I. Assemblers
 A. 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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------------------------------------------
       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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 B. goals of Assemblers and Assembly Language
        Having written machine code using numbers,
           why is that hard?
------------------------------------------
       GOALS OF ASSEMBLERS

- Relieve tedium of machine code by:





- Help communication between people with:






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    Does an assembly language programmer need to know how the
    machine's instructions work?
    Does a programmer in a higher level programming language need
    to know that?
 C. 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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  1. 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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  2. 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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II. Executable Files
 A. Sections of Executable Files
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        SECTIONS OF EXECUTABLE FILES

(header: info about each section)

Text Section:


Data Section:


Relocation Section:




(debugging sections:
   symbol table section: global labels
   debugging section: file + line info
------------------------------------------
------------------------------------------
        EXECUTABLE ELF FILE LAYOUT

  |------------------------|
  [ General info           ] (header
  [ Program name           ]  section)
  [ Start address of text  ]
  [ Length of text section ]
  [ Start address of data  ]
  [ Length of data section ]
  [ Start address of reloc.]
  [ Length of reloc. sect. ]
  |------------------------|
  |                        |
  |                        |
  |    Text Section        |
  |                        |
  |                        |  
  |------------------------|
  |                        |
  |                        |
  |    Data Section        |
  |                        |
  |                        |  
  |------------------------|
  |                        |
  |                        |
  |    Relocation Section  |
  |                        |
  |                        |  
  |------------------------|

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 B. Relocation
------------------------------------------
     RELOCATION OF EXECUTABLE FILES

Assemblers generate code assuming
   starting address is 0

Relocation data identifies
  parts of instructions that need offset
  added if the starting address is
    changed to 0 + offset
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III. Linkers
 A. Job of a Linker
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       WHAT A LINKER DOES

Combines object files
   resolving symbolic names
  For example, a program and
 


Object Code P              Executable
 [ header   ]              [ header    ]
 [ text P   ]              [ text P    ]
 [ data P   ] \            [ text L    ]
 [ sym tab P]  \           [ data P    ]
                > Linker ->[ data L    ]
Object Code L   >          [ sym tab P ]
 [ header   ]  /           [ sym tab L ]
 [ text L   ] /
 [ data L   ]
 [ sym tab L]
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    Why not refer to addresses in the library's code?
    Does linking support separate compilation?
    Is linking necessary for execution?
  1. kinds of linking
------------------------------------------
           KINDS OF LINKING

def: *static linking* is linking that
     happens



def: *dynamic linking* is linking that
     happens


Advantages:

  + static linking:




  + dynamic linking:



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     Which is better for computer security?
IV. Loaders
 A. Job of a Loader
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           WHAT A LOADER DOES

A loader places




Types of loaders:

 - Absolute loader:


 - Bootstrap loader:

 
 - Relocating loader:


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     Where does a program loaded by an absolute loader start execution?
     How does a relocating loader work?
     If we have a computer with virtual memory,
         do we still need relocating loaders?