IRQ steering

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Was reading about IRQ steering in the Mike Meyers book and feel confused about the subject could anyone explain it differently. How well do I have to understand this for the A+ OS exam. Any additional information or explanations on this subject would really help me alot.

Professor Falken

 

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Can you try and explain in your own words what you understand IRQ steering is?

This will help you clarify what you think you know, and help us to explain what you don't.

 

Traditionaly, because of a horrible mistake by IBM back in the dark ages of the first PC, two devices couldn't share an IRQ.

This was finaly fixed in the PCI specs, such that they *could* share under certain circumstances. Win9x marks such situations with the 'steering' note. It is a small driver that works out which of the devices actualy needs to handle the IRQ when it is shared.

Harry.

 

IBM is to blame, very interesting Harry, all those conflicts, hair pulling and frustration in Windows Win 95 with IRQ conflicts could have been avoided then

 

So basically it is devices sharing IRQs. Could someone explain a little bit more.

Professor Falken

 

Just that.

Not entirely sure what you are looking for here.

The basic electronic reason goes something like this.

IRQs can usualy be setup as edge-triggered, or level-triggered. On simple systems it doesn't matter very much which you use. But the moment you want to share IRQs, usualy because you have run out of IRQ lines on the system, then edge-triggering has problems.

If you leave a IRQ routine and another IRQ has happened then you have no way of knowing, as the 'edge' leaves no trace in the IRQ controller's registers. If, on the other hand, you use level-triggering you know it has had another IRQ, because the IRQ line is still in the triggered state.

The Amiga got this right, and made heavy use of shared IRQs. The original design of the PC got it wrong, and used edge-triggering on the short ISA bus of the day. From the descriptions I have seen of the design of that PC it was probably an oversight, as it was somewhat slapped together. However, we contiued to suffer from that mistake for many years until PCI came along.

Harry.

 

Agreed. I loved my Amiga 500.

 

Amigas ruled.