Transcript CSCI1412 - Introduction & Overview

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Operating Systems (CSCI2413)
Lecture 2
Overview
Operating-System Roles
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Resource management
Operating System Services
System Calls
Kernel
Virtual Memory
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Resource Management
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Process Management
Memory Management
File Management
I/O Management
Secondary Management
Protection
Command-Interpreter
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User
Application Program
O.S.
Process
Management
CPU
User Interface
Memory
Management
File
Management
Main
Memory
I/O
Management
File Store
I/O
Devices
Hardware Resources
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Process Management
• A process is a program in execution.
• A process requires
the CPU time, memory, files, and I/O
devices, to accomplish its task.
• OS is responsible for:
– Process creation and deletion.
– process suspension and resumption.
– Provision of mechanisms for:
• process synchronization
• process communication
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Memory Management
• Memory is a repository of quickly
accessible data shared by the CPU and
I/O devices.
• OS is responsible for:
– Keeping track of which parts of memory are
currently being used and by whom.
– Deciding which processes to load when
memory space becomes available.
– Allocating and de-allocating memory space
as needed.
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File Management
• A files represent programs (both source and
object codes) and data.
• OS is responsible for:
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File creation and deletion.
Directory creation and deletion.
Manipulating files and directories.
Mapping files onto secondary storage.
File backup on stable (nonvolatile) storage media.
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I/O Management
• The I/O system consists of:
– A buffer-caching system
– A general device-driver interface
– Drivers for specific hardware devices
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Secondary-Storage Management
• Generally, disks are used for both
programs and data.
• OS is responsible for:
– Free space management
– Storage allocation
– Disk scheduling
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Protection
• A mechanism for controlling access by
programs, processes, or users to both
system and user resources.
• Protection must:
– Distinguish between authorised and unauthorised
usage
– Specify the controls to be imposed.
– Provide a means of enforcing the control.
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Command Interpreter
• Commands are given to the OS by control
statements which mainly deal with:
– Resource management, as explained above
• The program that reads and interprets
control statements is variously called:
– Command-line-interpreter
– Shell (in UNIX).
• Its function is to get and execute the next
command.
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Services
• Program Execution – load a program into memory and
run it
• I/O operations – OS is responsible for performing I/O
operations.
• File Manipulation – capability to create, read, write, and
delete files.
• Communications – Exchange of information between
processes executing either on the same computer or on
a distant computer connected via network.
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Services (cont..)
• Error detection – detecting errors in
the CPU and memory, in I/O devices,
or in user program
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System Calls
• An interface between a running program and the
operating system.
• In another word, the means by which a process
requests a specific kernel service.
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User
Processes
Kernel
Response to many system calls from processes
Traps and
Virtual
Interrupts
faults
memory
CPU
Main
Memory
Network
Disk,
terminal
Network
interface
Hardware Resources
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Virtual Memory
• Virtual memory enables a computer to appear to
have more memory than its real physical memory
– the technique is employed by all modern OS’s
• All computers have more hard disk space than
real physical memory (RAM)
– hard disk space is used as ‘imaginary’ RAM
• When a process starts, rather than loading the
whole program into memory, the supervisor loads
instructions and data into RAM in small blocks
– these memory blocks are called pages
• modern page sizes are typically 2K or 4K bytes
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Virtual Memory Illustration
hard disk
program 3
program 2
program 1
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main memory
main memory
process 3 (page 1)
process 3 (page 1)
process 2
process 3 (page 2)
process 1
process 1
supervisor task
supervisor task
there is not enough free space in
memory for all of program 3 so,
initially, only page 1 is loaded in
(note process 1 is still writing to
hard disk in the background)
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process 3, page 2 is now
required, so process 2 has
been swapped out to hard
disk and the required page
has been loaded into memory
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Swapping
• When physical memory (RAM) becomes full the
supervisor searches for unused pages
– the unused page(s) (instructions or data pages from an
inactive process) are copied out to hard disk
– the required page is then loaded into RAM
• this is called swapping
• If a swapped-out page is required again another
page is copied out and the needed one copied in
– sometimes the OS can spend so much time swapping
pages that little or no useful processing is done
• this is called disk thrashing
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Virtual Machines
• The virtual machine concept takes virtual memory
one stage further
– a real machine simulates a number of virtual machines
– each virtual machine has its own operating system
(supervisor task) in its own block of virtual memory
– all machine interaction (input / output) takes place via
an underlying virtual machine controller task
• Intel processors have provided hardware support
for virtual machines since 486’s
– VMWare is a virtual machine program for PC’s
– allows e.g. Win 95, Win NT and Linux to run at once
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Virtual Machine Illustration
main memory
user process 3
supervisor task 3 (e.g. Linux)
user process 2
supervisor task 2 (e.g. Win NT)
user process 1
supervisor task 1 (e.g. Win 95)
virtual machine task
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