How INT2 works on AT style machines

[John Plocher]

In article <42700010 at osiris.cso.uiuc.edu> dpgerdes at osiris.cso.uiuc.edu writes:
>>First, as usual, Mr Plocher is correct in that INT 2 is unusable for all
>>PC/AT architecture machines (as opposed to PC bus.)  INT 2 is used to cascade
>    Thanks for the information.  I never challenged whether he was correct
>or not, but was merely expressing my frustration that every tech person
>that has ever made a statement like the above to me has not been able to

Let me expand on this thought.

First the terminology:

	INT x	   = The hardware BUS signal found on many plug in cards
        PICinput x = The pin on the PIC
	Intvec x   = A location in the software maintained array of ISR addresses
	ISR        = Interrupt Service Routine
	PIC        = Programmable Interrupt Controller

	When a hardware Interrupt is generated by a board (for e.g., INT 3)
	the PIC takes the INT 3 hardware signal and (handwaving here) with the
	help of the CPU generates a call to the routine whose address is
	found at intvec[ 3 ].

>From a low level kernel hackers view of the world like mine (interrupt
service routines, interrupt vector tables, and the like) there is no way for
a device on an AT BUS to generate a a signal which calls the ISR routine
whose address is stored in Intvec[ 2 ].  This is because the input pin on the
PIC which would usually be used to generate this action is used instead to add
a second PIC chip to the system.  In Intel terminology this is called cascading
interrupt controller chips.

>From a mid-level hardware point of view, the BUS SIGNAL (which was called INT 2
on PC and XT systems) gets wired to a pin on the new PIC.  This pin is the one
which generates calls to the routine at Intvec[ 9 ].  This means that any card
which generates a BUS SIGNAL INT 2 ends up activating the Intvec[ 9 ] routine.

>From a high level view, all you need to do to use cards that have a jumper
marked INT 2 is to pronounce it as "Interrupt Nine".  This means that you
set the jumper to *what is labeled as* INT 2, but set ALL your software to
use Intvec[ 9 ].


	    Very Much Overly Simplified block diagram of the
	    interrupt structure used on AT-style machines.


		BUS SIGNAL       PICinput
		----------       -----------
		  |||                |||
		  vvv                vvv
				    +---------+
		INT 2 ------------++| 9       |             \ 
				    |      P  |              \ 
		INT 10 -----------++| 10   I  |               \ 
				    |      C  |                \ 
		INT 11 -----------++| 11      |                 \ 
				    |      2  |                  > new in the AT
		INT 12 -----------++| 12      |                 / 
				    |         |                / 
		INT 13 -----------++| 13      |               / 
				    |         |              / 
		INT 14 -----------++| 14  OUT |++--+        / 
				    |         |    |
				    +---------+    /
						  /
				+-----------------
				|   CASCADE LINE
				|
				\   +---------+
				 -++| 2       |
				    |         |             \ 
		INT 3 ------------++| 3       |              \ 
				    |      P  |               \ 
		INT 4 ------------++| 4    I  |                \  PCs, XTs,
				    |      C  |                /  and ATs
		INT 5 ------------++| 5       |               / 
				    |      1  |              / 
		INT 6 ------------++| 6       |             / 
				    |         |
		INT 7 ------------++| 7   OUT |++------>>> To INT pin on CPU
				    |         |
				    +---------+


There.  A much more in depth answer.  Gawd, I hope it is right :-)

   -John Plocher