Lecture 3: OS Functions and Design Approaches

1
Lecture 3 OS Functions and Design Approaches

• OS duties
• process management
• memory management
• disk/file management
• protection security
• interaction with OS
• dual-mode operation
• system calls
• API, system programs, UI
• OS design approaches
• monolithic kernel
• microkernel
• virtual machine

2
Process Management

• OS manages many kinds of activities
• user programs
• system programs printer spoolers, name servers,
  file servers, etc.
• a running program is called a process
• a process includes the complete execution context
  (code, data, PC, registers, OS resources in use,
  etc.)
• a process is not a program
• program - a sequence of instructions (passive)
• process - one instance of a program in execution
  (active)
• many processes can be running the same program
  and one program may cause to create multiple
  processes
• from OS viewpoint process is a unit of work OS
  must
• create, delete, suspend, resume, and schedule
  processes
• support inter-process communication and
  synchronization, handle deadlocks

3
Memory Management

• primary (main) memory (RAM)
• provides direct access storage for CPU
• processes must be in main memory to execute
• OS must
• mechanics
• keep track of memory in use
• keep track of unused (free) memory
• protect memory space
• allocate, deallocate space for processes
• swap processes memory ? disk
• policies
• decide when to load each process into memory
• decide how much memory space to allocate to each
  process
• decide when a process should be removed from
  memory

4
Disk Management

• the size of the disk is much greater than main
  memory and, unlike main memory, disk is
  persistent (endures system failures and power
  outages)
• OS hides peculiarities of disk usage by managing
  disk space at low level
• keeps track of used spaces
• keeps track of unused (free) space
• keeps track of bad blocks
• OS handles low-level disk functions, such as
• schedules of disk operations
• and head movement

5
File Management

• disks provide long-term storage, but are awkward
  to use directly
• file - a logical named persistent collection of
  data maintained by OS
• file system - a logical structure that that is
  maintained by OS to simplify file manipulation
  usually directory based
• OS must
• create and delete files and directories
• manipulate files and directories - read, write,
  extend, rename, copy, protect
• provide general higher-level services - backups,
  accounting, quotas
• note the difference between disk management and
  file system management

6
Protection Security

• protection any mechanism for controlling access
  of processes or users to resources
• disk, memory, CPU,
• security defense of the system against internal
  and external attacks
• systems generally first distinguish among users,
  to determine who can do what
• user identities and privileges associated with
  this identifier
• user ID then associated with all files, processes
  of that user to determine access control

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Dual-Mode Operation

• to allow protection OS operates in dual-mode
• user mode and kernel mode
• mode bit provided by hardware
• user (1), kernel (0)
• enables OS to distinguish when system is running
  user code or kernel code
• some instructions designated as privileged, only
  executable in kernel mode
• changing modes
• modifying timers
• modifying interrupt service routines
• I/O device access

8
System Call Definition

• app. program can ask the OS to carry out service
  forit by invoking a system call
• to the application the invocation is similar to
  an ordinary function call
• example Unix write system call description

prompt man -S 2 write WRITE(2)
Linux Programmer's Manual
WRITE(2) NAME write - write to a file
descriptor SYNOPSIS include ltunistd.hgt
ssize_t write(int fd, const void buf,
size_t count) DESCRIPTION write()
writes up to count bytes to the file referenced
by the file descriptor fd from the buffer
starting at buf. RETURN VALUE On
success, the number of bytes written are returned

9
Mode Switch

• mode switch transition from user to kernel mode
• system call generates a trap (software
  generatedinterrupt)
• physical interrupt occurs (e.g. a timer
  interrupt)
• control is transferred to the interrupt service
  routine, which may pass control to the OS (in
  kernel mode) return it back to the user process
• in case OS needs to do extensive work, it needs
  to save the context (state) of the user program

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Function Invocation TraceAcross Modes

• Example function invocation in Solaris (Unix-like
  OS from Sun Microsystems)
• node the mode (User or Kernel) for function
  invocation

