Instruction set complexity - Two general models

CISC - complex instruction set computer
  Original design - evolved over time.
    Backwards compatibility increases number of instructions.

  Complex very flexible instructions
    Single instruction performs complex or compound instructions.
      6502: LDA (42),Y - load and add value pointed to by 16 bit address 
        retrieved from memory locations 42 and 43 after adding the contents
        of the y index to the retrieved address.

      808x: LOOPNZ here - decrement the CX register. If CX not 0, branch to
        the location 'here'.

      808x, IBM 360: Floating point math.

      IBM 360: ZAP - zero and add packed - Pack commands. Combines register
        indirect addressing, loop counting, autoincrementing addressing, and
        data conversion. 


      Curiously IBM 360 predates CISC/RISC concept 
      But has many RISC properties.
        Large number of GPR.
        Heavy dependence on Register based commands.
        Limited addessing modes (although regiser indirect predominant).
    
  Number of registers very greatly between datapath designs.
    6502 - 1 gpr, 2 index registers.

    8088 - 4 gpr, 4 Index/operand address registers.

    IBM 360 - 16 GPR, 4 double floating point registers.

    Z8 - 255 registers mixed general and special purpose registers.
         4k in newer design.

(Some view the 6502 as bordering on Risc because of its simple design)