Trying to understand a picture of computer buses

A bus is just a medium of communication with the following properties:

• Multiple entities can be connected to it
• If one entity sends a message or "does something" to the bus, every other entity can see it
• Bad things will happen if two entities try to communicate at the same exact time
• A protocol or set of rules is needed so that all components on the bus have a system where they can take turns using it. Usually this protocol is different according to the purpose and speed of the bus
• Some sort of addressing scheme is used where devices can say who they are and who they want to talk to
• Bad things will happen if multiple entities have the same address
• At the very least entites wanting to "talk" on the bus need to look to see if there is activity going on before they try to send data through it
• Entities wanting to "listen" on the bus generally need to listen for their own address and only snatch the data meaningful to them

If you have any knowledge of networking and most of this sounds familiar it's pretty much similar in concept.

The light blue lines represent a bus. The dark blue lines represent what is connected to the bus.

To answer your questions:

1. Looks to me like the CPU needs to go through the processor bus, northbridge and PCI bus to get to southbridge.
2. I believe they represent connects to the busses. To me it looks like the labels are identifying the thicker light blue lines. The diagram could be a bit better IMHO. Note that AGP stands for "Accelerated Graphics Port" - technically it's not a bus as multiple components don't come into play there (one of the whole reasons AGP was invented). To software it appears as another PCI bus, though.
3. I think so. IIRC device drivers, in order to access southbridge components, need to interact with PCI bus programmatically.
4. See my initial paragraph. It's possible for a bus to be connected to another bus and take on responsibility of forwarding data through it. These are what "PCI-PCI bridge" devices are if you've ever seen them in Windows Device Manager or lspci.

here's a picture from Ars Technica that may be clearer

There are only 3 "buses" in any computer: Data, Address and Control. That's it. That's a very simplistic top-down look at it. Data and address buses are pretty obvious and relatively simple. However, the control bus can get very complicated since it entails pretty much everything else including (and probably especially) timing.

What I'm seeing here is a basic systems chart. Certain things in the system are responsible for certain resources/processes. As you might expect, the CPU is at the top of the heap and in charge of pretty much everything. Right below (in hierarchy) there is the north bridge which directly controls video and RAM. The north bridge indirectly controls the south bridge via the PCI "bus" as well as LAN and SCSI systems too. However, the south bridge directly controls ISA, USB and IDE devices. So if you wanted to retrieve data on a IDE drive your CPU would go through the north bridge which then requests this over the PCI bus where to the south bridge in turn gets the IDE resource to provide it (or more accurately, the south bridge tells the IDE device when to place the info on the address/data bus - which the CPU is really in control of).

You're probably making this harder than it needs to be. The CPU is still the heart of every computer. Therefore, your diagram is a horrible example of any of the actual "buses" being used. In fact, the entire diagram might be considered to be a description of the control bus - and only the control bus. It is an excellent visual on what certain sub systems do and even what directly controls certain resources, but there is absolutely no indication of what's actually being hard wired or how an entire computer really works at least in terms of bus structure.

1. The northbridge ties the southbridge to the CPU and so there is no direct bus between the CPU and the SB.
2. Most of these "buses" have their own self-describing names, like ISA bus, PCI bus, etc. Others are less obvious such as the LPC bus which connects most of the low-bandwidth devices to the SB and thus the CPU (e.g. the Super I/O controller, BIOS, etc.).
3. No, this "bus" doesn't exist as described. But in the scenario of a communication to the devices in the lower-half of the diagram, data must pass through the "bus" from the CPU to the northbridge (I quote bus because the NB may be integrated on the CPU), and then again over what typically is a PCI bus to the SB, and vice versa for the round trip.
4. There's no one straightforward way to answer this question as processors today are becoming more complex and thus taking different approaches to memory, bus, and cache access. Most modern processors have integrated memory controllers, thus there is no need to talk to the northbridge for DMA. For instance, Intel's new processors with the QPI bus talk to a chip that is similar to a traditional northbridge except that it lacks a memory controller and talks to the CPU over the QPI bus which replaces the traditional front side bus [FSB].

I think this image from Wiki might be a more useful mneonmic device for you to learn from: http://upload.wikimedia.org/wikipedia/commons/b/bd/Motherboard_diagram.svg (can't embed SVG files).