Transcript Abstract View of System Components

Module 1: Introduction
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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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What is an Operating System?
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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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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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Abstract View of System Components
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Multiprogramming and Timesharing
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Multiprogramming: CPU is multiplexed (shared) among a number of jobs -- while
one job waiting for I/O, another can use CPU.
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Advantages:
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Disadvantages:
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CPU is kept busy.
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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Why need multiprogramming? I/O Times vs.
CPU times
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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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Parallel Systems
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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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Parallel Systems (Cont.)
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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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Symmetric Multiprocessing Architecture
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Distributed Systems
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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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Network vs. Distributed OS
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Network OS
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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
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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
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 less autonomy between computers
 gives the impression there is a single operating system controlling the network.
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Research vehicle
Examples: Bell Labs Inferno & Plan 9, Mach, Amobea by Tanenbaum, Chorus by CMU
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Computing Architectures
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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
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Cloud Computing
•http://www.youtube.com/watch?v=8RMWO9JxZjA &
http://www.youtube.com/watch?v=hplXnFUlPmg&feature=fvw
•beta.cloudos.com
•eyeos.info & eyeos.mobi (has and an internet browser)
•oos.cc
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Scalability vs. SSI
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Real-Time Systems
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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 shortterm 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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Operating System Services
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Program execution – system capability to load a program into
memory and to run it.
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I/O operations – since user programs cannot execute I/O
operations directly, the operating system must provide some
means to perform I/O.
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File-system manipulation – program capability to read, write,
create, and delete files.
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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.
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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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System Programs
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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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System Calls
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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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System Calls
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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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System Call Scenario (cont.)
File system
User
program
I/O
devices
memory
Operating System
User program is confined!
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Dual-Mode Operation
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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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Dual-Mode Operation
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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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Dual-Mode Operation (Cont.)
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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
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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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API vs. System Calls
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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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Example of Standard API
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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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API – System Call – OS Relationship
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Standard C Library Example
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C program invoking printf() library call, which calls write() system call
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System Call Implementation
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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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System Calls (cont.)
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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
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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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User vs. System Mode
case
“trap”
to
O.S.
i-call
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System
(or kernel)
memory
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code for read
trap n
User Program
(text)
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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System Call Scenario
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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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