Changing PCI cards without switching off?

[Bryan Hoover]

There are a number of variations to interrupt handling but the
original notion, from which the term gets its name, is just as it
says; an 'interrupt' to the normal execution flow.

I would guess it is an "interrupt request" at the hardware input pin
level.
Yes.

I.E. Upon assertion of the interrupt line the processor is jerked
away from whatever it's doing and sent into the interrupt service
routine. The processor does not 'poll' or otherwise 'monitor' the
interrupt line. The redirection is handled in the hardware.

That might be what I would have guessed if I knew something about the
hardware and software involved. I was thinking the term IRQ [number]
refers more to hardware than software. I don't think it's worth
arguing, but I suspect you disagree.

IRQ does refer to the hardware. Specifically, which interrupt line it is.

The original PC had an interrupt controller chip that provided 8 interrupt
lines. The AT added a second 8 line interrupt controller that connects to
the first one on IRQ2, so you have 7 on the first (IRQ2 being used already
for the chain to the second controller) and 8 on the second for 15 total.
Of the 15... 0, 1, 2, 8, and 13 are reserved for system use. That leaves
the ones most PC builders should be familiar with.

When an interrupt occurs on one of the IRQ lines the processor is
interrupted and the corresponding interrupt handler routine is vectored to
by the IRQ number, which is an offset into an IRQ table at the beginning of
memory. I.E. the IRQ number causes the hardware to load in the address
stored in location IRQ_VECTOR_BASE + IRQ#(times 4 because there are 4 bytes
per address vector) and code execution resumes at that location: the
interrupt routine. At the same time the interrupt is vectored, where the
processor *was* executing code is saved and, when the interrupt routine is
finished, it restores that and execution goes back to where it was when
interrupted.

Point is, the 'interrupt' is done by the hardware.

IRQ sharing is when more than one device is on the same IRQ line and, in
that case, the interrupt handling routine must then go and query the
card(s) to see which one made the interrupt request.

I see someone else has mentioned that the processor 'waits for the
beginning of the next instruction' and then claims this sounds like
'polling' to him. Regardless of what it might sound like, that is not
'polling' as the term is commonly used. That the interrupt occurs on an
instruction cycle boundary is, again, handled by the hardware (or rather,
the hardware and microcode in the processor) and not the software.

'Polling' generally refers to software having to perform the function of
repeatedly scanning the item(s) of interest to see if it/they need(s)
service because there's nothing to 'interrupt' the processor when
'something needs to be done'.

The reason for hardware interrupts should be rather obvious from that
description. With polling, processor time is used up by repeatedly scanning
whatever it is that's being checked and response time is limited by how
fast the processor does it, which you will want to do as little as possible
to avoid sucking up processor time. It's a catch-22 between response time
and processor time.

With hardware interrupts the processor can work it's little heart out on
whatever software you're running and is burdened only when something really
does need servicing.

If hardware interrupts are enabled, and one goes active, then the processor
*is* going to get interrupted and execution *will* be redirected to
whatever location is stored in the interrupt vector table regardless of
what 'code' one writes. The hardware will cause it.

With 'polling', nothing will happen unless you write the code to do it.

Excellent review of hardware interrupts. Brings to mind all that good stuff
about "event driven" programming, and such.

I like the terms "push", and "pull" to describe the difference between hardware
interrupts, and polling respectively (I don't think I'm confusing the terms
anyway). A "pull" is kind of like, "don't call us, we'll call you", wherein
the item of interest, is at regular intervals, polled by the, if you will,
attendant. And a "push" is the opposite of that, where the item of interest
calls the attendant when it needs attention, which is obviously much more
efficient.

Hardware interrupts rule. Software interrupts are opinionated, and often
repleat with ironic impulses with regard for neither chicken, nor tomato soup
-- bah-studs!

Bryan

 

[John Doe]

Actually, that's only one insight.

The other redundant insight is that "polling" and "similar to
polling" are two different things. I think you are on the verge of
that one.

