Computers (VC) - proprietary system:
provided the breakthrough needed for the emergence of computational science, buy they were only a partial answer.
Massively Parallel Processors (MPP) -proprietary systems:
high cost and a low performance/price ratio.
Symmetric Multiprocessors (SMP):
suffers from scalability
Distributed Systems:
difficult to use and hard to extract parallel performance.
Clusters - gaining popularity:
High Performance Computing - Commodity Supercomputing
High Availability Computing - Mission Critical Applications
Grid Computing
Cloud Computing
Grid computing (or the use of computational grids) is the combination of computer resources from multiple administrative domains applied to a common task, usually to a scientific, technical or business problem that requires a great number of computer processing cycles or the need to process large amounts of data.
One of the main strategies of grid computing is using software to divide and apportion pieces of a program among several computers, sometimes up to many thousands. Grid computing is distributed, large-scale cluster computing, as well as a form of network-distributed parallel processing [1]. The size of grid computing may vary from being small — confined to a network of computer workstations within a corporation, for example — to being large, public collaboration across many companies and networks. 
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13. Cloud Computing
Cloud computing is the E-busniess on-demand of the future. Huge VPS servers. CC is the provision of dynamically scalable and often virtualised resources as a service over the Internet.[1][2] Users need not have knowledge of, expertise in, or control over the technology infrastructure in the "cloud" that supports them.[3] Cloud computing services often provide common business applications online that are accessed from a web browser, while the software and data are stored on the servers.
The term cloud is used as a metaphor for the Internet, based on how the Internet is depicted incomputer network diagrams and is an abstraction for the complex infrastructure it conceals.[4]
A technical definition is that "cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction."[5] This definition states that clouds have five essential characteristics: on-demand self-service, broad network access, resource pooling, rapid elasticity, and measured service
Software as a service (SaaS)
Hardware as a service (HaaS)
Infrastracute as a service (IaaS)
Platform as a service (PaaS)
E-business.
&
beta.cloudos.com
eyeos.info & eyeos.mobi (has and an internet browser)
oos.cc
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14. Scalability vs. SSI
In distributed computing, a single system image cluster is a cluster of machines that appears to be one single system. [1] [2] The interest in SSI clusters is based on the perception that they may be simpler to use and administer than more specialized clusters. Different SSI systems may provide a more or less complete illusion of a single system.
As a result, all NUMA (non-uniform Memory Access) computers sold to the market use special-purpose hardware to maintain cache coherence and thus class as "cache-coherent NUMA", or ccNUMA.
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15. Real-Time Systems
Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.
Well-defined fixed-time constraints.
Hard real-time system. Deadline support
Secondary storage limited or absent, data stored in short-term memory, or read-only memory (ROM)
Conflicts with time-sharing systems, not supported by general-purpose operating systems.
Soft real-time system. No deadline support
Limited utility in industrial control or robotics
Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.
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16. Operating System Services
Program execution – system capability to load a program into memory and to run it.
I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.
File-system manipulation – program capability to read, write, create, and delete files.
Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.
Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.
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17. System Programs
System programs provide a convenient environment for program development and execution. The can be divided into:
File manipulation
Status information
File modification
Programming language support
Program loading and execution
Communications
Application programs
Most users’ view of the operation system is defined by system programs, not the actual system calls.
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18. System Calls
System calls provide the interface between a running program and the operating system.
Generally available as assembly-language instructions.
Languages defined to replace assembly language for systems programming allow system calls to be made directly
Three general methods are used to pass parameters between a running program and the operating system.
Pass parameters in registers.
Store the parameters in a table in memory, and the table address is passed as a parameter in a register.
Push (store) the parameters onto the stack by the program, and pop off the stack by operating system.
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19. System Calls
Provide "direct access" to operating system services (e.g., file system, I/O routines, memory allocate & free routines) by user programs.
System calls execute instructions that control the resources of the computer system, e.g., I/O instructions for devices.
We want such privileged instructions to be executed only by a system routine, under the control of the OS!
As we will see, system calls are special, and in fact, are treated as a special case of interrupts.
Programs that make system calls were traditionally called "system programs" and were traditionally implemented in assembly language.
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20. System Call Scenario (cont.)
File system
User program
I/O devices
memory
Operating System
User program is confined!
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21. Dual-Mode Operation
Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.
Provide hardware support to differentiate between at least two modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also supervisor mode or system mode) – execution done on behalf of operating system.
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22. Dual-Mode Operation
Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.
Provide hardware support to differentiate between at least two modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also supervisor mode or system mode) – execution done on behalf of operating system.
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23. Dual-Mode Operation (Cont.)
Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1).
Part of EFLAG in x86 architecture
Part of PSW in Motorola architecture
When an interrupt or fault occurs hardware switches to monitor mode.
Interrupt/fault
monitor
user
set user mode
Privileged instructions can be issued only in monitor mode.
All I/O instructions are privileged instructions.
Must ensure that a user program could never gain control of the computer in monitor mode (I.e., a user program that, as part of its execution, traps if it executes a privileged instruction).
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24. API vs. System Calls
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?
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25. 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
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26. API – System Call – OS Relationship
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27. Standard C Library Example
C program invoking printf() library call, which calls write() system call
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28. 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 callers 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)
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29. System Calls (cont.)
Now, system calls can be made from high-level languages, such as C and Modula-2 (to a degree).
Unix has about 32 system calls: read(), write(), open(), close(), fork(), …
using trap instructions:
i = read(fd,80, buffer)
push buffer
push 80
push fd
trap read
pop i
Each system call had a particular number. Instruction set has a special instruction for making system calls:
SVC (IBM 360/370)
trap (PDP 11)
tw (PowerPC) - trap word
tcc (Sparc)
break (MIPS)
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30. User vs. System Mode case i-call System (or kernel) memory
“trap” to O.S.
l :
n :
code for read
User Program
(text)
trap n
Special mode-bit set in PSW register:
mode-bit = 0 => user program executing
mode-bit = 1 => system routine executing
Privileged instructions possible only when mode-bit = 1!
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31. System Call Scenario User program executing (mode-bit = 0)
User makes a system call
hardware sets mode-bit to 1
system saves state of user process
branch to case statement in system code
branch to code for system routine based on system call number
copy parameters from user stack
execute system call (using privileged instructions)
restore state of user program
hardware resets mode-bit
return to user process
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