I. Overview
------------------------------------------
            SYSTEMS SOFTWARE

Helping people run programs


  Human     -----> [ Translator ]
  Readable         /    |
  Input           /     |
  (Prog. text)   /      |
                /       v
               v     Object
           Library   Code
           (Binary)  (Binary)
             \          |
              \         |
               \        v
                \->[ Linker/ Loader ]
                        |
                        |
                        v
                      Process
                        |
                        |
                        v
                    [ OS + Computer ]
                        |
                        |
                        v
                      Computation

------------------------------------------
        How would a debugger fit into this picture?
        What job or jobs does a translator have?
        What jobs would an IDE (like Visual Studio) do?
 A. Compiler vs. Interpreter
------------------------------------------
          RECALL DEFINITIONS

def: An *assembler* translates
     a low-level language that is close
     to machine code into machine code.



def: A *compiler* translates a
     (high-level) language into a form
     that can be (more easily)
     executed by a computer.


def: An *interpreter* executes a
     programming language directly,
     often without translating it
     into object code first



------------------------------------------
        Which of these is most like what a VM does?
        Does the VM you are writing do loading?
        What are some examples of languages with interpreters?
------------------------------------------
            COMPILER PICTURE

  Source -----> [ Lexical Analyzer ]
                        |
                        v
                 Token stream
                        |
                        v
                [     Parser       ]
                        |
                        v
                  Parse Tree (AST)
                   + Symbol Table
                        |
                        v
                 [ Static Analysis ]
                        |
                        v
                  Parse Tree (AST)
                   + Symbol Table
                        |
                        v
                 [ Code Generator ]
                        |
                        v
                   Object Code
                        |
                        v
                 [ Linker/ Loader ]
                        |
                        v
                      Process
                        |
                        v
                 [  OS + Computer ]

------------------------------------------
------------------------------------------
             INTERPRETER PICTURE

  Source -----> [ Lexical Analyzer ]
                        |
                        v
                 Token stream
                        |
                        v
                [     Parser       ]
                        |
                        v
                  Parse Tree (AST)
                   + Symbol Table
                        |
                        v
                [ Interpreter     ]
                        |
                        v
                 [  OS + Computer ]

------------------------------------------
  1. advantages and disadvantages
------------------------------------------
       ADVANTAGES AND DISADVANTAGES

Compiler
  Advantages:









  Disadvantages:





Interpreters
  Advantages:









  Disadvantages:





------------------------------------------
  2. hybrids of compilers and interpreters
------------------------------------------
   HYBRID COMPILER/INTERPRETER PICTURE

  Source -----> [ Lexical Analyzer ]
                        |
                        v
                 Token stream
                        |
                        v
                [     Parser       ]
                        |
                        v
                  Parse Tree (AST)
                   + Symbol Table
                        |
                        v
                 [ Static Analysis ]
                        |
                        v
                  Parse Tree (AST)
                   + Symbol Table
                        |
                        v
                 [ Code Generator ]
                        |
                        v
                      VM Code
                        |
                        v
       /------->[ VM + JIT Compiler ]
       |statistics      |
       |                v
       \--------[  OS + Computer ]

------------------------------------------
------------------------------------------
   HYBRID ADVANTAGES AND DISADVANTAGES

Advantages:









Disadvantages:







------------------------------------------
 B. compiler structure
------------------------------------------
        STANDARD COMPILER ARCHITECTURE
  (Based on Apel's book "Modern Compiler
                         Implementation")

  Source code (text)
         |
         v
  [    Lexer        ]
         |
         v
   Stream of tokens
         |
         v
  [    Parser       ]
         |
         v
        AST -------------\
         |                \
         v                 v
  [ Static Analyzer ] <- Symbol Table
         |                    /
         v                   /
   Intermediate Rep.        /
         |                 /
  [  HL Optimizer   ]     /
         |               /
         v              /
   Intermediate Rep.   /
         |            /
         v           v
  [ Code Generator  ]
         |
         v
    Instruction Rep.
         |
         v
  [  LL Optimizer   ]
         |
         v
    Machine Code

------------------------------------------
  1. tokens
   a. definitions
------------------------------------------
          TOKENS

Represent distinct symbols in the input





    including punctuation and operators







Typically, comments are ignored

Reserved words:





Keywords:


White space delimits identifiers/
    aNumber   vs.  a Number
------------------------------------------
   b. data structures
    i. token types
------------------------------------------
          TOKENS

Defined in *.tab.h file
  produced by bison

Example from the SSM assembler (asm.tab.h)

#ifndef YYTOKENTYPE
# define YYTOKENTYPE
  enum yytokentype
  {
    YYEMPTY = -2,
    YYEOF = 0,        /* "end of file" */
    YYerror = 256,    /* error  */
    YYUNDEF = 257,    /* "invalid token" */
    eolsym = 258,     /* eolsym */
    identsym = 259,   /* identsym  */
    unsignednumsym = 260,
    plussym = 261,     /* "+"  */
    minussym = 262,    /* "-"  */
    commasym = 263,    /* ","  */
    dottextsym = 264,  /* ".text"  */
    dotdatasym = 265,  /* ".data"  */
    dotstacksym = 266, /* ".stack"  */
    dotendsym = 267,   /* ".end"  */
    colonsym = 268,    /* ":"  */
    lbracketsym = 269, /* "["  */
    rbracketsym = 270, /* "]"  */
    equalsym = 271,    /* "="  */
    noopsym = 272,     /* "NOP"  */
    addopsym = 273,    /* "ADD"  */
    subopsym = 274,    /* "SUB"  */
    /* ... */
    wordsym = 318,     /* "WORD"  */
    charsym = 319,     /* "CHAR"  */
    stringsym = 320,   /* "STRING"  */
    charliteralsym = 321,
    stringliteralsym = 322
  };
  typedef enum yytokentype yytoken_kind_t;
#endif


Examples:

    Input      yytokentype
=======================================
    ident      identsym
    34         unsignednumbersym
    +          plussym



------------------------------------------
        What would be the token types for the input
              WORD x = +24
             wordsym identsym equalsym plussym unsignednumsym

        Q: How are reserved words represented?
        How are reserved words represented?
  2. symbols and symbol table
       What part of a compiler should populate the symbol table?
       Why should the that be the tool?
   a. symbols
------------------------------------------
               SYMBOLS

What information should remembered
  for identifiers?








------------------------------------------
        What would be a suitable data structure for a symbol?
   b. symbol table
------------------------------------------
          SYMBOL TABLE

A *symbol table*


Does each scope have its own symbol table?



What operations would a symbol table need?



What data structure would be good?



------------------------------------------