But only redundant if the person I was replying to knew it and it's
not at all clear that he does.

It's clear to me, in both cases.

You're assuming a 'hardware clock' is needed to 'test' the pin but,
while that might be one way to accomplish the task, it isn't a
'requirement'.

A digital circuit does nothing with an input until a clock
transition. I think that's a fundamental difference between
digital/analog circuits.

If the schooling had taken we wouldn't have people calling a
hardware interrupt 'polling'.

And we are not, Masta.

 

[w_tom]

Some busses are selectively powered down to avoid power
spikes. This is the 'kludge' solution used for hot poppable
PCI. But then other busses have no such feature - USB and a
Mil Std buss used in a nuclear weapons system. The ability to
hot pop without warning (in those latter busses) even made
that system more reliable in combat. In summary, hot popping
is a function of how much hardware is installed. A truly hot
poppable buss is more complex and more expensive.

Properly noted - a CMOS digital IC connected to data lines
before being powered will power itself using data bus
electricity. Yes I am saying it will power itself via data
pins rather than from power pins. This causes strange and
undesirable operation. How power is automatically applied and
removed during hot popping is but another *complication* for
hot pop busses where cards can be removed without warning. To
avoid that complication, the hot poppable enhancement to PCI
involves LEDs so that technician is informed by hardware when
board removal is safe. And involves selected IC so that noise
on bus is not a problem.

The hot poppable PCI uses a simpler 'kludge' method. All
depends on complexity of that buss hardware. PCI hot poppable
bus is a simplistic and cheaper method. First Administrator
must tell computer to power down that bus. When LED says go,
only then can technician remove bus card.

BTW we have not even discussed another complication with hot
popping. On power up, the computer must do a complex
initialization on the PCI card. Simple modems would not have
the complex configuration that other peripheral cards may
have. But again, just because one card was hot popped without
damage does not mean all cards can be hot popped. Some cards
will not properly configure if not in the machine when PnP
software configures hardware or if configuration information
is lost when removed.

Notice that I post without profane language. Suggest you
learn same since you really don't know what a hot poppable bus
must do.

 

[John Doe]

To the O.S. programmer, O.S. because that's where interrupt
handling will be done, there is no 'polling' code to write
because it is done by 'the hardware' [...]

Yes, as I said before: "In effect, yes, interrupt lines are polled
in hardware."

Coming up next...David Maynard argues his meaning of "it".

 

[David Maynard]

news:3DAzd.572117$Pl.189475@pd7tw1no...

[snip]

Suppose, as I suggested, pending interrupts are acted upon at the
start of the next instruction - that is, the processor's interrupt
mechanism kicks in at the point where the next instruction would begin
to be executed (I don't know of anything that doesn't work like this).
That means that based on whether there is a pending interrupt, the
processor periodically (albeit very frequently) chooses whether to
execute the next instruction or begin the process for handling the
interrupt (eg saving the PC and jumping to some predefined address).
Sounds very similar to polling to me.

You're mixing hardware and software together.

Only to point out the conceptual similarity.

Except it isn't.

To the O.S. programmer, O.S. because that's where interrupt handling
will be done, there is no 'polling' code to write because it is done by
'the hardware' [...]

Yes, as I said before: "In effect, yes, interrupt lines are polled in
hardware."

And how do you 'poll' an edge trigger?

Secondly, you're calling every bit and portion of a processor 'the
processor' and that's like saying you 'decide' to snap your leg up when
someone whacks your knee in the right spot testing reflex actions. [...]
The address unit of the processor, note that is a 'part' of a processor,
can get the next address from a number of sources. One is to simply
increment the current address. Another is code altering what the next
address is, as in a branch. And another is the next address
automagically appearing because the interrupt unit, another 'part' of
the processor, shoved it in there.

Try looking at it the other way round: the new PC (incremented PC, branch
target or interrupt vector) is selected (ie chosen), based on control
signals within the processor (affected by, amongst other things, interrupt
lines).

