还是自己来：
The processor generates a single-step debug exception if (while an instruction is
being executed) it detects that the TF flag in the EFLAGS register is set. The excep-
tion is a trap-class exception, because the exception is generated after the instruc-
tion is executed. The processor will not generate this exception after the instruction
that sets the TF flag. For example, if the POPF instruction is used to set the TF flag, a
single-step trap does not occur until after the instruction that follows the POPF
instruction.
