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Introduction to operating system structure and computer systems

What is an Operating System?

operating system (OS) is the software that acts as an intermediary between user applications and computer hardware

computer system components

hardware: provides basic computing resources (CPU, memory, I/O devices)

operatingsystem: controls and coordinates use of hardware among application programs and I/O for devices

applicationprograms: solve computing problems of users


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users: people, machines and other computers

operating system roles

referee: resource allocation, isolation of different users and applications, communication between users and applications

illusionist: each application appears to have entire machine to itself

glue: reduces cost of developing software with libraries and user interface widgets

example: file systems

referee: prevent users from accessing each other’s files without permission

illusionist: files can grow arbitrarily large

glue: named directories

operating system challenges: reliability, availability, security, privacy, performance, portability

key idea: device files as the boundary

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I/O sharing implemented in operating system kernel

Operating Systems History

early operating systems: bare machine (1950s), expensive hardware and cheap human

structure: large machines run from console, single user system with programmer/user as operator and punched cards

secure execution and inefficient use of expensive resources (low CPU utilization and high setup time)

simple batch systems (1960s)

reduce setup time by batching jobs with similar requirements and hire an operator (user is NOT the operator)

automatic job sequencing with rudimentary OS

solutions to speed up I/O: offline processing

spooling (simultaneous peripheral operation on-line)

introduces job pool allowing OS to choose next job to run as to increase CPU utilization

direct memory access (DMA)

data moved directly between I/O devices and memory thus CPU can work on other tasks

used for high speed I/O devices to transmit information at close to memory speeds

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I/O completion by polling: device sets a flag when it is busy and program tests the flag in a loop waiting for completion of I/O

interrupts: on completion of I/O, device forces CPU to jump to specific instruction address with ISR

after interrupt processed, CPU returns to code it was executing prior to servicing interrupt

multi-programmed batch systems

use interrupt to run multiple programs simultaneously

when program performs I/O, instead of polling, execute another program till interrupt is received

requires secure memory and I/O for each program, intervention if program indefinite loops and CPU scheduling to choose

next job to run

time-sharing systems ( cheaper hardware and more expensive human )

programs queued for execution in FIFO order and timer device interrupts after quantum (timeslice)

interrupted program returned to end of FIFO and next program taken from head of FIFO

control card interpreter replaced by command language interpreter

interactive (action/response), file systems required for users to access data and code, and virtual memory

personal and mobile computer systems ( cheap hardware and expensive human )

portable, single user systems with I/O devices and NO need for advanced CPU utilization or protection features

advantages of user convenience, responsiveness and ubiquitous

Computer System and Operating System Structure

computer system organization

execution sequence: fetch instruction at PC, decode, execute, write results to registers/memory, increase PC and repeat

I/O devices and CPU execute concurrently

each device controller is in charge of a particular device type with local buffer

CPU moves data from/to main memory to/from local buffers and device controller interrupts CPU on completion of I/O

interrupt: transfers control to interrupt service routine (segments of code that determine action to be taken for interrupt)

polling: same interrupt handler called for all interrupts thus polls all devices to figure out reason for interrupt

interrupt vector table: different interrupt handlers will be executed for different interrupts

OS preserves state of CPU by storing registers and program counter

incoming interrupts disabled while another interrupt being processed to prevent a lost interrupt

types of I/O processing

synchronous I/O: after I/O requested, wait until I/O is done thus program will be idle

asynchronous I/O: after I/O requested, control returns to user program without waiting for I/O completion

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limit process rights by hardware protection

dual mode operation: provide hardware support to differentiate between at least two modes of operation

user mode (execution done on behalf of user) and kernel mode (execution done on behalf of OS)

“privileged” instructions only executable in kernel mode, otherwise in user mode would trap into kernel mode (interrupt)

trap (exception): software generated interrupt caused by error or user request (system call)

mode bit added to hardware to indicate current mode, kernel = 0 and user = 1

(when interrupt OR trap occurs, switch to kernel mode)

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memory protection: provided for interrupt vector and interrupt service routines

when process is running, only memory in that process address space must be accessible BUT kernel has unrestricted access

to ALL memory when executing in kernel mode

to provide memory protection, add two registers that determine range of legal addresses a program may address

base register holds smallest legal physical memory address and limit register contains size of the range

virtually addressed base and bounds

(memory outside defined range is protected)

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CPU protection: prevent a process from executing indefinitely

timer: interrupts computer after specified period to ensure that OS maintains control, decremented every clock tick

when timer reaches value of 0, an interrupt occurs

commonly used to implement time sharing and to compute current time (load timer is privileged instruction)

I/O protection (ALL I/O instructions are privileged instructions)

must ensure that user program could never gain control of computer in kernel mode

user perform I/O via system calls, method used by process to request action by OS

process: an instance of a program, running with limited rights

thread: a sequence of instructions within a process, potentially many threads per process

address space: set of rights of a process and memory that the process can access with permissions process has

kernel: computer program that is core of computer’s OS, with complete control over everything in the system

handles I/O requests from software and translate them into data-processing instructions for central-processing unit (CPU)

connects application software to hardware of computer and performs tasks in protected memory

isolation of memory access

address space = set of accessible addresses + state associated with them (for 32-bit processor, 232 = 4 billion addresses )