I have no idea what 'significance' you're trying to imply by 'selected'.

Software that is polling, when there are no interrupts.

There is no software doing 'polling'.

Rene wrote: "There is one reason why hardware interrupts were invented, and
that is so that polling wouldn't be necessary anymore."

Apparently, both you and Noozer missed that.

Rene is correct.

That's either true or false, depending what you mean. > I know what you mean.
I meant asynchronous like an asynchronous set/clear on a flip-flop.

Yes, the event capture is asynchronous.

I only said it was *like* polling. John Doe summed it up nicely.

The attempt to claim interrupts are 'like polling' ignores the purpose and
point of interrupts. You might as well say, in a conversation about modes
of transportation, that a car 'is like' a tin can because they're both 'man
made' of sheet metal with an exterior and an interior. At some level there
may be a 'similarity' but it's not useful for the context.

 

[David Maynard]

The other redundant insight is that "polling" and "similar to
polling" are two different things. I think you are on the verge of
that one.

I use dissimilar 'similarities' for comedic purposes. Such as, "interrupts
are 'like' usenet posts. They occur whenever they happen." It's as true as
"interrupts are 'like polling'."

It's clear to me, in both cases.




A digital circuit does nothing with an input until a clock
transition. I think that's a fundamental difference between
digital/analog circuits.

Feed the interrupt line to the trigger input of a flip flop so that it is
latched. No "hardware clock transition testing an input pin" required.

 

[David Maynard]

To the O.S. programmer, O.S. because that's where interrupt
handling will be done, there is no 'polling' code to write
because it is done by 'the hardware' [...]

Yes, as I said before: "In effect, yes, interrupt lines are polled
in hardware."

Coming up next...David Maynard argues his meaning of "it".

And here you were doing so well at explaining how you got the 'meaning' of
everything.

"It" is the interrupt. There is no 'polling' code to write because "the
interrupt" is done by 'the hardware'.

Let's put this a different way. What useful 'insight' do you and Alex claim
is provided by claiming interrupts are 'like polling'? What 'confusion'
does this clear up and/or how does this improve one's understanding?

 

[David Maynard]

David Maynard wrote:

There are a number of variations to interrupt handling but the
original notion, from which the term gets its name, is just as it
says; an 'interrupt' to the normal execution flow.


I would guess it is an "interrupt request" at the hardware input pin
level.
Yes.



I.E. Upon assertion of the interrupt line the processor is jerked
away from whatever it's doing and sent into the interrupt service
routine. The processor does not 'poll' or otherwise 'monitor' the
interrupt line. The redirection is handled in the hardware.


That might be what I would have guessed if I knew something about the
hardware and software involved. I was thinking the term IRQ [number]
refers more to hardware than software. I don't think it's worth
arguing, but I suspect you disagree.

IRQ does refer to the hardware. Specifically, which interrupt line it is.

The original PC had an interrupt controller chip that provided 8 interrupt
lines. The AT added a second 8 line interrupt controller that connects to
the first one on IRQ2, so you have 7 on the first (IRQ2 being used already
for the chain to the second controller) and 8 on the second for 15 total.
Of the 15... 0, 1, 2, 8, and 13 are reserved for system use. That leaves
the ones most PC builders should be familiar with.

When an interrupt occurs on one of the IRQ lines the processor is
interrupted and the corresponding interrupt handler routine is vectored to
by the IRQ number, which is an offset into an IRQ table at the beginning of
memory. I.E. the IRQ number causes the hardware to load in the address
stored in location IRQ_VECTOR_BASE + IRQ#(times 4 because there are 4 bytes
per address vector) and code execution resumes at that location: the
interrupt routine. At the same time the interrupt is vectored, where the
processor *was* executing code is saved and, when the interrupt routine is
finished, it restores that and execution goes back to where it was when
interrupted.

Point is, the 'interrupt' is done by the hardware.

IRQ sharing is when more than one device is on the same IRQ line and, in
that case, the interrupt handling routine must then go and query the
card(s) to see which one made the interrupt request.

