code: function dec (cpu, n) {
      const r = n - 1;
  
      cpu.f &= ~0xe0;
      if ((r & 0xff) == 0) cpu.f |= FLAG_Z;
      cpu.f |= FLAG_N;
      if ((n & 0xf) == 0) cpu.f |= FLAG_H;
  
      return r;
  }
docs: DEC n
  
  Description:
  Decrement register n.
  
  Use with:
  n = A, B, C, D, E, H, L, (HL)
  
  Flags affected:
  Z - Set if reselt is zero.
  N - Set.
  H - Set if no borrow from bit 4.
  C - Not affected.
  
  Opcodes:
  Instruction    Parameters    Opcode    Cycles
  DEC            A             3D        4
  DEC            B             05        4
  DEC            C             0D        4
  DEC            D             15        4
  DEC            E             1D        4
  DEC            H             25        4
  DEC            L             2D        4
  DEC            (HL)          35        12