COP 3402 meeting -*- Outline -*-

* Assemblers
       Developed in the 1940s
          Kathleen Britten (in 1947)
            see ref. 7 in chapter 7 of Montagne's book
            
** Overview
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      ASSEMBLY LANGUAGE AND ASSEMBLERS

def: *Assembly language* is a language





def: An *assembler* translates
     from
     to 

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     ... that directly manipulates a (virtual) machine's state
         and in which each statement corresponds
         to one machine instruction

     ... assembly language
     ... machine code

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       ASSEMBLERS VS. COMPILERS

Statements correspond to

 In assembly language:



 In a higher-level programming language:



Expressions correspond to:

 In assembly language:




 In a higher-level programming language:




Names are abstractions of

 In assembly language:





 In a higher-level programming language:






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   ... one machine instruction

   ... several machine instructions

   ... must be explicitly programmed
       with many machine instructions
       and explicit use of temporary storage

   ... are implicitly computed
       with many machine instructions
       and implicit use of temporary storage

   ... of locations (of data)
       and addresses in program space

   ... of variables, constants
         or computations (procedures, functions)

** goals of Assemblers and Assembly Language
        Originally assemblers were called "auto-coders"...
          why?
          what do they automate?

        Q: Having written machine code using numbers,
           why is that hard?
             - Have to remember opcode numbers
             - Have to count for jumps, loads, stores, etc.

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       GOALS OF ASSEMBLERS

- Relieve tedium of machine code by:





- Help communication between people with:






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  ...
  - translating names to locations
  - translating mnemonics to opcodes
  - translating decimal numbers to binary

  ...
  - comments
  - symbolic names (labels)
  - translating decimal (or hex/octal)
    to binary

    Q: Does an assembly language programmer need to know how the
    machine's instructions work?
      Yes!
    Q: Does a programmer in a higher level programming language need
    to know that?
      No, except perhaps to make programs run faster

** How assemblers work
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          BASIC PROBLEM FOR ASSEMBLERS

  ...
         JMP ahead  ; forward reference
  ...
  ahead:            ; label

How can assembler know the address of
the label "ahead"?


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     ... it uses two passes...

*** Two pass design
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       TWO PASS DESIGN OF ASSEMBLER

Pass 1:
   count instructions
   determine address of each label


Pass 2:
   check that all labels are defined
   generate machine code


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   ... enter label's address in symbol table
   ... looking up labels as needed

*** Relative Jumps
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          RELATIVE JUMPS

Additional task for assembler:

  If computer architecture has jumps
   to (absolute) addresses

  Then translate jumps relative to PC into

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    ... jumps to (absolute) addresses

        We did this in the VM, but the assembler could do it also