code: function swap (cpu, n) {
      const r = n << 4 | n >> 4;
  
      cpu.f = 0;
      if ((r & 0xff) == 0) cpu.f |= FLAG_Z;
  
      return r;
  }
docs: Miscellaneous 
  SWAP n
  
  Description:
  Swap upper & lower nibles of n.
  
  Use with:
  n = A, B, C, D, E, H, L, (HL)
  
  Flags affected:
  Z - Set if result is zero.
  N - Reset.
  H - Reset.
  C - Reset.
  
  Opcodes:
  Instruction    Parameters    Opcode    Cycles
  SWAP           A             CB 37     8
  SWAP           B             CB 30     8
  SWAP           C             CB 31     8
  SWAP           D             CB 32     8
  SWAP           E             CB 33     8
  SWAP           H             CB 34     8
  SWAP           L             CB 35     8
  SWAP           (HL)          CB 36     16