Networking and the Internet 5

1
Networking and the Internet (5)

• Last Week
• CPU design and Performance (Coope ch.6, Willis
  ch.9-8)
• Memory Virtual and Real (Coope, chapter 4)
• HTML Tables and Practical work
• Week 5 Focus
• Compilers and linkers interpreters (Willis,
  ch.7, sect 7.7)
• Interrupts and status processor modes (Coope,
  chapter 9)
• Module assignment
• Frames HTML to display multiple pages at once
• HTML Practical

2
Programming Languages

• In the beginning
• machine code(1948 to around 1950)
• Then Assemblers developed
• translate 11 to machine code
• Processor-dependent
• As complex as the processor (at least)
• Still in use today
• High-level compiled languages (from 1956)
• Processor-independent
• FORTRAN, Cobol, ALGOL, Pascal, C, PL/I

• Interpreted languages (from 1960s)
• Translated as program runs
• Basic, APL, REXX
• Report Generators (from 1970)
• Also called 4GL 4th generation
• Based on definitions of input and output
  ......not procedures to transform
• Object-oriented
• Aims to model real world, defining methods for
  objects
• Visual Basic, C, Java

3
Producing Machine Code

• What runs in the CPU is a series of numeric codes
  in memory machine code
• Weve already seen those (BS2911w1.doc)
• Problem is that humans are lousy with numbers
• So we represent them with mnemonics like LOAD
  ADD
• Also use symbols to represent data
  addressesDefinitions A, B, C DS
  INT4 Executable code LOAD A ADD B STORE C
• Assemblers produce machine code in two passes
• First determines which symbols and labels
  existin this case, it has to reserve three words
  for integer data, perhaps A at 00 0F 0C, B at 00
  0F 10 and C at 00 0F 14
• Second pass fills in the addresses for the symbols

4
Assembler Example

• Lets write a program to addC A B
• Assume the following codes
• 01 is written as LOAD
• 02 written as STORE
• 03 written as ADD
• 04 written as SUB and so on
• So we program as LOAD A ADD B STORE C(more
  code would go in here)A DS INT4B DS INT4C DS I
  NT4

• First pass, we translate codes and prepare data
  storage (1-byte code, 3-byte addresses)
• Lets start from cell 00 0C 00C00 01 C01 needs
  AC04 03 C05 needs BC08 02 C09 needs C(assume
  more code up to F0C holds AF10 holds BF14 hold
  s C
• Now we can resolve the symbols to machine
  codeC00 01 00 0F 0CC04 03 00 0F 10C08 02 00 0F
  14
• The processor can run this

5
High Level Languages

• Writing assembler still needs you to think about
  how machine works
• 11 correspondence of mnemonic to code
• plus macros mnemonics that produce several
  codes
• Better if we can write in something closer to
  English
• Machine independent
• Calculations written like algebraic formulae
  CAB
• Choices written as structures IF xx THEN yy
  ELSE zz
• Loops handled in structured way WHILE xx DO
  END
• One formula generates many machine instructions
• Process of converting to machine code is
  Compilation
• Slow to compile, but MUCH faster to write

6
Running a program

• Compiling high-level language needs more than 2
  passes
• Declarations, internal subprogram calls, syntax
• Then compiler needs to do what the Assembler does
• Object code produced still has unresolved
  references such as starting points of external
  subprograms
• These must be resolved before pointing CPU at the
  code
• In real life, machine code runs under OS control
• Program calls built-in and OS subprograms
• Location is determined by OS at run-time
• So Assembler cant produce absolute addresses
• Most computers have hardware features to assist
• OS sets a Base Register to hold the starting
  address, and compiled program contains offsets
  from that

7
Running the Compiled Program

• Before running, machine code has to be loaded
  into memory
• As object module is read, the loader does two
  things
• relocates the module by adjusting addresses to
  take account of starting point
• makes a list of unresolved references names
  used but not defined inside the module
• It then tries to resolve references using
  entry-points in known modules (OS, function
  libraries, etc.)
• where to look may be a defined in a LIBPATH
• in Windows, main resolution is through DLLs
• All this takes time, so we may try to do it once,
  creating a load module with a Linkage Editor or
  Builder

8
Compile or Interpret?

• Compilers typically generate efficient code, but
• Change cycle is long Edit/Compile/Link/Load/Run
• Each compilation takes time and resource
• Source code isnt usually available for run-time
  debugging
• Wouldnt it be nice to compile on the fly?
• Interpreters do this, for example APL, REXX,
  Basic
• Generally limited ability to look-ahead
• Undeclared variables get typed by what goes
  into them
• Each statement is read and converted to machine
  code when its needed
• Some interpreters redo this every time a
  statement is run
• Excellent diagnostics available if code fails,
  source can be displayed with values of variables
  in use

