Preliminaries

The Translation Process - Getting from Source code to Execution

• Source is translated to machine code
• Machine code is then executed on machine

• Source code is fed to a software program (interpreter) and is directly executed.
• The interpreter is a software simulator of a machine whose machine language is the high-level language

The above are the two extremes with all sorts of variations in between

• Massage source removing unnecessary whitespace and comments and then interpret
• Compile the source to assembler
• Compile the source to some other high-level language
• Compile the source to some for of intermediate language
• Compile the source to some other interpreted machine language

• Hardware computer- actual computer
• Software simulator - virtual computer

Preprocessing

• textual replacement of macros
• Can provide
  • a manifest constant facility
  • a file inclusion facility
  • conditional compilation
• Often (usually?) separate from the language proper

Linking

• 'Editing' of separately compiled modules
• Provides
  • Ability to create code modules
  • Library facilities

Program Structure as Related to Translation and Execution

Lexical Structure of a Program

The rules regarding the words and symbols of a program (think of it like spelling). The words and symbols are known as tokens.

• Identifiers: names of various entities (things) in the program
  • variables
  • function names
  • keywords: identifiers reserved for use by the language: if, while, int
• Symbols: non-alphabetic sequences:
  • Operators: symbols that accept one or operands and produce a result: +, &&, --
  • punctuation: ;, ., "
• literals:
  • value that represents itself
  • value that occurs in a program that cannot be changed
  14, "Hello", 9.08, 'A'

• "Hello unterminated literal
• 12GB invalid token
• @ invalid (unknown) token

Syntactic Structure of a Program

The rules regarding the 'sentences' and 'paragraphs' of the program (think of it like grammar).

• Construct: a grammatical phrase (think of sentences, paragraphs, clauses): if construct function construct, expression

• if a == b) printf("Hi"); missing '(' -- syntax error
• else printf("Hi"); 'else' without 'if'
• a = c + d missing ';' -- syntax error

Semantics of a Program

The rules regarding valid 'meaning' a program (whether the program 'makes sense').

• Types must be correct
• Declaration appears before usage

• Type incompatibility:

  int x;
  char *s;
  ...
  x = s;
  		

• Declaration error

  			x = 3;
  			int x;
  		

Logic of a Program

What's correct/incorrect is up to the programmer.

/* This program reads in a number and prints the number and its square */

main() {
	int x;
	scanf("%d", &n);
	printf("The square of %d is %d\n"
	

Binding and Binding Times

We would like to be able to speak of the point at which a program element is bound (i.e., associated with) to a particular characteristic or property.

Binding time is the point at which this association (or choice) is made. Various Binding Times and Examples

• Execution time
  • Variables to values
  • Variables and data structures to particular storage locations
  Furthermore, execution time may be subdivided:
  • On entry to a function or block
    • Arguments are bound to formal parameters
    • Local variables are bound to their storage locations
  • At arbitrary points of execution
    • At any point of the execution, an assignment binds the lhs variable to the rhs value
• Translation or compile time
  • Variables to types
  • Sizes of structures
  • Some particular categories of variables to values or locations

• Java is compiled then interpreted
• Compilation phase
  • Checks for various lexical/syntax/semantic errors
  • Generates Java byte code
• Interpreter
  • Emulates the Java Virtual Machine (JVM)
  • Executes the byte code on the virtual machine