Design of a two pass assembler

Design of a two pass assembler

• 1.
  SYSTEM PROGRAMMING ALA VARIANTS OFASSEMBLERS, DESIGN OF TWO PASS ASSEMBLER Dhananjaysinh Jhala 170410107027 TY CE 1 Batch B
• 2.
  DESIGN OF 2PASS ASSEMBLER • Tasks performed by the passes of a two pass assembler are: • Pass 1: • Separate the symbol, mnemonic opcode and operand fields • Build the symbol table • Perform LC processing • Construct intermediate representation • Pass 2: • Synthesize the target program
• 3.
  DESIGN OF 2PASS ASSEMBLER •Pass 1 uses the given data structures: • OPTAB - table of mnemonic opcodes, class (IS,DL,AD) and mnemonic info • SYMTAB - symbol table • LITTAB - table of literals used in the program • POOLTAB - table of information concerning literal pools
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  PROGRAM EXAMPLE
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  ALGORITHM FOR PASS1
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  INTERMEDIATE CODE FORMS •IC Consists of a sequence of IC units having 3 fields : • Variant forms of intermediate codes, specifically the operand and address fields are used. Information in mnemonic code field has the same representation in all variants.
• 9.
  VARIANT 1 • Firstoperand is represented by a single digit number which is either code used for register or condition code itself. • Second operand is a memory operand represented by a pair of the form (operand class, code) where operand class is C,S or L(constant, symbol or literal).
• 10.
  Note that forvariant 1, 2 kinds of entries exist in the symbol table at any time: (for defined symbol) and (for forward references) i.e. We need to enter A in the symbol table, suppose at entry number 'n', but at this point address and length fields of A are not known.
• 11.
  VARIANT 2 • Itdiffers from variant one because here the operand field of intermediate code maybe in the processed form or in the source form itself. For an AD/DL, operand has to be in the processes form to support LC processing. • Symbolic references in the operand field of intermediate statements are put in the source form itself.