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	AddICRVector -- attach an interrupt handler to a CIA bit.

	interrupt = AddICRVector( Resource, iCRBit, interrupt )
	D0                        A6        D0      A1

	struct Interrupt *AddICRVector( struct Library *, WORD,
		struct Interrupt * );

	Assign interrupt processing code to a particular interrupt bit
	of the CIA ICR.  If the interrupt bit has already been
	assigned, this function will fail, and return a pointer to the
	owner interrupt.  If it succeeds, a null is returned.

	This function will also enable the CIA interrupt for the given
	ICR bit.

	iCRBit          Bit number to set (0..4).
	interrupt       Pointer to interrupt structure.

	interrupt       Zero if successful, otherwise returns a
	                    pointer to the current owner interrupt

	A processor interrupt may be generated immediately if this call
	is successful.

	In general, it is probably best to only call this function
	while DISABLED so that the resource to which the interrupt
	handler is being attached may be set to a known state before
	the handler is called. You MUST NOT change the state of the
	resource before attaching your handler to it.

	The CIA resources are special in that there is more than one
	of them in the system. Because of this, the C language stubs
	in amiga.lib for the CIA resources require an extra parameter
	to specify which CIA resource to use. The synopsis for the
	amiga.lib stubs is as follows:

	interrupt = AddICRVector( Resource, iCRBit, interrupt )
	D0                        A6        D0      A1

	struct Interrupt *AddICRVector( struct Library *, WORD,
		struct Interrupt *);


	Never assume that any of the CIA hardware is free for use.
	Always use the AddICRVector() function to obtain ownership
	of the CIA hardware registers your code will use.

	Note that there are two (2) interval timers per CIA.  If
	your application needs one of the interval timers, you
	can try to obtain any one of the four (4) until AddICRVector()
	succeeds.  If all four interval timers are in-use, your
	application should exit cleanly.

	If you just want ownership of a CIA hardware timer, or register,
	but do not want interrupts generated, use the AddICRVector()
	function to obtain ownership, and use the AbleICR() function
	to turn off (or on) interrupts as needed.

	Note that CIA-B generates level 6 interrupts (which can degrade
	system performance by blocking lower priority interrupts).  As
	usual, interrupt handling code should be optimized for speed.

	Always call RemICRVector() when your code exits to release
	ownership of any CIA hardware obtained with AddICRVector().

	cia.resource/RemICRVector(), cia.resource/AbleICR()

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