Backward compatibility and changes.
  On current 64-bit OS systems, compatibility is implemented via
    emulatation - virtual 8086 mode.

Most signficant changes.

Protected memory. Code no longer has direct access to system hardware.

   Early updates allowed CPU to toggle between legacy and protected mode.
   Requires emulation to run older code.

Change from segmented to linear address.

   Requires emulation to run older code.

Wider data-path. 
   
   Less disruptive than other updates but still requires emulation
     for very old software.

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   C programming language.
     Developed 1969-73
     Most core C instructions written in assembler.
       Most of the more advanced features written in C which means 
       C is self compiling once the core is built.
  
   Introduction of the C language for main coding language for application
     made recompiling code for new architecture much more affordable.