INT 2F U - DOS 3+ internal - PERFORM CRITICAL ERROR INTERRUPT
        AX = 120Ah
        DS = SS = DOS DS (must be using a DOS internal stack)
        STACK: WORD extended error code
Return: AL = user response (0=ignore, 1=retry, 2=abort, 3=fail)
        CF clear if retry, set otherwise
        STACK unchanged
Notes:  can only be called during a DOS function call, as it uses various
          fields in the SDA to set up the registers for the INT 24
        reportedly sets current DPB's first root directory sector to 1