system-software-tools

Chapter 5: System Software Tools
Text Editors(Editing Process & Features)
User Interface
Debugging Systems

Text Editors[1]
• An interactive text editor has become an important part of almost
any computing environment.
• Text editor acts as a primary interface to the computer for all type of
“knowledge workers” as they compose, organize, study, and manipulate
computer-based information.

Text Editors[2]
• A text editor allows you to edit a text file (create, modify etc…)
• Text editors on Windows OS
- Notepad, WordPad, Microsoft Word
• Text editors on UNIX OS
- vi, emacs, jed, pico,

Common Editing Features
• Moving the cursor
• Deleting
• Replacing
• Pasting
• Searching
• Searching and replacing
• Saving and loading
• Miscellaneous (e.g. quitting)

Overview of the Editing Process[1]
• An interactive editor is a computer program that allows a user to create and
revise a target document.
• Document - includes objects such as computer diagrams, text, equations
tables, diagrams, line art, and photographs.
• Here we restrict to text editors, where character strings are the primary
elements of the target text.

• Document-editing process in an interactive user-computer
dialogue has four tasks.
1. Select the part of the target document to be viewed and manipulated.
2. Determine how to format this view on-line and how to display it.
3. Specify and execute operations that modify the target document.
4. Update the view appropriately.

• The above tasks involves traveling, filtering and formatting
• Traveling – locate the area of interest
• Filtering – extracting the relevant subset
• Formatting – visible representation on a display screen
• Editing phase involves – insert, delete, replace, move, copy, cut,
paste, etc…

• Manuscript-oriented editor – characters, words, lines, sentences
and paragraphs
• Program-oriented editors – identifiers, keywords, statements
• User wish – what he/she wants - formatted

User Interface
• The user interface is concerned with:
• The input devices
• Are used to enter elements of text being edited, to enter commands.
• The output devices
• Lets the user view the elements being edited and the results of the editing operations
• The interaction language
• Communication with the editor

Input Devices
• Text Devices – Keyboard
• Button Devices – Special function keys, symbols on the screen
• Locator Devices – Mouse, data tablet
• Voice input devices – Translates spoken words to their textual equivalents

Output Devices
• Teletypewriters - first output devices
• Glass teletypes - Cathode ray tube (CRT) technology
• Advanced CRT terminals
• TFT Monitors
• Printers – Hard-copy

Interaction Language
• Typing oriented or text command oriented – oldest editors, use of
commands, use of function keys, control keys etc.,
• Menu-oriented user interface – menu is a multiple choice set of
text strings or icons. Display area for text is limited. Menus can be
turned on or off.

Text Editors in Windows Environment
• Edit - MS-DOS editor, menu oriented, options are selected with specified
Alphabets
• Notepad - A basic text editor that you can use to create simple documents.
Menu oriented, use of control keys
• WordPad - We can create and edit simple text documents or documents
with complex formatting and graphics, uses menu as well as icons
• Microsoft Word - A sophisticated word processor, menu as well as use of
icons

Text Editors in Unix Environment
• vi - text editor, old, reliable, present on every Unix machine, uses
two modes, command mode, text mode
• emacs - the Extensible, Customizable, Self-Documenting, Real-
time Display Editor

Typical editor structure
Editor Structure[1]

Editor Structure[2]
• Most text editors have a structure similar to shown in the figure regardless of features and
the computers
• Command language Processor
• – accepts command
• – uses semantic routines
• – performs functions such as editing and viewing.

Editor Structure[3]
• The semantic routines involve traveling, editing, viewing and display
functions.
• Editing operations are specified explicitly by the user and display operations
are specified implicitly by the editor
• Traveling and viewing operations may be invoked either explicitly by the
user or implicitly by the editing operations.

Document Editing
• The start of the area to be edited is determined by the current editing
pointer maintained by the editing component
• Editing component is a collection of modules dealing with editing tasks
• Current editing pointer can be set or reset due to next paragraph, next
screen, cut paragraph, paste paragraph etc..,
• When Editing Command is Issued:
• Editing component invokes the editing filter – generates a new editing buffer –
contains part of the document to be edited from current editing pointer
• Filtering and editing may be interleaved, with no explicit editor buffer being created

Document Viewing
• The start of the area to be viewed is determined by the current viewing
pointer maintained by the viewing component
• Viewing component is a collection of modules responsible for determining
the next view
• Current viewing pointer can be set or reset as a result of previous editing
operation

When Display Needs to be Updated…[1]
• Viewing component invokes the viewing filter – generates a new viewing
buffer – contains part of the document to be viewed from current viewing
pointer.
• Line editors – viewing buffer may contain the current line.
• Screen editors - viewing buffer contains a rectangular cutout of the
quarter plane of the text.

