Software Language Levels

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10-6
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Software Language Levels

• Machine Language (Binary)
• Assembly Language
• Assembler converts Assembly into machine
• High Level Languages (C, Perl, Shell)
• Compiled C
• Interpreted Perl, Shell

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Compilation Convert Source to Object

• SUM A B
• Compiles to Machine language of
• LDR R1, A
• LDR R2, B
• ADD R1, R1, R2
• STR R1, C

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Some Terms

• Source
• The language program was written in
• Object
• The machine language equivalent of the program
  after compilation
• Compiler
• A software program that translates the source
  code into object code
• Assembler is a special case of compiler where the
  source was written in Assembly language

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Programming Stepsfor Compilation

• Create/Edit source
• Compile source
• Link object modules together
• Test executable
• If errors, Start over
• Stop

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Example Good Morning

• Use text editor to create source file print.c
• includeltstdio.hgt
• int main()
• printf(Good Morning\n)
• return 0

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Compilation process

• Invoke compiler on source program to generate
  machine language equivalent
• Compiler translates source to object
• Saves object output as disk files
• Large Systems may have many source programs
• Each has to be compiled

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Link object modules together

• Combine them together to form executable
• Take multiple object modules
• LINKER then takes object module(s) and creates
  executables for you
• Linker resolves references to other object
  modules
• Handles calls to external libraries
• Creates an executable

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Interpretation

• No linking
• No object code generated
• Source statements executed line by line

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Steps in interpretation

• Read a source line
• Parse line
• Do what the line says
• Allocate space for variables
• Execute arithmetic opts etc..
• Go to back to step 1
• Similar to instruction cycle

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Example Good Morning

• Perl script print.pl
• print Good Morning\n
• Shell script print.sh
• echo Good Morning\n

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Compilation Advantages

• Faster Execution
• Single file to execute
• Compiler can do better diagnosis of syntax and
  semantic errors, since it has more info than an
  interpreter (Interpreter only sees one line at a
  time)
• Compiler can optimize code

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Compilation Disadvantages

• Harder to debug
• Takes longer to change source code, recompile and
  relink

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Interpreter Advantages

• Easier to debug
• Faster development time

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Interpreter disadvantages

• Slower execution times
• No optimization
• Need all of source code available
• Source code larger than executable for large
  systems

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Some Interpretive Systems

• Languages
• BASIC, Perl, Lisp
• OS Interfaces
• C Shell, Bourne Shell
• WINEXEC

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Conclusion

• Many different ways of developing systems
• There is always a tradeoff between system
  development time, testing, debugging and
  performance, support etc.