OPERATING SYSTEMS

1
OPERATING SYSTEMS

• BU01

2
Main tasks of Operating System

• To hide HW specifics (abstract layer for
  programs)
• Processes maintenance
• Memory maintenance
• Files maintenance
• I/O maintenance (peripheral devices)
• Network maintenance
• Permissions and access
• User Graphics Interface

3
Processes

• Process - executed program containing sequence of
  instructions realizing some algorithms. Each
  process consumes some computer resources
• time of processor
• part of operating memory
• some peripheral devices
• disk space
• etc.
• Operating system is responsible for following
  manipulating with process
• Creating (loading) and running of process
• Terminating and deleting of process
• Suspending and resuming of process

4
States of processes

• new
• process is loaded into memory, creating a new
  record in table of processes
• run
• processor executes instructions of process
• wait
• processor does not executes instructions of
  process, process is waiting for some event (e.g.
  finishing of I/O device operation)
• ready
• process is loaded and waiting for starting
• terminated
• process has been finished but not deleted yet
• Operating system provides to the process
  requested services (access to the devices,
  increases amount of the memory for process,
  arranges communication between processes etc.)
• Process description, process attributes, and
  state are stored in the table of processes PCB
  Process Control Block.

5
Properties of process

• Important attributes of process
• Unique Identifier PID (process ID). Integer
  number depends on the history.
• Permissions of process. Each process runs with
  permissions of some user. Full permissions has
  the user "administrator, "system", resp. "root".
• Priority of process. According to this level the
  resources are assigned to the process in
  comparison with another ones.
• Name of process. Derived from the name of the
  file that process was loaded from.
• OS can evaluate CPU and memory consumption of
  running process.
• OS can also evaluate number of bytes sent by
  process to/from particular I/O device.
• Process can create subprocess with attributes
  inherited from parent process. (Parental child
  process).
• OS consists of system processes (they run in
  privileged and normal regime).

6
Preemptive and cooperative architecture

• Preemptive OS provides to processes only limited
  time interval of processor. After this interval
  (some ms) is process changed into the
  "wait-status" and another process in the queue is
  activated. During the suspension of process OS
  has to store all information allowing the
  process to be resumed ( great overhead, but
  enables multitasking) .
• Various OS use various algorithms for processes
  planning and processor assigning (process
  scheduling). Process priority and OS requests
  must be evaluated. More processor kernels
  complicates this evaluating. Main goal using the
  processor in the best wait with minimal overhead
  of process maintenance.
• Cooperative OS assigns the processor to the
  process (jump instruction), but the process must
  return the processor to the OS in its own
  overhead (jump instruction to entry point of OS).

7
Threads

• Process is container of threads. Each thread runs
  relatively independent and inherits from process
  its resources and permissions.
• Each process contains one thread at least.
  Instructions performed by processor in fact
  belongs rather to the thread than process.
• Processes are isolated and can communicate only
  through services of operating system. Unlike
  processes, threads of one process can communicate
  in direct way by sharing of memory space.
• Threads can also create, terminate or suspend
  another threads of the same process. In PC with
  many processors or multikernel processor each
  kernel is assigned to various thread
  automatically by OS.
• Useful technique is to create particular thread
  for resources demand computation to protect
  another activity (especially GUI) from blocking.
• E.g. For each request to the web server there is
  one independent thread created to handle the
  request and send a response.

8
Memory maintenance

• Instructions can contain addresses of data or
  another instruction (jumps). There is a problem
  of address adjusting without knowledge of future
  location of process in operating memory.
• Solution during programming process the
  programmers use logical (virtual) addresses. Real
  (physical) address consists of logical address
  adding the content of special relocation register
  of processor. This register is set up by OS for
  each process to secure their own memory.
• Difference between logical and physical address
  schematically
• Processor obtains jump instruction to address 123
• Contain of relocation register 80000
• Resulting address in the memory where the jump
  will be realized to, is the sum 80123
• While content of relocation register is constant
  during the process existence each
  address-oriented instructions will work correctly
  despite of random placing the process.
• Memory fragmentation problem

9
Memory paging

• To protect the memory from fragmentation paging
  mechanisms is used
• Physical memory is divided into the frames of
  the same size (4 KB).
• Logical memory is divided by the same way into
  the pages of the same size.
• Before starting the process OS analyzes needed
  number of frames. The table of pages is created
  addresses of free frames are assigned to each
  logical page.
• There is continuous logical space for each
  process, each address in instructions is
  relocated into the addresses in the space of free
  frames. Physical memory space where process is
  placed can be discontinuous.
• Maximal unused space for process is 4 KB.
• This mechanisms of memory paging is supported by
  modern processors starting Intel 80386
• Process cannot use frames outside its space
  guaranteed by processor
• Manipulating of relocation registry content is
  evidently dangerous. That is why these
  instructions are performed only for OS not for
  standard processes. Process of OS runs in
  privileged regime.

