Desc: automatically asserted by the UART when COM1 needs attention, if the
UART has been programmed to generate interrupts
BUG: this vector is modified but not restored by Direct Access v4.0, and
may be left dangling by other programs written with the same version
of compiled BASIC
Notes: on many PC's, COM3 shares this interrupt
may be masked by setting bit 4 on I/O port 21h