Generate machine language

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Generate machine language

  • Generate machine language

    • Translate mnemonic operation codes to machine code
  • Assign addresses to symbolic labels used by the programmer

Generate an image of what memory must look like for the program to be executed.

  • Generate an image of what memory must look like for the program to be executed.

  • Interpret assembler directives (Pseudo-Instructions)

Input Assembly Code

  • Input Assembly Code

  • Output

    • Assembly Listing
    • Object Code
  • Intermediate files

    • Assembly Listing
    • LOCCTR
    • Instruction Length
    • Error Flags

One Pass: Line by Line

  • One Pass: Line by Line

    • forward reference?
  • Two Passes:

    • Pass 1
    • Pass2
  • Intermediate files

The main problem is forward reference.

  • The main problem is forward reference.

  • Eliminating forward reference

    • Simply ask the programmer to define variables before using them.
  • However, ?!

    • Backward jumps is too restrictive.
    • Forward jumps (Subroutine calls, Loops)

For any symbol that has not yet been defined

  • For any symbol that has not yet been defined

    • 1. omit the address translation
    • 2. insert the symbol into SYMTAB, and mark this symbol undefined
    • 3. the address that refers to the undefined symbol is added to a list of forward references associated with the symbol table entry
    • 4. when the definition for a symbol is encountered, the proper address for the symbol is then inserted into any instructions previous generated according to the forward reference list

At the end of the program

  • At the end of the program

    • any SYMTAB entries that are still marked with * indicate undefined symbols
    • search SYMTAB for the symbol named in the END statement and jump to this location to begin execution
  • The actual starting address must be specified at assembly time

Pass 1

  • Pass 1

    • Validate Opcodes
    • Assign addresses to all statements in the program
    • Scan the source for labels and save their values
    • Perform some processing of assembler directives
      • Determine the length of areas defined by DC, DS
  • Pass 2

    • Translate/assemble the instructions
    • Generate Data Values defined by DC
    • Process the rest of the assembler directives
    • Write the Object Code and Assembly Listing

Operation Code Table (OPTAB)

  • Operation Code Table (OPTAB)

    • Opcode, Instruction format, and length
    • Pass 1: Validate opcodes
    • Pass2: Assemble instructions
  • Symbol Table (SYMTAB)

    • Label name and value, error flags
    • Pass 1: Created!
    • Lookup symbols to insert in assembled instr.
  • Location Counter

    • Initialed to the Org or End

OPTAB is static (access)

  • OPTAB is static (access)

  • SYMTAB (add, access)

    • Insertion and Retrieval efficiency
    • Key: Label Name
      • LOOP1, LOOP2, LOOP3…, A, X, Y, Z…


  • Content

    • The mapping between mnemonic and machine code. Also include the instruction format, available addressing modes, and length information.
  • Characteristic

    • Static table. The content will never change.
  • Implementation

    • Array or hash table. Because the content will never change, we can optimize its search speed.
  • In pass 1, OPTAB is used to look up and validate mnemonics in the source program.

  • In pass 2, OPTAB is used to translate mnemonics to machine instructions.


  • Content

    • Include the label name and value (address) for each label in the source program.
    • Include type and length information (e.g., int64)
    • With flag to indicate errors (e.g., a symbol defined in two places)
  • Characteristic

    • Dynamic table (I.e., symbols may be inserted, deleted, or searched in the table)
  • Implementation

    • Hash table can be used to speed up search
    • Because variable names may be very similar (e.g., LOOP1, LOOP2), the selected hash function must perform well with such non-random keys.

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