INT 0C - IRQ4 - SERIAL COMMUNICATIONS (COM1)

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
	DOS 3.3+ revectors IRQ4 to a stack-switching routine unless STACKS=0
	  has been set in CONFIG.SYS.  MS/PC-DOS use the IBM Interrupt Sharing
	  Protocol (see #02568) when hooking this IRQ
SeeAlso: INT 0B"COM2",INT 54"DESQview",INT 5C"DoubleDOS",INT 7C"GO32"