• Viewing buffer is then passed to the display component of the editor,
which produces a display by mapping the buffer to a rectangular subset of
the screen – called a window
• The editing and viewing buffers may be identical or may be completely
disjoint
• Identical – user edits the text directly on the screen
• Disjoint – Find and Replace
• There are 150 lines of text, user is in 100th line, decides to change all
occurrences of ‘text editor’ with ‘editor’

• The editing and viewing buffers can also be partially overlap, or one may be
completely contained in the other
• Windows typically cover entire screen or a rectangular portion of it
• May show different portions of the same file or portions of different file
• Inter-file editing operations are possible

• The components of the editor deal with a user document on two levels:
 In main memory and in the disk file system
 Loading an entire document into main memory may be infeasible – only part is
loaded – demand paging is used – uses editor paging routines.
• Documents may not be stored sequentially as a string of characters
 Uses separate editor data structure that allows addition, deletion, and modification
with a minimum of I/O and character movement.

• Editors function in three basic types of computing environments:
1. Time sharing
2. Stand-alone
3. Distributed
• Each type of environment imposes some constraints on the design of an
editor.

• In time sharing environment
 Editor must function swiftly within the context of the load on the computer’s
processor, memory and I/O devices
• In stand-alone environment
 Editors on stand-alone system are built with all the functions to carry out editing and
viewing operations.
 The help of the OS may also be taken to carry out some tasks like demand paging.
• In distributed environment
 Editor has both functions of stand-alone editor, to run independently on each user’s
machine and like a time sharing editor, contend for shared resources such as files.

Interactive Debugging Systems
• Debugging is a methodical process of finding and reducing the number of
bugs, or defects, in a computer program.
• An interactive debugging system provides programmers with facilities that
aid in testing and debugging of programs.
• Here we discuss
 Introducing important functions and capabilities of IDS.
 Relationship of IDS to other parts of the system
 The nature of the user interface for IDS

Debugging Functions and Capabilities
• Debugging system should also provide functions such as tracing and
traceback.
• Traceback can show the path by which the current statement in the program
was reached.
• It can also show which statements have modified a given variable or
parameter.
• The statements are displayed rather than as hexadecimal displacements.

Debugging Process
• Print debugging is the act of watching (live or recorded) trace statements, or
print statements, that indicate the flow of execution of a process.
• In computers, debugging is the process of locating and fixing or bypassing
bugs (errors) in computer program code or the engineering of a hardware
device.
• Remote debugging is the process of debugging a program running on a
system different than the debugger.
• Post-mortem debugging is the act of debugging the core dump of process.

Program-display Capabilities[1]
• A debugger should have good program-display capabilities.
• Program being debugged should be displayed completely with statement numbers.
• The program may be displayed as originally written or with macro expansion.
• Keeping track of any changes made to the programs during the debugging session.
• Support for symbolically displaying or modifying the contents of any of the
variables and constants in the program.
• Resume execution – after these changes.

• The context being used has many different effects on the debugging interaction.
• The statements are different depending on the language
• Example:
• assignment statements
Cobol - MOVE 6.5 TO X
Fortran - X = 6.5
• the condition that X be unequal to Z may be expressed as
Cobol - IF X NOT EQUAL TO Z
Fortran - IF ( X.NE.Z)
C - IF ( X <> Z)

• It is also important that a debugging system be able to deal with optimized
code. Many optimizations like
• Invariant expressions can be removed from loops
• Separate loops can be combined into a single loop
• Redundant expression may be eliminated
• Elimination of unnecessary branch instructions

Relationship with Other Parts of the
System[1]
• The important requirement for an interactive debugger is that it always be
available.
• Must appear as part of the run-time environment and an integral part of the
system.
• When an error is discovered, immediate debugging must be possible.
• The debugger must communicate and cooperate with other operating system
components such as interactive subsystems.

Relationship with Other Parts of the
System[2]
• Debugging is more important at production time than it is at application-
development time.
• When an application fails during a production run, work dependent on that
application stops.
• The debugger must also exist in a way that is consistent with the security and
integrity components of the system.
• The debugger must coordinate its activities with those of existing and future
language compilers and interpreters.

