🚀 go-pugleaf

In article <1992Aug6.154949.164290@dstos3.dsto.gov.au> egg@dstos3.dsto.gov.au writes:
>
>>No Josh, your wrong.  Some VGA board create a vertical retrace interrupt
>>whenever they enter the retrace period.  A program would merely have to
>>trap the interrupt to know when the retrace period is happening.
>
>I beleive its not a good idea to use the vertical retrace interrupt because
> according to "Programmers Guide to PC and PS2 video systems" hardware
>interrupts on IRQ2 cannot be distinguished from Vertical interrupts. I'm not
>that cluey on the subject but if its in the book ???

Shouldn't you be able to distinguish interrupts based on the status of
the CRT controller register? There's a bit which is set during the
vertical blank, and reset otherwise. It seems to me that the interrupt
service routine could check the value of this bit; if the bit is set, it
would assume that the interrupt was generated by the video card, if
reset it would assume that the IRQ was generated by some other hardware
and chain to the next ISR in the vector chain. Not foolproof, but might
be useful in certain situations where the chances of other hardware
causing the IRQ are small or the other hardware has a positive method
for determining whether or not IT caused the interrupt.

For example, the Microsoft InPort card (a.k.a. the bus mouse card) has
an Interrupt Identification Register which can be checked to determine
if the InPort card (mouse) has caused an interrupt since the last time
it was checked. Other hardware (UARTs, for example) have similar
capabilities.