$0314-$0315/788-789:   Vector: Hardware IRQ Interrupt Address

   Points to $EA31

This vector points to the address of the routine that is executed when an IRQ interrupt occurs (normally 59953 ($EA31)).

At power on, the CIA #1 Timer B is set to cause an IRQ interrupt to occur every 1/60 second. This vector is set to point to the
routine which updates the software clock and STOP key check, blinks the cursor, maintains the tape interlock, and reads the
keyboard. By changing this vector, the user can add or substitute a machine language routine that will likewise execute every
1/60 second.

The user who is writing IRQ interrupt routines should consider the following:

	It is possible for an IRQ interrupt to occur while you are changing this vector, which would cause an error from which no
	recovery could be made. Therefore, you must disable all IRQ interrupts before changing the contents of this location, and
	reenable them afterwards, by using the 6510 SEI and CLI instructions, or by using the Kernal VECTOR routine (64794, $FD1A)
	to set this vector.

	There is some code in ROM that is executed before the interrupt routine is directed through this vector. This code checks
	whether the source of the interrupt was an IRQ or BRK instruction. If first preserves the contents of all the registers by
	pushing them onto the stack in the following sequence: PHA, TXA, PHA, TYA, PHA. It is up to the user to restore the stack at
	the end of his routine, either by exiting through the normal IRQ, or with the sequence: PLA, TAY, PLA, TAX, PLA, RTI.

	There is only one IRQ vector, but there are many sources for IRQ interrupts (two CIA chips, and several VIC chip IRQs).
	If you plan to enable IRQs from more than one source, the IRQ routine here must determine the source, and continue the
	routine in the appropriate place for an IRQ from that source.

In the same vein, if you replace the normal IRQ routine with your own, you should be aware that the keyboard's scanning and
clock update will not occur unless you call the old interrupt routine once every 1/60 second. It is suggested that if you plan
to use that routine, you save the old vector address in some other location. In that way, you can JuMP to the keyboard interrupt
routine through this alternate vector, rather than assuming that the ROM address will never change and that it is safe to jump
into the ROM directly.

Kernal-Reference:

 LDA $0314   : $F895
     $0314,Y : $FD20
 STA $0314   : $FCB3 $FCC0
     $0314,Y : $FD29

 CMP $0315   : $F8C1
 LDA $0315   : $F89B
 STA $0315   : $FCAD $FCC6

Kernal-Reference:

 JMP ($0314) : $FF58

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