9
Processing without Interruption
Clock
Arithmetic and Logic Unit
Central Processor Unit
1 Load 2 Store 3 Add 4 Subtract 5
Multiply
Memory C00 1 1 F0C C04 1 2 F10 C08 3 1 2 C0C 2 1
F14 .. .
Processing happens one instruction after another
C00, C04
...
10
Interrupts

• Interrupts are a hardware facility for
  controlling software
• All current processors depend heavily on
  interrupts
• Essentially, work by swapping the Instruction
  Counter
• Interrupts can be caused by many things
• I/O hardware ready (device end after moving data)
• External event (such as timer or keyboard)
• Addressing exception (DAT cant give real
  address)
• Program check (e.g. because instruction unknown)
• this is a neat way to invoke software emulation
  of an instruction not present on this particular
  model of CPU
• Supervisor call (e.g. task asks OS for a service)
• Interrupts are the basis of much task switching,
  as well as detecting errors that the OS must
  deal with

11
Interrupt handling

• Lets assume we have finished our time-slice, and
  the timer has just signalled the fact and
  interrupted us
• Interrupt hardware will save current IC value in
  (say) loc 40
• Then set IC to the address given in (say)
  location 60
• CPU then executes interrupt handler code at this
  address
• First it has to save state vector of interrupted
  task
• registers, privilege level (e.g. supervisor
  state)
• then do whats needed to service the
  interruptprobably jump into the scheduler to
  pick next ready task
• IBM S/370 maintains state information in PSW
• Hardware swaps whole Program Status Word, not
  just the instruction counter
• Each interrupt class loads its own new PSW

12
Processor modes

• Clearly we could lose our return address if an
  interrupt handler is interrupted (by the same
  class of interrupt)
• So we need to Mask against this class
• and usually against lower-priority interrupts as
  well
• This mask is another component of the S/370 PSW
• defines one of the processors modes
• Other processor modes (examples again for S/370)
• Supervisor state Needed to run privileged
  instructions
• Allows OS to do things not permitted to
  applications
• Certain instructions need current PSW in
  Supervisor state
• If its not, you get a Program interrupt
• Storage key lets you protect your code and data
  from being overwritten by another task

13
Finishing Survey of HTML

• Last week Creating Tables
• Tables allow you to present tabular data without
  worrying about the size of the viewers window
• Dividing a window into Frames
• Frames let you present more than one WWW page at
  a time

14
Web Site development Assignment

• Web Site means a collection of related
  pages,put together in a logical way
• Development means you go through a process
• Decide what your site is to do
• Make a high-level design what the pieces are
  and how they should fit together
• Design the individual pages and their linkages
• Code up the content of the pages
• Please dont use a tool like Front Page
• It doesnt simplify the task of understanding and
  designing
• Does make it easy to produce visually-interesting
  pages
• Doesnt give you understanding of HTML
• And you lose marks

15
Marking Scheme

• Two thirds for quality of web-site, including
• Success in achieving what you said you set out to
  do
• Usability, including ease of navigation between
  pages
• Accuracy minimal impact from errors
• One third for report showing evidence of
  understanding and logical development
• Clear statement of scope what is and isnt to
  be included
• Review of what you discovered in research
  development
• Reflection on how you developed your site
• Both aspects will be affected by how ambitious
  youve been, but you can usually do better with a
  simpler site you get right than aiming for the
  sky and falling short

16
Netscape Frames

• Typical Example of how HTML has grown
• Netscape extended HTML to address a navigation
  issue
• Other browsers climbed on to the bandwagon
• HTML standards committee added the facility
• Idea is that we can attach pages within logical
  frames inside our page
• Gives common look and feel to site
• Makes navigation information permanently
  available
• Lets you include alien material in your website
• BUT
• Burns up valuable screen real-estate

17
Frames

• Frames are a useful way to organize content of
  screen
• You can have a fixed bit, perhaps for navigation
• and what the user does there can control what
  appears in another frame
• E.g. we might have choice of what appears in the
  RH pane
• Normal way to organize modern web-sites
• Details on the web at http// sharkysoft.com/tutor
  ials/frames/
• Summary in your notes

• Basic structure is a page that defines
• how it is split into frames,
• hyperlinks to the resources that go into each
  frame
• Awkward shapes can be produced by binary chopping
• Each frame can be named
• You can control which named frame gets updated
  with next resource
• or even create a new browser window
• Best way forward is to try it

