code: function sbc (cpu, n) {
      const cy = cpu.f >> 4 & 1;
      const r = cpu.a - n - cy;
  
      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: SBC A,n
  
  Description:
  Subtract n + Carry flag 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
  SBC            A,A           9F        4
  SBC            A,B           98        4
  SBC            A,C           99        4
  SBC            A,D           9A        4
  SBC            A,E           9B        4
  SBC            A,H           9C        4
  SBC            A,L           9D        4
  SBC            A,(HL)        9E        8
  SBC            A,#           DE        8