INT F1 - Common ISDN API(Application Program[ming] Interface) The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running.  Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes. (CAPI) v1.1 - "API-SET-SIGNAL" - SIGNAL HANDLING
	AH = 05h
	DX = application ID (see AH=01h)
	ES:BX -> signal handler or 0000h:0000h to disable
Return: AX = status (0000h,1002h) (see #04069)
Notes:	the caller is required to provide at least 512 bytes of stack space
	the signal handler is called as though it were an interrupt, with
	  interrupts disabled; the handler must preserve all registers and
	  return with an IRET
SeeAlso: AH=01h