18
Learning about Frames

• As usual, the best thing is to try it with a
  simple example
• Download frames.zip from Week 5 of the Learning
  Network
• Extract everything in it into a folder on your
  machine
• Edit the file framed.html using Notepad
• Can you work out what its meant to do?
• You may need to look inside the components it
  links to
• Open framed.html
• Does it behave as you expected?
• What happens when you resize the window?
• Try to understand exactly whats happening

19
Frames Exercise

• Create a page called xxx.htm (you choose the xxx)
• Divide it into three frames
• Split the page vertically into two equal parts
• Left initially displays a trivial page you create
  call xxxL.htm
• Split the Right-hand half horizontally 7030
• Top 70 of the RH, displays a trivial page called
  xxxTR.htm
• Bottom 30 of the RH is for navigation store as
  xxxBR.htm
• In Bottom-right frame
• Provide hyperlinks to load target pages into the
  left-hand and top-right frames
• And heavier-duty hyperlinks to replace whole page
• Associate graphics with each hyperlink, e.g.
  arrows
• You can base all future frame-sets on this!

20
HTML Summary

• HTML is a pretty small language
• Youve seen most of it already Frames was all
  thats left
• Easy to pick up structure from a good site on
  the web,then introduce your own content
• Writing your own HTML puts you in control
• How pictures and text are related
• What happens when you update the page
• WYSIWYG tools are getting better, but still leave
  you guessing and often generate wasteful HTML
• A good HTML tutorial can be found
  athttp//www.w3schools.com/html/html_intro.asp

21
Assignment Workshop

• Chance to check understanding of
• Document structure lthtmlgt ltheadgt lt/headgt ltbodygt
• Basic HTML lthngt ltpgt ltulgt ltligt etc
• Hyperlinking lta hrefgt lta namegt
• Tables lttablegt lttrgt lttdgt
• Handling graphics ltimg srcgt
• Frames ltframesetgt ltframegt
• Attributes used inside a tag
• Width, height not both in an ltimggt tag!
• href dont forget http// in remote links
• border1
• Try to avoid direct reference to presentation
• Font, bold, italic

22
The Mechanics of Writing HTML

• Need at least two windows open
• A browser to test your page in e.g. Internet
  Explorer
• An editor to update it in Notepad is fine
• Internet Explorer View source opens file in
  Notepad
• Can then modify the file, Save it,
• Then check the effects in IE
• Reality is that most pages are based on existing
  ones
• Get file into Notepad
• Save As with new name (make sure extension is
  right )
• Modify the content as you wish including the
  Title
• You never have to bother typing ltHTMLgt ltHEADgt etc

23
Reminder Creating Hyperlinks

• Any hyperlink reference needs to say what it is
  linking to
• In HTML, we use an Anchor tag with the HREF
  attribute
• ltA href"http//www.pdq.edu/freds.htm"gtQuick
  Univlt/Agt
• ltA href"freds.htm"gtLocal referencelt/Agt
• You can reference any resource on the Internet
  Just give its URL
• Any lack of specificity means look locally, so
  missing out the domain means it is in the local
  file system
• Always leave out as much as you can
• Makes it much easier to move the site
• Can also use relative references, e.g. to a
  sub-folder ltA href./subdir/fred2.htm"gtrelative
  referencelt/Agt

24
Some HTML Primers on the Web

• The original NCSA primer has been removed, but
  theres a printable archive on our web-site
• Two good primers are available on-line at
• http//www.w3schools.com/html/html_intro.asp
• http//www.web-nation.com/lessons/html-pri.htm
• This is Tony Drewrys clear exposition of HTML
  tagshttp//www.cems.uwe.ac.uk/tdrewry/html/index
  .htm
• A very wide-ranging site, covering everything
  from simple HTML to XML and databases
  http//www.w3schools.com/
• Any more favourites?

25
HTML Table Exercise

• Starting with the zip file of samples from Week 4
  of the web-site (http//www.winchester.ac.uk/bm/
  )
• Make a copy of the source on My Documents,
  preferably with a new name (samples\XXXtable.htm)
• Start Windows Explorer and get your file visible
  on it
• Open it with the NOTEPAD Accessory (right-click)
• Open it with Internet Explorer (double-click)
• Open Firefox and Netscape as well if you have
  them
• You can drag the file from Explorer into each
  browser
• Has anything gone wrong?
• Make changes in NOTEPAD, Save, and look at the
  effect in both the browsers (dont forget to
  Refresh/Reload)