User-interface Criteria[1]
• Debugging systems should be simple in its organization and familiar in its
language, closely reflect common user tasks.
• The user interaction should make use of full-screen displays and windowing-
systems as much as possible.
• With menus and full-screen editors, the user has far less information to enter
and remember.

User-interface Criteria[2]
• There should be complete functional equivalence between commands and
menus – user where unable to use full-screen IDSs may use commands.
• The command language should have a clear, logical and simple syntax;
command formats should be as flexible as possible.
• Any good IDSs should have an on-line HELP facility. HELP should be
accessible from any state of the debugging session.

Dilemma ?
• Since for system software which is machine dependent, there is a need for
real machine. However, most real machines have certain characteristics which
are unusual or unique.
• Very difficult to distinguish the features which are fundamental and those
which are idiosyncrasies of a particular machines.
• In addition, so many machines
• So Hypothetical Machine: SIC, SIC/XE . . .

(SIC and SIC/XE)
Machine Architecture
• Memory
• Registers
• Data Formats
• Instruction Formats
• Addressing modes
• Instruction Set
• Input/Output:

Sections of Major Chapters
• Fundamental functions
• Features depending on machine
• Features which are common and machine-independent
• Major design principles and options
• Such as single pass or multiple passes.
• Implementation

Introduction Summary
• Computer Architecture
• Memory, register, data format, instruction (format and set), addressing mode, I/O
• Sequential, parallel, pipeline, dual or multi core.
• Languages: machine, assembly, high
• System software
• OS, assembler/linker/loader, compiler, editor, …
• SIC and SIC/XE architecture
• Addressing modes
• Relations among different components of computer
• Hardware and software
• Different software

Assembler Summary
• Basic Functions
• Mnemonic  Opcode
• Labels  Addresses
• Extended
• Program relocation
• Different instruction formats, addressing
modes.
• Literals, EQU, Expression, Blocks, Control
Sections. # and = differences.
• Data structures
• Tables: OPTABLE, SYMTAB, LITTAB, ESTAB, … , hash/or
linked list
• Location counter: LOCCTR, PROGADDR, CSADDR, …
• Design Options
• Directives:
• BYTE, WORD, RESB, RESW, BASE, EQU, USE,
LTORG, CSEC, EXTDEF, EXTREF,
• Object records:
• Header, End, Text, Modification, Define, Refer, …
• Relations among source program, (intermediate file),
object code, and object program.
• Relations among assembler, loader, and linker.
• Different Addressing modes

Linker/Loader Summary
• Loader, Linker, Linking loader,
• Linkage editor, simple loader, dynamic linking, absolute loader, bootstrap loader.
• Functions
• Pass 1: Assign addresses to external symbols
• Pass 2: loading, relocation, linking
• Important: how relocation and linking is implemented.
• Data Structures: ESTAB, PROGADDR, CSADDR, EXECADDR
• Library search and linking
• Linking options
• Dynamic linking

Macro Processor Summary
• Basic functions
• Definitions and expansions
• Features
• Labels, nested definitions, recursive invocations.
• Conditional macro processing.
• IF…ENDIF, WHILE…ENDW, macro-time variables and instructions
• Keyword parameters
• Data structures and algorithms
• NAMTAB, DEFTAB, ARGTAB
• For recursive invocation, STACK.
• Relation between macro processors and assemblers

Actual Implementation for Real
Machines
System Software Examples
Assembler MASM assembler for Pentium
SPARC assembler
AIX assembler for PowerPC
Loader & Linker MS-DOS linker
SunOS linkers
Cray MPP linker
Macro Processor MASM Macro Processor
ANSI C Macro Language
ELENA Macro Processor

My Expectation
• Why System Software and System Programming
• Hypothetical Machine
• Assembler
• Loader and Linkers
• Macro Processor
• Others

Have You Met the Course Objectives?
[Self Assessment Questions]
• Do I understand:
• What, why and how of hypothetical machine?
• The fundamental process models of high-level languages ?
• About various functions of assemblers and how to design and implement it ?
• How loaders and linkers are designed and implemented as a system software ?
• How the macro processors designed and programmed ?
• About system software tools like editors, user interface, and editor structure;
• Debugging functions and relationship with user interface criteria.

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