System architectural views.  
  (modeled on ISO/OSI 1977  
   - international standards organization/open systems interconnection )
  a. Physical level (Digital logic). - L0
  b. Micro-architectural level. - L1
  c. Instruction set architecture. - L2
  d. Low-level programming. - L3
  e. Operating system level. - L4
  f. High-level programming. - L5
  g. Application level - L6
  h. Application level programming - L7

Physical level - L0
  [CSCI 463]
  Base level

  Voltage levels, signal timing and clock speeds, path distances.

  Physical materials.

  Binary, analog, multiple discrete levels (Flash memory 4 value per cell)

  Basic (atomic) logic units called gates:
    Bit level memory.

    Simple decision circuits (NAND, NOR, NOT).

  Simple functional circuits:
    Standard memory units - registers, byte or word sized memory cells.

    Simple computational units - adder circuit, comparator, shift multiplier.

Micro-architectural or control level - L1
  [CSCI 463, CSCI 480]
  Functional units arranged from physical material.

  Memory units assigned specific function:
    Memory address registers, memory data registers, accumulators,
     cache, general purpose registers.

  Computation units:
    Arithmetic Logic Unit (ALU).
      Group adder, multiplier, comparator together.

      SIMD - single instruction, multiple data

      * Floats often done by separate core or GPU (graphical processor unit).

    Instruction Decoder - logic to select calculating function.
      Hardwired or programmable decoding and execution.

      Instruction may be decoded either.
        By a hardwired decoder.
          RISC : simpler logic makes for easy implementation.
               + faster execution. 
               - complex actions performed at software level.

        By a programmable decoder (micro-interpreter).
          CISC : used on more complex CPUs. 
               + define complex instructions at hardware level.
               + allows repair or upgrade.
               - not all gates in use for particular instruction.
               - slower than hard-coded design.

          Modern Pentium/AMD chips are at least partially micro-programmable.

    Memory Management Unit.