I see someone else has mentioned that the processor 'waits for the
beginning of the next instruction' and then claims this sounds like
'polling' to him. Regardless of what it might sound like, that is not
'polling' as the term is commonly used. That the interrupt occurs on an
instruction cycle boundary is, again, handled by the hardware (or rather,
the hardware and microcode in the processor) and not the software.

'Polling' generally refers to software having to perform the function of
repeatedly scanning the item(s) of interest to see if it/they need(s)
service because there's nothing to 'interrupt' the processor when
'something needs to be done'.

The reason for hardware interrupts should be rather obvious from that
description. With polling, processor time is used up by repeatedly scanning
whatever it is that's being checked and response time is limited by how
fast the processor does it, which you will want to do as little as possible
to avoid sucking up processor time. It's a catch-22 between response time
and processor time.

With hardware interrupts the processor can work it's little heart out on
whatever software you're running and is burdened only when something really
does need servicing.

If hardware interrupts are enabled, and one goes active, then the processor
*is* going to get interrupted and execution *will* be redirected to
whatever location is stored in the interrupt vector table regardless of
what 'code' one writes. The hardware will cause it.

With 'polling', nothing will happen unless you write the code to do it.

Excellent review of hardware interrupts. Brings to mind all that good stuff
about "event driven" programming, and such.

I like the terms "push", and "pull" to describe the difference between hardware
interrupts, and polling respectively (I don't think I'm confusing the terms
anyway). A "pull" is kind of like, "don't call us, we'll call you", wherein
the item of interest, is at regular intervals, polled by the, if you will,
attendant. And a "push" is the opposite of that, where the item of interest
calls the attendant when it needs attention, which is obviously much more
efficient.

Yeah. I've seen that description used, usually in the context of higher
level functions such as SMP and cluster messaging.

I've been around hardware interrupts so long that the terms seem self
explanatory.

 

[Noozer]

Yes, as I said before: "In effect, yes, interrupt lines are polled in

hardware."

And with this statement you are 100% wrong...

Are the lightbulbs in your house polled? Do the bulbs light up because
something inside checks the switch periodically and tells the bulb to light
up when it see's a change? No.

IRQ's are the same

An IRQ line is pulled low. Because of this, the CPU will not grab it's next
instruction. Instead it will get a new address (that of the IRQ handler) and
start executing code from that point in memory. When that code is done, the
CPU will resume where it was when it was interrupted.

There is *NOTHING* in the CPU that is watching for the IRQ line to change.
It's automatic. A trigger. A gate. NO SOFTWARE INVOLVED. NO POLLING INVOLVED
AT ALL.

 

[John Doe]

"David Maynard" <[email protected]> wrote in message

To the O.S. programmer, O.S. because that's where interrupt
handling will be done, there is no 'polling' code to write
because it is done by 'the hardware' [...]

Yes, as I said before: "In effect, yes, interrupt lines are
polled in hardware."

Coming up next...David Maynard argues his meaning of "it".

And here you were doing so well at explaining how you got the
'meaning' of everything.
"It" is the interrupt. There is no 'polling' code to write because
"the interrupt" is done by 'the hardware'.

I told you so.
That's funny IMO.

David Maynard. If you're going to put marks around every third word
or phrase, you should put marks around every instance of "it".

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From: David Maynard <[email protected]>
Newsgroups: uk.comp.homebuilt,alt.comp.hardware,alt.comp.hardware.pc-homebuilt
Subject: Re: Changing PCI cards without switching off?
Date: Mon, 27 Dec 2004 21:25:36 -0600
Organization: Posted via Supernews, http://www.supernews.com
Message-ID: <[email protected]>
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<[email protected]> <[email protected]>

 

[John Doe]

I use dissimilar 'similarities' for comedic purposes.

Must be some very dark, pedantic sort of comedy.

 

[David Maynard]

To the O.S. programmer, O.S. because that's where interrupt
handling will be done, there is no 'polling' code to write
because it is done by 'the hardware' [...]

