code: function add_sp_n (cpu, mmu) {
      const n = mmu.readByte(cpu.pc + 1).signed();
      const r = cpu.sp + n;
  
      const op = cpu.sp ^ n ^ r;
  
      cpu.f = 0;
      if ((op & 0x10) != 0) cpu.f |= FLAG_H;
      if ((op & 0x100) != 0) cpu.f |= FLAG_C;
  
      return r;
  }
docs: LD HL,SP+n
  LDHL SP,n
  
  Description
  Put SP + n effective address into HL.
  
  Use with:
  n = one byte signed immediate value.
  
  Flags affected:
  Z - Reset.
  N - Reset.
  H - Set or reset according to operation.
  C - Set or reset according to operation.
  
  Opcodes:
  Instruction    Parameters    Opcode    Cycles
  LDHL           SP,n          F8        12