[Open-graphics] Video interrupt enable/disable

[James Richard Tyrer]

Timothy Normand Miller wrote:
> On 4/17/07, James Richard Tyrer <tyrerj at acm.org> wrote:
> 
>> Have I been the victim of Intel hype?  They make hardware that would
>> make it possible to have enough interrupts for each PCI card slot to
>> have 4 interrupts or at least to have 24 hardware interrupts available
>> to be assigned with a cross point.
> 
> On a PCI bus, there are four interrupt lines.  All four are connected
> to each slot, but in a different order.

Yes, that was one design but it is also possible to use PNP to assign 
interrupt numbers.

> In some Sun hardware, each slot has its own set of interrupt lines.
> But they do other weird stuff too.

That is the way all PCI systems should be. :-D

> Also, in general, there are many sources of interrupts in a PC, but
> they all route to one priority encoder that hands a binary number to
> the CPU that tells it which one to pay attention to.  Then it comes
> down to the ISR looking to figure out which source it is and so forth.
> 
> You're never going to get away from the fact that you have to query
> devices to figure out which one asserted the interrupt, because you
> cannot tell just from the interrupt number.
> 
Actually, the Intel PIC chip has the interrupt vector in a register so 
when that interrupt is asserted, execution jumps to that address, IIRC, 
this vector points to an address in memory somewhere.  So, on a PC 
running DOS, the jump to the interrupt routine is handled by hardware.

The standard PC AT hardware contained 2 PICs the first one with 7 
interrupts (one is used to cascade) and the second one with 8.  Some of 
these were assigned to specific purposes on the MB.

Since they could all be cascaded, it was possible with that hardware to 
make a system with 64 hardware vectored interrupts with 2 levels of PCI 
chips.  Why this wasn't done when VLSI chips became common, and it would 
no longer take 9 chips, is a mystery since it would be backward compatible.

Then Intel introduced the APIC (which I don't have printed documentation 
for) which was supposed to support protected mode and more interrupts 
per chip.  But, the question is whether or not current PCs still have 
only the standard 2 PICs or whether they have an APIC.  Intel hyped the 
fact that PCI would have individual hardware vectored interrupts.

My current motherboard assigns interrupts to PCI slots which would 
appear to mean that all four interrupts in the slot feed to the same 
interrupt vector which seems to defeat the purpose of having 4 PCI 
interrupts.  When you run out of interrupts (as I appear to have) the 
Kernel first applies the interrupt sharing driver to determine which PCI 
slot has raised the interrupt and then that slot's vector is called. 
This process is transparent to the driver.

I only have a basic understanding of this, but could look into it 
further if needed.

-- 
JRT