Understanding PCI Express

[The Coder]

The newest top of the line boards are using them.
What is the objective of PCI-E? I have a few questions:

1.) It is obviously replacing AGP, but will it eventually replace all PCI slots?

2.) I see there are various PCI-E slots - 16x, 1x. What is the difference between the different ones - 16 x, 1x? What kinds of devices plug into each slot type?

3.) PCIE video cards seem to plug into the PCIE 16x slot. Can this slot hold anything but a video card?

[bwkaz]

1) That's the idea, yes.

2) The difference is the size of the connector, and the speed that the interface runs at. I believe there's more than one (serial) channel on the higher speed slots, that's how they achieve higher speeds. It's kind of like having dual-channel DDR memory versus single-channel -- with dual-channel memory, you can have two memory accesses going on at once. (But you need two CPUs to do dual-channel, I think, whereas you don't need 16 CPUs to do PCI-X x16.)

Any device can plug into any slot, as long as it was designed for that slot to begin with. Video cards use x16 slots, basically because those slots are the fastest. I haven't seen any other PCI-X devices, but I'm sure they'll eventually exist. I suspect modems, sound/network cards, etc. (low-traffic stuff) will use the lower-speed slots.

3) The slot can hold anything that was made for it. At the moment, I haven't seen anything but video cards made for the x16 slot, though, so at the moment, the answer is no. But eventually I can see higher-speed devices being run through that slot. (For example, a very-high-speed SCSI card might use it instead of the 64-bit 66MHz PCI slots that they use now.)

[The Coder]

Thanks. You have answered most of my questions. In addition I have also been reading a lot about the subject on line. The main thing I can't understand is this: With regular PCI you have all slots the same standard size and any device that says it's PCI an plug into any slot. With PCI express and all these different slot sizes(1x, 4x, 16x) only certain devices can plug into certain slots. ANd if you have PCI express, but don't have a certain slot size, then you are screwed right? Like if you have a winmodem that can only plug into 1x slots but you have 2 16x slots.

[MorphiusFaydal]

As far as I have been able to find out, the 16x slots are the AGP-replacement, and the 1x slots are the PCI replacement.

I think the idea is that you have 1, maybe 2 16x slots (SLI video) and then have 4 or 5 1x slots, and they just don't make the other theoretical sizes (2x, 3x, 4x...) And as a 1x slot is several times faster than AGP 8x, you could plug in your SCSI card into the 1x slot... altthough some server boards may end up having some 4x slots for say.. SCSI or Fiber-channel cards, i think that all the desktop boards will have the 1x and 16x slots, and most stuff will go through 1x slots.

bwkaz: you dont need dual CPU's to do dual-channel memory, most P4's and Athlon XPs make in the last few years allow dual-channel... I'm not sure how it works, but im glad it does...

http://en.wikipedia.org/wiki/Dual-channel doesnt really say much....

at any rate... im not gonna worry about PC-E until AMD does DDR-2 in 2006.

[The Coder]

Yeah, I guess it is hard for anyone to say for sure about the way things will go. I was looking at the new MSI board with the nForce4 Ultra chipset and it has a 16x, 1x and 4x slots for PCIE. You are right the 2 16x slots seem to be used for SLI. I just think that having different different size slots won't be as simple and clean as the current PCI.

I am not really sure with the dual channel memory, but it seems the 939 socket CPUs can use it.

[MorphiusFaydal]

Well... if desktop boards have 4x slots, it must have a specific purpose... 1x for low-end devices.. 4x for medium grade, and the 16x for video....

Ethernet and disk controlers in the 4x slots anyone?

audio and others in the 1x slots?

we'll have to see....

But i'm sure that there is some sort of set standard somewhere for what is supposed to go into each type of slot...

[bwkaz]

Originally posted by MorphiusFaydal
bwkaz: you dont need dual CPU's to do dual-channel memory, Hmm... I remember reading that somewhere at one point. Maybe my source was just confused.

Oh well. ;)