INT 0B - IRQ3 - SERIAL COMMUNICATIONS (COM2)

Desc:	automatically asserted by the UART when COM2 needs attention, if the
	  UART has been programmed to generate interrupts
Notes:	the TOPS and PCnet adapters use this interrupt request line as an
	  alternate
	on PS/2s, COM2 through COM8 share this interrupt; on many PC's, COM4
	  shares this interrupt
	may be masked by setting bit 3 on I/O port 21h
	DOS 3.3+ revectors IRQ3 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 0C"COM1",INT 53"DESQview",INT 5B"DoubleDOS",INT 7B"GO32"