UNIX

1
UNIX
1 History of UNIX 2 Overview of UNIX 3
Processes in UNIX 4 Memory management in UNIX 5
The UNIX file system 6 Input/output in UNIX
2
History of UNIX

• MULTICS and UNICS (Thompson)
• PDP-11 UNIX (B C, Richie Tompson)
• Portable UNIX
• Berkeley UNIX
• Standard UNIX
• Linux (Linus Torvald)

3
UNIX Goals

• An interactive system designed to handle multiple
  pro-cesses and multiple (sophisticated) users at
  the same time
• Group of experienced users ! personal computer
  model
• Experienced users want
• simplicity, elegancy, consistency
• power and flexibility
• avoid useless redundancy

4
Interfaces to UNIX
UserInterface

• The layers of a UNIX system.

5
UNIX Shell (F?????)

• Shell a command line interface ! X Windows
• Faster to use, more powerful, easily extensible
• When the shell starts up initializes it self,
  types a prompt character. User types a command
  line. Shell extract first word -gt program to
  execute. Shell is just another program
• Commands may take arguments, e.g. cp a1.txt
  b1.txt
• Other aspects flags, wildcards e.g. 2 , /?
• Standard input, output, error. Redirection ( lt,
  gt).
• e.g. sort lt in gt tmp head 30 lt temp rm temp
• Output of a program is input to another. Pipes
  ()
• e.g. sort lt in head 30
• Multiple commands e.g. sort lt in gt tmp . Shell
  scripts.

6
UNIX Utility Programs

• A few of the more common UNIX utility programs
  required by POSIX

7
Processes in UNIX

• Processes are created using the fork system call
• Parent and child processes have their own,
  private memory images. Open files are shared
  between the two processes
• How do the processes know which one should run
  the parent code and which one should run the
  child code?

8
POSIX

• Processes communicate with each other
• using a form of message passing (pipes) and with
  signals

• The signals required by POSIX.

9
System Calls for Process Management

• s is an error code
• pid is a process ID
• residual is the remaining time from the previous
  alarm

10
POSIX Shell

• A highly simplified shell

11
UNIX Processes Implementation

• Two data structures process table and user
  structure
• Process table (keeps information for all
  processes)
• Scheduling parameters (process priority, amount
  of CPU time consumed recently, sleeping time
  recently)
• Memory image (pointers to text, data and stack
  segments)
• Signals (masks showing treatment of signals)
• Miscellaneous (process state, event being waited
  for, PID, etc.)
• User structure (only if process in memory and
  runnable)
• Machine registers
• System call state
• File descriptor table (used to locate the
  i-nodes)
• Accounting (CPU used so far)

12
The ls Command

• Steps in executing the command ls type to the
  shell

13
UNIX Scheduler

• Designed to provide good response to interactive
  processes
• Two-level algorithm
• Low-level picks the process to run next from the
  runnable ones
• High level moves processes between memory and
  disk
• Low level algorithm
• Uses multiple queues, each with a range of
  priorities values
• Processes in user mode have positive values,
  kernel mode negative
• A process run for a quantum (usually 100msec) or
  until it blocks and then it is put on the end of
  the queue
• Every second priority CPU_usage nice base

14
UNIX Scheduler

• The UNIX scheduler is based on a multilevel queue
  structure

15
Booting UNIX
The boot program located in the MBR runs first
and loads the O.S.
cp

• The sequences of processes used to boot some
  systems

16
Handling Memory
Process A
Process B

• Process A's virtual address space (text,data and
  stack)
• Physical memory
• Process B's virtual address space (text, data and
  stack)

17
System Calls for Memory Management

• s is an error code
• b and addr are memory addresses
• len is a length
• prot controls protection
• flags are miscellaneous bits
• fd is a file descriptor
• offset is a file offset

18
Implementation of Paging in UNIX

• Earlier UNIX systems used swapping (low-level
  scheduling)

• On-demand
• No prepaging
• Page daemon
• OS, core map and page frames

The core map has an entry for each page
19
Page Replacement in UNIX

• When a page fault occurs and the there is no
  available page, process is suspended until page
  daemon has freed some.
• Page replacement algorithm is executed by page
  daemon every 250 msec it checks if free pages gt
  lotsfree parameter
• If not, it transfers pages to disk so free pages
  gt lotsfree
• Modified version of the clock algorithm global
  replacement
• Two-hand clock algorithm the page daemon
  maintains two pointers into the core map. When it
  runs, it first clears the usage bit at the front
  end and then checks the usage bit at the back
  end, after which it advances both hands.

20
The UNIX File System

• Some important directories found in most UNIX
  systems

21
The UNIX File System

• Before linking.
• After linking.

(a) Before linking. (b) After linking
22
The UNIX File System

• Separate file systems
• After mounting

(a)
(b)
(a) Before mounting. (b) After mounting
23
Locking Files

• (a) File with one lock
• (b) Addition of a second lock
• (c) A third lock
• Exclusive and Shared Locks similar to read and
  write locks

24
System Calls for File Management

• s is an error code
• fd is a file descriptor
• position is a file offset

25
The lstat System Call

• Fields returned by the lstat system call.

26
System Calls for Directory Management

• s is an error code
• dir identifies a directory stream
• dirent is a directory entry

27
UNIX File System

• Disk layout in classical UNIX systems

28
UNIX File System

• Directory entry fields.

Structure of the i-node
29
UNIX File System

• Example n read (fd, buffer, nbytes)
• When kernel gets control, all it has is just 3
  parameters. It has to locate the i-node. Also
  store the file position.
• One of the internal tables file descriptor array
• One possibility have a pointer to the i-node at
  the file descriptor and keep file position in the
  i-node. FAILS (?)
• Another possibility keep file position in the
  file descriptor array. What happens if a child
  process modifies the file?
• Introduce a new table, the open file description
  between the file descriptor table and the i-node
  table.

30
UNIX File System

• The relation between the file descriptor
  table, the open file description

31
UNIX I/O System

• All I/O devices are made to look like files and
  are accessed with the same read/write system
  calls.
• UNIX integrates I/O devices to the file system as
  special files. E.g. /dev/hd1 (hard disk),
  /dev/lp1 (printer), /dev/net.
• Programs can read/write special files the same
  way with regular files. E.g. cp account.txt
  /dev/lp1
• Special files
• block sequence of numbered blocks (e.g. disks)
• character input or output character streams
  (e.g. keyboard, printer)

32
Networking

• Sockets are analogous to mailboxes or wall
  sockets and are used for networking
• Sockets are created (returns a file descriptor)
  and destroyed dynamically
• Each socket supports
• reliable connection-oriented byte stream
  (equivalent of a pipe between 2 processes)
• reliable connection-oriented packet stream
  (preserves packet boundaries)
• unreliable packet transmission (access to the raw
  network no guarantees)
• Before a socket can be used, it must have an
  address

33
Terminal Management

• The main POSIX calls for managing the terminal

34
UNIX I/O

• Some of the fields of a typical cdevsw table

35
UNIX I/O

• The UNIX I/O system in BSD