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system call

user code can issue a system call which causes a trap to be handled by kernel (ex: read(), write(), open() …)

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interface between running program and OS: assembly language instruction and direct system calls by high level language

passing parameters between running program and OS via register, memory tables or stack

system services/programs: convenient environment for program development and execution and defines user view of OS

ex: command interpreter (parses/execute other system programs), file manipulation (copy/print), file modification (edit) …

command interpreter system: commands that are given to OS via executable command statements

ex: process creation and deletion, I/O handling, file system access …

programs that read and interpret control statements and obtains next command and executes it

ex: control card interpreter, command-line interpreter and shell

system design and implementation

establish design goals (user/system) and software engineering (separate mechanism from policy, what vs. how)

choose high-level implementation language (faster implementation, compact and easier to debug)

system generation

OS written for class of machines thus must be configured for each specific site

process to customize OS for exact hardware: customizable source code, pre-compiled modules and table to choose execution

system boot

bootstrap program: stored in ROM, loader program loads kernel which loads rest of OS

storage architecture

main memory: only large storage media that the CPU can access directly

secondary storage: extension of main memory that has large nonvolatile storage capacity

magnetic disk: rigid metal-glass platters covered with magnetic recording material

disk surface logically divided into tracks/sectors & disk controller determines logical interaction betw. device & computer

storage systems organized in hierarchy based on speed, cost and volatility

caching: process of copying information into faster storage system (main memory = fast cache for secondary storage)

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operating systems organization

simple (1-2 levels of code) and layered (lower levels independent of upper levels)

modular (core kernel with dynamically loadable modules) and microkernel (OS built from many user-level processes)

MS-DOS: provides a lot of functionality in little space, NOT divided into modules,

interfaces and levels of functionality NOT well separated

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original unix system structure: limited structuring with two separable parts

systems programs and kernel (below system call interface and above physical hardware)

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layered os structure: OS divided into number of layers (bottom is hardware and highest is user interface)

each layer uses functions and services of only lower-level layers

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modules-based structure: most modern os implement modules where each core component is separte, each talks to other over known interfaces and each is loadable as needed within the kernel

similar to layers BUT more flexibility

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monolithic vs. microkernel os

mono have large kernels with lots of components (Linux, Windows, Mac)

whereas micro moves as much from kernel into “user” space with small core OS running at kernel level

communication between modules with message passing

benefits: easier to extend a microkernel and to port OS to new architecture, more reliable (less code running in kernel mode)

fault isolation (parts of kernel protected from other parts) and more secure

detriments: performance overhead severe from naïve implementation

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OS tasks managements

process: fundamental concept in OS as instance of a program in execution and needs resources (CPU time, memory, files…)

OS responsible for following process management activities

process creation and deletion, suspension and resumption, synchronization and interprocess communication & interaction

main memory: volatile array of addressable words OR bytes that is quickly accessible

OS responsible for following memory management activities

allocation/deallocation, managing multiple processes within memory and determineing which processes to load when memory becomes available

secondary storage: required for backup as primary form of disk

OS responsible for storage allocation, free-space management and disk scheduling

I/O system consists of buffer caching and management, device driver interface that abstracts device details and drivers for

specific hardware devices

file system management: file = collection of related information defined by creator, represents programs and data

OS responsible for file creation and deletion, directory creation and deleted, mapping files to disks and …

protection and security: protection mechanisms control access of programs and processes to user and system resources

THUS protects user from himself, user from other users and system from users

protection mechanism: distinguish between authorized and unauthorized use

specify access control to be imposed on use

provide mechanism for enforcement of access control

security mechanisms provide trust in system and privacy (authentication, certification…)

networking: connecting processor in distributed system which is collection of processors that do NOT share memory or clock

processor connected via communication network

advantages: allows use and system to exchange information

provide computational speedup

increase reliability and availability of information

virtual machines

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use cases: resource configuration, running multiple OS (either same or different OS) and run existing OS binaries

I/O devices

port: connection point with a device bus: set of wires and common protocol for communication with devices

controller: digital interface to physical device

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system communications

I/O devices and CPU can execute concurrently BUT CPU must be aware of when devices should be interacted with

CPU queries device using polling OR device controller informs CPU that it is ready by causing an interrupt

polling: CPU is used by device drive to continually check device (registers) to see whether device is ready for input or output

( frequent polling = wasted CPU cycles, whereas infrequent polling = buffer overrun and data loss )

interrupts: device controller sets interrupt request line (different lines for different device types)

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interrupt handling: when interrupt triggered, control is transferred to interrupt service routine (ISR) through

interrupt vector which contains address of all service routines

( ISR must save address of interrupt instruction and program state )

for fast synchronous devices (display driver) use polling and for slow asynchronous devices (network driver) use interrupts

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direct memory access (DMA)

devices requiring big amounts of data to be copied from/to device memory to/from main memory

if CPU handles copy, slow and waste cycles

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device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention

( interrupt is generated when transfer is done )


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