code: function bit (cpu, b, n) {
      const r = n & (1 << b);
  
      cpu.f &= ~0xe0;
      if (r == 0) cpu.f |= FLAG_Z;
      cpu.f |= FLAG_H;
  
      return r;
  }
docs: Bit Opcodes 
  BIT b,r
  
  Description:
  Test bit b in register r.
  
  Use with:
  b = 0 - 7, r = A, B, C, D, E, H, L, (HL)
  
  Flags affected:
  Z - Set if bit b of register r is 0.
  N - Reset.
  H - Set.
  C - Not affected.
  
  Opcodes:
  Instruction    Parameters    Opcode    Cycles
  BIT            b,A           CB 47     8
  BIT            b,B           CB 40     8
  BIT            b,C           CB 41     8
  BIT            b,D           CB 42     8
  BIT            b,E           CB 43     8
  BIT            b,H           CB 44     8
  BIT            b,L           CB 45     8
  BIT            b,(HL)        CB 46     16