10
Virtual memory

• Through the mechanisms of physical and logical
  pages OS can provide to the process the amount of
  memory greater than its physical limit.
• Rarely used pages can by removed from the memory
  to the special disk file (swapfile). Released
  space can be used by the new process (swapping).
• Table of pages has the flag for each page
  indicating that the page is in memory space (and
  can be used) or the pages is swapped and must be
  resumed before using. In that case the page is
  reloaded into the free frame and the page table
  is repaired.
• There is a great number of algorithms to minimize
  transport of pages between memory and disk.
• In case of running too many process
  simultaneously the swapping process is active and
  the system performance goes down. On the other
  hand this situation is better than crash. In any
  case we can some of the unused process terminate.

11
Filesystem System of files

• Information stored on peripheral memory is
  organized as system of files (logical units of
  data or programs).
• Structure of this system of files is different
  according to
• Operating system
• Type of peripheral memory
• File is identified by its name and included into
  the hierarchy of folders (directories)
• Access to the File System (moving the file
  into/from the system memory) is allowed only
  through the services of operating system.
• Operating system contain abstract layer hiding
  specific organization and file structure. Files
  can be stored not only on magnetic disks, as well
  as other devices (flash drive, CD, DVD, magnetic
  tape, file system on another computer across a
  network, etc.).
• Programs working with files can use the same
  operating system services (so-called logical or
  Virtual file system).

12
Access layers to filesystem

• At the lowest level are custom disk controller
  drivers that transmit data between disks and
  memory.
• Subroutines of the next layer use the services of
  drivers to allow reading and writing the
  required sectors (Basic File System).
• Another layer already contains information about
  the general distribution of occupied sectors and
  their belonging to the files (File Organization
  Module).
• The last layer, which is then used by user
  programs (so called application layer), organizes
  files into directories, assigns file attributes
  (owner, permissions, time of creation, etc.) and
  allows programs own reading and writing files
  (Logical File System).
• The first three layers are usually unavailable
  for normal programs.

13
Filesystems Metadata

• Information of the distribution of files on
  individual sectors of the disk, and their
  attributes are also stored on the disk in the
  system tables, called metadata.
• When working with a file an application layer
  creates entry in the metadata - a special data
  structure describing the file to disk (FCB - File
  Control Block).
• FCB contains the file attributes (owner, access
  permissions, size, creation time, etc.) and a
  list of sectors in which the file is located.
• Layer hierarchy of operating system for file
  system ensures that when changing the contents of
  the file structure FCB is changed synchronously
  (delete, transfer of data, create a new text
  file, etc.)
• Because of acceleration is a copy of FCB stored
  in the memory and only after the closing of the
  file is copied to the disk.
• Missing file closing can lead to the file
  damaging.

14
Allocation unit

• FAT file system disk is divided into allocation
  units (clusters) consisting of the constant
  number of consecutive sectors. Metadata contains
  the FAT (File Allocation Table) - list of cluster
  numbers containing the file (concatenated list).
• For each file is required one cluster at least.
  If the file does not fit exactly into a number of
  allocation units, the capacity of the disk
  remains unused.
• The numbers in the table allow addressing FAT
  clusters according to their range (e.g. the
  numbers stored in 16 bits (FAT16) can address up
  to 216 65 636 clusters)
• The minimum possible size of the cluster grows
  proportional to the total capacity of the disk.
  Larger clusters, however, always lead to worse
  effective capacity.
• One of the methods to introduce smaller clusters
  on large drives the division of the total disk
  capacity in the area (partitions) that behave as
  independent disks (they contain their own tables
  FAT).
• Another method is based on larger numbers
  addressing clusters (FAT32 can contain about
  4,000,000,000 clusters).
• Disk fragmentation.