Bus Speed/Bandwidth Bottlenecks

[dvogel]

I've finally come to an understanding of the frequency limits on processors (pipeline lengths, etc). Now I would like to learn about the bottlenecks involved with pushing the bandwidth limits. Personally, I am a little ticked that after 30 years, we don't have 1Mbit/1GHz bus speeds. I hope I will be a bit more empathetic toward the engineers of the world if I understood why we don't.

Also, if we can't get a PCI bus past 64bit/66MHz (32bit/33MHz in most home systems), why not at least provide an extra PCI bus? I understand this, again, adds to the complexity of the chipset, but it sure would help. (disclaimer: I know some chipsets provide more than one PCI bus, I'm just a little ticked more designers don't follow suit).

--Drew Vogel

 

[CTho9305]

<< Also, if we can't get a PCI bus past 64bit/66MHz (32bit/33MHz in most home systems), why not at least provide an extra PCI bus? I understand this, again, adds to the complexity of the chipset, but it sure would help. (disclaimer: I know some chipsets provide more than one PCI bus, I'm just a little ticked more designers don't follow suit). >>


adding another bus would double the number of traces needed for PCI. look at current motherboards - see how packed they are? there are already many 6-layer motherboards, adding more increases the cost. if you're willing to pay, you can have anything you want 😉

IMHO, the PCI bus is not a major bottleneck. after all, what is on PCI? sound, network, hard drive. sound cards work fine on ISA - they don't need much bandwidth. gigabit NICs could use a lot of bandwidth, as could extremely fast hard drives (SCSI RAID), but neither are very mainstream yet. until you can get gigabit net connections easily, and fast storage, there is probably not much of a bottleneck in the bus itself.

 

[dvogel]

there are already many 6-layer motherboards, adding more increases the cost. if you're willing to pay, you can have anything you want

A motherboard with 6 PCI slots on 3 PCI buses wouldn't be much more packed than the same motherboard with all 6 PCI slots on one PCI bus.

...as could extremely fast hard drives (SCSI RAID), but neither are very mainstream yet.

And why aren't they? Because a single drive on an Ultra160 SCSI controller can easily fill two 32bit/33MHz PCI buses.

sound cards work fine on ISA - they don't need much bandwidth

Agreed... if you don't mind hisses and pops and wierd torn-record type sounds when the ISA bus is saturated.

 

[AndyHui]

Clock skew, complexity and expense. That's the short answer.

Now I'll just sit back and wait for Sohcan to blow all our answers out of the water.