Yes, as I said before: "In effect, yes, interrupt lines are
polled in hardware."

Coming up next...David Maynard argues his meaning of "it".

And here you were doing so well at explaining how you got the
'meaning' of everything.
"It" is the interrupt. There is no 'polling' code to write because
"the interrupt" is done by 'the hardware'.

I told you so.

I wouldn't have if you hadn't implied you didn't understand what 'it' was.

That's funny IMO.

Not really. It's a self fulfilling prophecy.

David Maynard. If you're going to put marks around every third word
or phrase, you should put marks around every instance of "it".

I do, when it's appropriate.

 

[David Maynard]

Must be some very dark, pedantic sort of comedy.

Now you're just being silly for the sake of arguing about SOMEthing, anything.

Entire comedic careers have been based on 'strange (dis) similarities'.

 

[John Doe]

John Doe wrote:
To the O.S. programmer, O.S. because that's where interrupt
handling will be done, there is no 'polling' code to write
because it is done by 'the hardware' [...]

Yes, as I said before: "In effect, yes, interrupt lines are
polled in hardware."

Coming up next...David Maynard argues his meaning of "it".

And here you were doing so well at explaining how you got the
'meaning' of everything.
"It" is the interrupt. There is no 'polling' code to write
because "the interrupt" is done by 'the hardware'.

I told you so.
That's funny IMO.

Not really.
It's a self fulfilling prophecy.

"You're a part of me I can't do without...la la la"

I do, when it's appropriate.

Citation please.

 

[Alex Fraser]

Alex Fraser wrote: [snip]

That's either true or false, depending what you mean. I know what you
mean. I meant asynchronous like an asynchronous set/clear on a
flip-flop.

Yes, the event capture is asynchronous.

It may be (eg for an edge-triggered interrupt), but the action (being
"pulled away") is not, in all CPUs I've come across at least.

The attempt to claim interrupts are 'like polling' ignores the purpose
and point of interrupts.

Not in the slightest. It is still happening in hardware, avoiding what is
sometimes (often, perhaps) significant software overhead. As far as I'm
concerned, that is "the purpose and point of interrupts". Did you have
something else in mind?

Alex

 

[Alex Fraser]

Are the lightbulbs in your house polled? Do the bulbs light up because
something inside checks the switch periodically and tells the bulb to
light up when it see's a change? No.

No, but suppose there was. Stick a microprocessor (no interrupts in the
circuit, running software that reads the position of the switch repeatedly,
turning the light on or off depending on the result. You'd call that
polling, right?

If the switch is "on" when you read its position, the light turns (or stays)
on; if it's "off", the light turns (or stays) off.

Replace the microprocessor with a D-type flip-flop.

If the switch is "on" when there is a clock edge, the light turns (or stays)
on; if it's "off", the light turns (or stays) off.

Ultimately, in a CPU, the signal from an interrupt pin, or perhaps a latched
version, will reach some clock-driven logic. Just like the polling example
above samples the switch position, the clock-driven logic samples the
interrupt line.

Alex

 

[Noozer]

No, but suppose there was. Stick a microprocessor (no interrupts in the
circuit, running software that reads the position of the switch repeatedly,
turning the light on or off depending on the result. You'd call that
polling, right?

If the switch is "on" when you read its position, the light turns (or stays)
on; if it's "off", the light turns (or stays) off.

That's polling...

Replace the microprocessor with a D-type flip-flop.

If the switch is "on" when there is a clock edge, the light turns (or stays)
on; if it's "off", the light turns (or stays) off.

That is *NOT* polling. There is nothing constantly checking the flip-flop.

Ultimately, in a CPU, the signal from an interrupt pin, or perhaps a latched
version, will reach some clock-driven logic. Just like the polling example
above samples the switch position, the clock-driven logic samples the
interrupt line.

Uhm no... Wrong... I give up.

 

[David Maynard]

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[David Maynard]

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[Noozer]

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