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liaoo

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Why Core processor PCI Express Root Port disappear until device attached ?

I have one question about the behavior of v2/3rd Gen Core processor PCI Express Root Port - 0155 ( where 0155 is the device id for this root port in Intel platform...)

@ platform information:
- OS: Win7 Pro 32bit
- MB: ASUS P8 Z77 VLE Plus
- CPU : Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz

@ phenomenon:
- if one PCIe device is attached(plug) to PCI Express slot 5, then I can see "...v2/3rd Gen Core processor PCI Express Root Port - 0155" in [Device Manager].

- But if no device plug into slot 5, then this root port is absent in  [Device Manager]

@ My questions are:

(1) why this root port is "disabled" without device attached ? ( for other pcie root ports, I found it is still present in [Device Manager] even though no device attached ! )

Can I enable it manually in Ex. DOS ?

(2) how to absolutely identify PCIe root port ?

Ex. PCIe root port is the PCI device in which:
a. class code is 0x6( bridge device)
b. PCIe capability exist

Am I correct ?
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Al Jee
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Looking at ASUS' specs for your motherboard I see only (4) PCIe slots along with (3) legacy PCI expansion slots.

From their specs page:
1 x PCIe 3.0/2.0 x16 (blue) *3
1 x PCIe 2.0 x16 (x4 mode, black) *4
2 x PCIe 2.0 x1
3 x PCI


Where are you finding an additional PCIe slot?


BTW you are asking more than one question in this Question.
That makes it very difficult [if not impossible] to provide a satisfactory "answer".
Perhaps you would consider opening multiple Questions with one question each?
That is the preferred way at EE.
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liaoo

ASKER

[EDIT]
For the question (2) I used below way to identify:
1) Its base class code and sub-class code should be 06/04h according to PCI spec
2) there should be PCIe capability(ID = 0x10) in the capability list
3) The port/type field in the PCIe capability should be 0100b, indicating this is the root port of RC

* Thus (2) is solved for me...

As for the additional PCIe slot(slot 5) please see the attached file quoted from ASUS manual.

To summarize,
a. Slot 5 is one PCIe slot on mother board.
b. When no PCIe device inserted on this slot, no Root Port appear in the [Device Manager]
c. But if insert PCIe device (Ex. Gen1 and x1 device) then I can see "...v2/3rd Gen Core processor PCI Express Root Port - 0155" in [Device Manager]

Could anyone know the reason why ?

* Now I just want to know the answer to question (1) and please ignore (2) !

Thanks !
slot5.jpg
ASKER CERTIFIED SOLUTION
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Al Jee
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Just as a matter of information, why wasn't my answer worth an "A"?
What about it was incomplete, in your view?
liaoo>  I have been asked to clarify my last post.

Your question was: "why this root port [from PCIe slot #5] is 'disabled' without device attached ?"

The reason I believe I deserve an "A" is that my explanation describes exactly the reason that your motherboard disables the  PCIe slot #5 if it is not occupied.

By your actions, you seemed to agree with me because you immediately accepted my explanation without asking for further details.
I have requested that the answer grade be reviewed by the Moderators.