CS 502 SYSTEMS PROGRAMMING - Simple Two Pass Assembler Pass 2

Simple Two Pass Assembler

Pass 2:
BEGIN
  initialize Scnt, Locctr, Skip, and Errorflag to 0
  write assembler report headings
  WHILE Sourceline[Scnt] is a comment
    BEGIN
      append to assembler report
      increment Scnt
    END {while}
  Breakup Sourceline[Scnt]
  IF Opcode = 'START' THEN
    BEGIN
      convert Operand from hex and save in Locctr
      append to assembler report
      increment Scnt
      Breakup Sourceline[Scnt]
    END
  ENDIF
  format and place the load point on object code array
  format and place ENDval on object code array, index ENDloc
  WHILE Opcode <> 'END'
    BEGIN
      IF Sourceline[Scnt] is not a comment THEN
        BEGIN
          Xsearch Opcodetab for Opcode
          IF found THEN
            DO CASE
              1. Opcode is 'RESW' or 'RESB'
                BEGIN
                  increment Locctr by Storageincr
                  place '!' on object code array
                  replace the value at index ENDloc with loader address
                  format and place Locctr on object code array
                  format and place ENDval on object code array, index ENDloc
                  set Skip to 1
                END
              2. Opcode is 'WORD' or 'BYTE'
                BEGIN
                  increment Locctr by Storageincr
                  Dostorage to get Objline
                  IF error THEN
                    set errors flag in Errors[Scnt]
                  ENDIF
                END
              3. OTHERWISE
                BEGIN
                  increment Locctr by Opcodeincr
                  Doinstruct to get Objline
                  IF error THEN
                    set errors flag in Errors[Scnt]
                  ENDIF
                END
            ENDCASE
          ELSE
            /* directives such as BASE handled here or */
            set errors flag in Errors[Scnt]
          ENDIF
        END
      ENDIF
      append to assembler report
      IF Errors[Scnt] <> 0 THEN
        BEGIN
          set Errorflag to 1
          append error report to assembler report
        END
      ENDIF
      IF Errorflag = 0 and Skip = 0 THEN
        BEGIN
          place Objline on object code array
        END
      ENDIF
      IF Skip = 1 THEN
        set Skip to 0
      ENDIF
      increment Scnt
      Breakup Sourceline[Scnt]
    END {while}
  place '!' on object code array
  IF Errorflag = 0 THEN
    transfer object code array to file
  ENDIF
END {of Pass 2}

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CS 502 SYSTEMS PROGRAMMING - Simple Two Pass Assembler Pass 1

Simple Two Pass Assembler

Pass 1:
BEGIN
  initialize Scnt, Locctr, ENDval, and Errorflag to 0
  WHILE Sourceline[Scnt] is a comment
    BEGIN
      increment Scnt
    END {while}
  Breakup Sourceline[Scnt]
  IF Opcode = 'START' THEN
    BEGIN
      convert Operand from hex and save in Locctr and ENDval
      IF Label not NULL THEN
        Insert (Label, Locctr) into Symtab
      ENDIF
      increment Scnt
      Breakup Sourceline[Scnt]
    END
  ENDIF
  WHILE Opcode <> 'END'
    BEGIN
      IF Sourceline[Scnt] is not a comment THEN
        BEGIN
          IF Label not NULL THEN
            Xsearch Symtab for Label
            IF not found
              Insert (Label, Locctr) into Symtab
            ELSE
              set errors flag in Errors[Scnt]
            ENDIF
          ENDIF
          Xsearch Opcodetab for Opcode
          IF found THEN
            DO CASE
              1. Opcode is 'RESW' or 'RESB'
                BEGIN
                  increment Locctr by Storageincr
                  IF error THEN
                    set errors flag in Errors[Scnt]
                  ENDIF
                END {case 1 (RESW or RESB)}
              2. Opcode is 'WORD' or 'BYTE' THEN
                BEGIN
                  increment Locctr by Storageincr
                  IF error THEN
                    set errors flag in Errors[Scnt]
                  ENDIF
                END {case 2 (WORD or BYTE)}
              3. OTHERWISE
                BEGIN
                  increment Locctr by Opcodeincr
                  IF error THEN
                    set errors flag in Errors[Scnt]
                  ENDIF {case 3 (default)}
                END
            ENDCASE
          ELSE
            /* directives such as BASE handled here or */
            set errors flag in Errors[Scnt]
          ENDIF
        END {IF block}
      ENDIF
    increment Scnt
    Breakup Sourceline[Scnt]
  END {while}
  IF Label not NULL THEN
    Xsearch Symtab for Label
    IF not found
      Insert (Label, Locctr) into Symtab
    ELSE
      set errors flag in Errors[Scnt]
    ENDIF
  ENDIF
  IF Operand not NULL
    Xsearch Symtab for Operand
    IF found
      install in ENDval
    ENDIF
  ENDIF
END {of Pass 1}

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