Processor
---------

  The GameBoy uses a computer chip similar to an Intel 8080.
 It contains all of the instructions of an 8080 except there
 are no exchange instructions. In many ways the processor is
 more similar to the Zilog Z80 processor. Compared to the
 Z80, some instructions have been added and some have been
 taken away.

 The following are added instructions:

  ADD  SP,nn             ;nn = signed byte
  LDI  (HL),A            ;Write A to (HL) and increment HL
  LDD  (HL),A            ;Write A to (HL) and decrement HL
  LDI  A,(HL)            ;Write (HL) to A and increment HL
  LDD  A,(HL)            ;Write (HL) to A and decrement HL
  LD  A,($FF00+nn)
  LD  A,($FF00+C)
  LD  ($FF00+nn),A
  LD  ($FF00+C),A
  LD  (nnnn),SP
  LD  HL,SP+nn           ;nn = signed byte
  STOP                   ;Stop processor & screen until button press
  SWAP r                 ;Swap high & low nibbles of r

 The following instructions have been removed:

  Any command that uses the IX or IY registers.
  All IN/OUT instructions.
  All exchange instructions.
  All commands prefixed by ED (except remapped RETI).
  All conditional jumps/calls/rets on parity/overflow and sign flag.

 The following instructions have different opcodes:

  LD  A,[nnnn]
  LD  [nnnn],A
  RETI