INTEGRATIVE QUESTION

How would you organize the code
  for an assembler?
  
To write an assembler...
  1. tokenize using lexical analyzer
  2. parser that builds ASTs
  3. Build symbol table for identifiers
     (labels, names of data)
       attributes: type
                   address
  4. translate statements into instructions
      also mnemonics to op codes
           literals (numbers) into binary

           COMPILER CONCEPTS

What is a scope?
   an area of program (its text) where a declaration
   has effect and duplicate declarations are forbidden

What syntactic features start scopes in C?
   Curly braces ('{' and '}'), function declarations,
   files

What starts a scope in PL/0?
   procedures


How is a symbol table used?
  Checking for duplicate declarations or
  for undeclared identifier uses

Is there just one symbol table?
   no (it changes over time)



           CODE GENERATION

What did we use for an IR in HW4?
   enhanced ASTs
      - with labels (to proc_decl)
        identifier ASTs for uses of identifiers
	   with id_use structs and attributes inside
	      (with lexical address info for locations)


What information from a name's use is
   needed to generate code in PL/0?
    - lexical address
       offset and number of static links to follow


In PL/0, why not have the parser determine
   the lexical address of each variable?

    when parsing don't know what declaration
       an identifer refers to
    parser isn't tracking scopes
       so would be hard for it to determine
          # of static links to follow







      CODE GENERATION FOR STATEMENTS

How is the code generator written in C?
   recursive over the ASTs,
   follow the grammar style

What does the C code look like for
   generating code for PL/0 statements?

   switch based on AST type,
     calling more specific statement functions
       for each kind of statement (AST)

What does the C code look like for
   generating code for a PL/0
   while statement?

    generate code for the condition;
    jump conditionally 2 (around a next jump)
    jump around the body (to exit)
    generate code for the body of the loop
    jump back to beginning of condition




 GENERATING CODE FOR IF STATEMENTS

In C there are 2 kinds of if-statements
   with syntax if (Exp) Stmt
   and  if (Exp) Stmt1 else Stmt2
How would these be compiled?
  could have 2 different kinds of ASTs

  code to evaluate expression
  conditional jump 2 (around next jump)
  jump to else part (if any) or around body (if no else)
  code for the then part body
  (jump to after the else part, if there is one)
  code for the else part (if any)

What would the generated code look like
  for a C switch statement?

  evaluate expression,
    put result in register (or on top of stack)
  code like if-then-else

 or
 
  build a jump table, addresses of each part
   evaluate expression
   check that it's in range of jump table,
      if not, jump code for default case
   use the expression as index into table
     jump to the address found there

indirect jumps on OO method calls:
   o.m()

  

          LABELS

Would labels (as in HW4) be useful
  if the VM's ISA required absolute
  addresses for jumps?





How would that work?
  in a first pass determine address
  use a second pass to fix the instruction addresses
    with information from the label