INT 01h (1)              Single Step

Generated after each instruction if the Trap Flag (TF) of the CPU is

The Trap Flag (TF) is turned off when any type of interrupt is
executed. Therefore, single stepping will not occur within the INT 01h
handler. After the INT 01h handler is exited, the flags are restored
and single stepping will continue.

Debuggers such as DEBUG use this service to single-step through

INT 01h (1) Single Step