Int 01  C - Cpu-generated - Single Step                                    [C]

Desc:  generated after each instruction if TF (trap flag) is set; TF is
     cleared on invoking the single-step interrupt handler

Notes: interrupts are prioritized such that external interrupts are invoked
     after the INT 01 pushes CS:IP/FLAGS and clears TF, but before the
     first instruction of the handler executes
   used by debuggers for single-instruction execution tracing, such as
     MS-DOS DEBUG's T command

See Also: INT 03"CPU"