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INT 01H           Single Step

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

    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
    instructions.