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System Call Example

• a typical app. program invokes system calls
  repeatedly
• example copying a file

12
Application Program Interface

• raw system calls tend to be difficult to use
• application program interface (API) defines
  functions that are more convenient to use
• API functions
• can be invoked by app. programmer
• run in user mode
• directly invoke system calls
• implemented in system libraries that come with
  the OS and are linked to the app. program
• API examples POSIX API (implemented by most Unix
  systems, MacOS X), Java API, Win 32 API windows
• ex printf (C function that prints formatted
  output, part of POSIX API) repeatedly invokes
  write

13
System Programs and UIs

• system programs come with the OS
• they are designed for end users the personfor
  whom the computer/OS/app. programs are designed
• cf. application programmers, OS programmers
• operate in user mode
• typical tasks file manipulation, status info,
  file modification (editors), programming language
  support (compilers/assemblers), configuration,
  communication, etc.
• user interface (UI) is a way the end-user
  interacts with system programs (and through them
  with the OS),
• command-line interface (CLI) interaction
  through command interpreter (shell) a
  text-based system program
• graphical-user interface (GUI) mouse-heavy with
  a lot of graphics
• most popular OSes support both kinds of UI

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Monolithic Kernel OS Design
critical OS data structures and device registers
are protected from user programs

• advantages speed and ease of operation
  (everything is at hand)
• disadvantages
• hard to develop, maintain, modify and debug
• kernel gets bigger as the OS develops

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Microkernel

• small kernel implements communication (usually
  messages)
• when system services are required microkernell
  calls other parts of OS running in user modes and
  passes the request there

• advantages reliability, ease of development,
  modularity - parts can be replaced and tailored
  to the architecture, user requirements etc.
• disadvantages slow?
• examples MacOS X, Windows XP

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Virtual Machine

• OSs system callsare considered an enhancement
  ofhardwaresinstruction set
• extend further virtual machine
• each user task is provided with an abstract
  (virtual machine) which OS hardware implement
• IBM pioneered
• Java VM modern example
• adv. portability at binary-level, security,
  greater language flexibility
• dis. speed(?)

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Java Virtual Machine (JVM)

• Java source code is translated into an
  architectureindependent java bytecode
• bytecode is executed by JVM
• JVM can be implemented purely in software or in
  hardware
• JVM verifies bytecodes correctness and then
  either interprets (translates the code into
  machines instructions one by one)
• or just-in-time (JIT) compiles to optimize, or
  both
• adv. portability at binary-level, security,
  greater language flexibility
• dis. speed(?)

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Lecture Review

• major OS duties are
• process management
• memory management
• disk/file management
• OS interacts with users through system calls, to
  ease interaction
• API - for app. programmers
• GUI or CLI for end-users
• OS is a big and complex program traditional
  monolithic kernel design approach yields OSes
  that are fast but hard to develop, modify and
  debug other approaches have been suggested
• microkernel
• virtual machine

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Future network computers?

• JavaOS
• no disk
• OS is only capable of running Java Virtual
  Machine
• all the programs (including OS components) are
  downloaded over the network
• advantages ease of administration and rollout
  (installation), platform independence
• disadvantages ??

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Monolithic Kernel OS Design
critical OS data structures and device registers
are protected from user programs can use
privileged instructions

• advantages speed and ease of operation
  (everything is at hand)
• disadvantages
• hard to develop, maintain, modify and debug
• kernel gets bigger as the OS develops

21
Layered Design

• divide OS into layers
• each layer uses services provided by next lower
  layer yet the implementation of these services
  are hidden from the upper layer
• THE Operating system layer structure
• user programs
• buffering for input and output devices
• operator-console device driver
• memory management
• CPU scheduling
• hardware
• advantages easier development and implementation
• disadvantages not always easy to break down on
  layers, slower (each level adds overhead)
• ex CPU scheduler is lower than virtual memory
  driver (driver may need to wait for I/O) yet the
  scheduler may have more info than can fit in
  memory
• examples THE, OS/2