Design of a two pass assembler
- 1. SYSTEM PROGRAMMING ALA VARIANTS OF ASSEMBLERS, DESIGN OF TWO PASS ASSEMBLER Dhananjaysinh Jhala 170410107027 TY CE 1 Batch B
- 2. DESIGN OF 2 PASS 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 2 PASS 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
- 6. ALGORITHM FOR PASS 1
- 8. 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 • First operand 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 for variant 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 • It differs 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.