code: function sub (cpu, n) {
      const r = cpu.a - n;
  
      cpu.f = FLAG_N;
      if ((r & 0xff) == 0) cpu.f |= FLAG_Z;
      if (((cpu.a ^ n ^ r) & 0x10) != 0) cpu.f |= FLAG_H;
      if ((r & 0x100) != 0) cpu.f |= FLAG_C;
  
      return r;
  }
docs: SUB n
  
  Description:
  Subtract n from A.
  
  Use with:
  n = A, B, C, D, E, H, L, (HL), #
  
  Flags affected:
  Z - Set if result is zero.
  N - Set.
  H - Set if no borrow from bit 4.
  C - Set if no borrow.
  
  Opcodes:
  Instruction    Parameters    Opcode    Cycles
  SUB            A             97        4
  SUB            B             90        4
  SUB            C             91        4
  SUB            D             92        4
  SUB            E             93        4
  SUB            H             94        4
  SUB            L             95        4
  SUB            (HL)          96        8
  SUB            #             D6        8