Chapter3: Language Translation issues Programming language Syntax Key criteria concerning syntax Basic syntactic concepts Overall Program-Subprogram structure Stages in Translation Analysis of the source program Synthesis of the object program Bootstrapping

What is Syntax The syntax of a programming language describes the structure of programs without any consideration of their meaning.

Key criteria concerning syntax Readability – a program is considered readable if the algorithm and data are apparent by inspection. Writeability – ease of writing the program. Verifiability – ability to prove program correctness (very difficult issue) Translatability – ease of translating the program into executable form. Lack of ambiguity – the syntax should provide for ease of avoiding ambiguous structures

Basic syntactic concepts Character set – The alphabet of the language. Several different character sets are used: ASCII, EBCIDIC, Unicode Identifiers – strings of letters of digits usually beginning with a letter Operator Symbols – +-*/ Keywords or Reserved Words – used as a fixed part of the syntax of a statement

Basic syntactic concepts Noise words – optional words inserted into statements to improve readability Comments – used to improve readability and for documentation purposes. Comments are usually enclosed by special markers Blanks – rules vary from language to language. Usually only significant in literal strings

Basic syntactic concepts Delimiters – used to denote the beginning and the end of syntactic constructs Expressions – functions that access data objects in a program and return a value Statements – these are the sentences of the language, they describe a task to be performed

Overall Program-Subprogram Structure Separate subprogram definitions: Separate compilation, linked at load time E.G. C/C++ Separate data definitions: General approach in OOP. Nested subprogram definitions: Subprogram definitions appear as declarations within the main program or other subprograms. E.G. Pascal

Overall Program-Subprogram Structure Separate interface definitions: C/C++ header files Data descriptions separated from executable statements. A centralized data division contains all data declarations. E.G. COBOL Unseparated subprogram definitions: No syntactic distinction between main program statements and subprogram statements. E.G BASIC

Stages in Translation Analysis of the source program Synthesis of the object program Bootstrapping

Analysis of the source program Lexical analysis (scanning) – identifying the tokens of the programming language: keywords, identifiers, constants and other symbols In the program void main() { printf("Hello World\n"); } the tokens are void, main, (, ), {, printf, (, "Hello World\n", ), ;, }

Syntactic and semantic analysis Syntactic analysis (parsing) – determining the structure of the program, as defined by the language grammar. Semantic analysis - assigning meaning to the syntactic structures Example: int variable1; meaning: 4 bytes for variable1 , a specific set of operations to be used with variable1.

Basic semantic tasks The semantic analysis builds the bridge between analysis and synthesis. Basic semantic tasks: Symbol–table maintenance Insertion of implicit information Error detection Macro processing Result : an internal representation, suitable to be used for code optimization and code generation.

Synthesis of the object program Three main steps: Optimization - Removing redundant statements Code generation - generating assembler commands with relative memory addresses for the separate program modules - obtaining the object code of the program. Linking and loading - resolving the addresses - obtaining the executable code of the program.

Optimization example Statements in yellow can be removed Assembler code not optimized: LOAD_R B ADD_R C STORE_R Temp1 LOAD_R Temp1 ADD_R D STORE_R Temp2 LOAD_R Temp2 STORE_R A Intermediate code: Temp1 = B + C Temp2 = Temp1 + D A = Temp2 Statements in yellow can be removed

Bootstrapping The compiler for a given language can be written in the same language. a program that translates some internal representation into assembler code the programmer manually re-writes the compiler into the internal representation, using the algorithm that is encoded into the compiler. From there on the internal representation is translated into assembler and then into machine language.