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I am trying to understand computer architecture. In particular: - Physical view i.e. what all is packed inside the motherboard and what all outside - Conceptual view. how processor,memory,peripherals are connected.

I am getting confused among various buses like local bus,PCI bus, SCSI bus,ISA bus,USB bus. I am looking for block diagrams. How is the USB port connected to processor ultimately? through PCI bus etc? Why do we have so many buses? What was it like before SCSI/IDE came?

Does the diagram at :

http://www.vaughns-1-pagers.com/computer/pc-block-diagram.htm

look correct? It shows no connection between PCI bus and SCSI bus. Is that correct?

I have found another at:

www.christiealwis.com/Knowledge%20Sharing/computer_buses.pdf

If disks connected through SCSI interface are anyway in turn go to PCI bus, then speed would be limited by PCI bus then why not connect these disks directly to PCI bus. Why is SCSI bus needed in the first place?

I would greatly appreciate any other link especially of block diagrams/anatomy and not just textual writeup.

Thanks,

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I stopped counting after 6 errors. – Ignacio Vazquez-Abrams Jan 10 '11 at 13:46
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"updated on 2010-03-19" my ass. That hasn't been updated since the early 90s. And even then it would be very wrong. – Ignacio Vazquez-Abrams Jan 10 '11 at 13:55
@Ignacio Vazquez-Abrams Well if you even know of one correct diagram, it'd be constructive to say. – barlop Jan 10 '11 at 14:15
I am not sure about now and things do depend on technology of the time.. and I would be suprised if anybody gave you a good answer covering different times. You could look up for a particular motherboard chipset, see a northbridge and a southbridge, and the busses. others with a northbridge and southbridge would probably be similar. Certainly in the P4 and AMD Athlon X2 days, but I know then AMD came up with one, maybe the Athlon 64, that didn't use that style. You can get a pic of that too. – barlop Jan 10 '11 at 14:21
I can tell you, ISA is old long gone. ISA is ancient. ISA was before PCI. PCI replaced it. You won't see ISA on any motherboard now. Even over 5 years ago ISA was ancient.. even 10 years ago ISA was at least old. ISA is totally finished. – barlop Jan 10 '11 at 14:35
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alt text

That above is a nice pic. From the P4 days, so a bit old, but simple and clear. I have included a more modern one too but this one helps understand the basic structure a bit more since there's less on it, and it's good to have a few pics, and you rightly wanted a few.

Nothing on there looks very strange to me, it looks as I remember from previous pictures i've seen!

You see the processor/CPU. Then you see 2 squares. The one near the CPU is called the northbridge. The one below it is called the southbridge.

To answer your questions "I am getting confused among various buses like local bus,PCI bus, SCSI bus,ISA bus,USB bus. I am looking for block diagrams. How is the USB port connected to processor ultimately? through PCI bus etc? Why do we have so many buses? What was it like before SCSI/IDE came?..." "vaughn diagram It shows no connection between PCI bus and SCSI bus. Is that correct?".. "If disks connected through SCSI interface are anyway in turn go to PCI bus, then speed would be limited by PCI bus then why not connect these disks directly to PCI bus. Why is SCSI bus needed in the first place?"

"local bus" , the term you use, might not mean anything.

The bus connecting the CPU to the northbridge, is I think called the FSB (front side bus)

ISA is way too old, not on this pic(And this is an old pic P4 is old). ISA was replaced by PCI long long ago. Even 15 years ago ISA was probably going out.. it has been finished for years.. you won't see it now.

SCSI is not on this diagram. It was common in servers.. http://en.wikipedia.org/wiki/SCSI says that with the advent of SATA, SCSI is being discontinued. I suppose IDE/(P)ATA had weaknesses that SCSI didn't.

AGP is connected to the northbridge. Though AGP is old. I guess that line in the diagram is the AGP bus

RAM is connected to the northbridge. I suppose that "line" connecting it is called the memory bus.

ATA is on the southbridge. IDE is a colloquial even amongst techies, way of referring to PATA/P-ATA. People started calling IDE "PATA" instead of just ATA, once SATA came out.

PCI is on the southbridge.

LAN is on the southbridge.

Really there isn't a lot connected to the northbridge, other than RAM. THere was AGP but it is old. Now though there is PCIe..

PCIe i'm not sure it may be possible for it to be on either.. see this pic
alt texthttp://www.pcstats.com/articleimages/200801/foxdigtalifeX38a_X38Block.gif

Chipset designs do vary, so it's worth looking for a few diagrams and trying to see if there are similarities. But I think you may find that anything with a northbridge and southbridge follows that pattern.

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Thanks a ton! Awesome reply. Pretty useful. One question that immediately strikes me is why is Flash BIOS connected to southbridge? Wouldn't it be better if it was connected to fast northbridge. Flash BIOS is needed for initial startup of system, right? and not usually used once the system gets started. Don't we want the initial start-up fast? – p2pnode Jan 10 '11 at 15:11
Also is there something like we have a separate math/floating co-processor apart from the main processor? Does this sentence mean anything. I vaguely remember these keywords. – p2pnode Jan 10 '11 at 15:17
@learnerforever Re BIOS,I suppose it's more the territory of an engineer,and so most of us including both of us,are in the dark, but i'm pretty sure that speed of access to the BIOS isn't an issue. Have you ever found BIOS access slow?of course not.There isn't much data there anyway and it's probably not frequently accessed.I suppose the benefit of the northbridge having not much on it other than RAM,is that it can focus on that without as much interruption or resource use as it'd get with many other things connected to it. Huge amounts of data need to frequently travel between the CPU and RAM – barlop Jan 10 '11 at 15:35
@learnerforever Re maths. Yes, there are pictures in old CS(computer science) textbooks I have. I think even then though, what you refer to, they call the ALU(arithmetic and logic unit). And that is part of the processor. So there are diagrams showing CPU(processor) registers, and other parts e.g. the CU, the ALU yoram.info/ICE/images/Pictures%20and%20Diagrams/… – barlop Jan 10 '11 at 15:40
@learnerforever: Way back when (and on a few obscure modern platforms as well, I think) the FPU used to be discrete from the CPU, and there was a dedicated bus between the CPU and the FPU. – Ignacio Vazquez-Abrams Jan 11 '11 at 3:27
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