Unleash the tiger!
EnthusiastPC

Basic PC architecture

1


The basic components that make your PC work


Okay, a little while ago I asked for input on what guides to write next and thank all of you for a lot of reactions! Most asked for were graphics cards, motherboards, SSDs and a basic overview. I will address all of these (and more) in separate writeups, but to be able to understand why one component may be better than another we'll need to do the overview first.

Basic components


I think most of us know what the basic components inside the computer are but I'll list them first and then go into detail on each of them:


The CPU, mainboard and memory are closely tied together so we'll be discussing these three in this article. Your choice of CPU architecture determines what type of mainboard you'll need and mainboard architectures between AMD and Intel are different. The CPU choice also determines the type of memory you'll need.

The mainboard (AMD)


For AMD mainboards we'll mostly be talking about the latest generation of socket AM3+ mainboards. Most of the design however is identical to older AMD mainboards. Please bear in mind that the only real AM3+ boards are the ones with an AMD 9xx based chipset. When we talk about the various features of AMD boards here we are referring to 9xx based boards. If you have an older chipset you may not have all of these features.

AMD based mainboards have 2 chips that comprise the mainboard chipset: the northbridge chip and the southbridge chip.

The Northbridge


The northbridge on AMD mainboards is mainly home to the PCI express controller. This chip therefore determines how many total PCI express lanes can be used on a particular mainboard. This is why 990FX boards can do x16/x16 dual graphics and 990X and 970 boards can only do x8/x8 or single x16 graphics. The northbridge also houses the IOMMU unit which is a type of memory controller that allows other chips direct access to the computer's main memory in a flexible manner.

The Southbridge


The southbridge on AMD mainboards houses the AMD SATA controller, AMD USB2 controller, the controller for the legacy PCI bus and hosts and several single lane PCI express connections. It also connects to the onboard audio chip on the mainboard and the onboard network controller(s) on the mainboard as well as the legacy I/O controller chip which houses things like the old parellel printer port, serial port and PS/2 mouse/keyboard connectors. The way these items are wired up to the southbridge is either via an internal PCI bus connection or an internal PCI express connection. To the operating systems these devices appear the same way as when they would be on an expansion card in a PCI or PCI express slot.

In AMD architecture the CPU houses the memory controller and so the CPU connects directly to the DIMM slots from it's socket. The CPU also connects directly to the northbridge chip but communicates with the southbridge chip and any devices wired to it via the northbridge chip. For reference, here is a schematic overview of how it all ties together:
Socket AM3 / AM3+ architectural overview (890FX/990FX).

In AMD architecture the connection between the CPU and the northbridge is what is called a HyperTransport link. This name is not unique to this connection, in multiprocessor (multiprocessor, not multicore!) AMD systems the links between the processors are also HyperTransport links.

The communication link between the northbridge and southbridge chips is called Alink Express (version 3 in the case of the 9 series chipsets). This name is unique to that specific connection.

The mainboard (Intel Sandy Bridge socket LGA 1155 with P67/Z68 chipset)


One of the things you'll immediately notice when looking at the Intel mainboard diagram is the absence of a northbridge chip:

Socket LGA1155 Sandy Bridge architectural overview (P67/Z68).

Sandy Bridge socket 1155 systems don't have a northbridge chip and instead the PCI express connections that are used for the graphics card(s) run directly from the CPU to the slots. The southbridge chip connects directly to the CPU and houses the same basic functionality as the southbridge chip in AMD based systems.

On the 1155 platform the link between the CPU and the northbridge (Z68/P67) chip is called a DMI link. This same link is used on the older X58 enthusiast platform for communication between the northbridge and southbridge chips.

The mainboard (Intel socket 1366 with X58 chipset)


The older (but still very much up to date for it's age) socket 1366 with the X58 chipset strongly resembles the current AMD architecture. There is a Northbridge and a Southbridge chip, both of which house the same basic functionality as they do in the AMD Northbridge and Southbridge chips.

Socket LGA1366 architectural overview (X58).

The mainboard (Intel Sandy Bridge Extreme, socket LGA 2011 with X79 chipset)


Sandy Bridge E with the new and massive socket LGA 2011 as well as the new X79 chipset is a lot beefier than the mainstream LGA 1155 platform in almost every respect. It has a high number of PCI Express lanes running directly from the processor, double the memory bandwidth and the processors come in quad and hexacore variants. The CPUs all have 10MB cache, an almost 67% increase over the mainstream Core i5! The PCI Express controller on the CPU die is wired up slightly differently so that it has lower latency than it's older Sandy Bridge counterpart and does a speedy 1GB/s per PCI Express lane of which it has a total of 40! The diagram shows a clear difference in all the stuff connecting directly to the CPU although the X79 Southbridge has the same facilities as the current P67 and Z68 chipsets. The exception here is probably the SSD caching feature although that appears to be an all-software solution that is programmed to work only on Z68.

Socket LGA2011 architectural overview (X79).

In general


For all architectures discussed here (AM3+, LGA 1155, X58 and LGA 2011) processors the memory controller resides on the CPU. The CPU therefore determines whether dual channel, triple channel or quad channel memory is supported. Obviously the communication lines from the CPU to the memory need to be on the mainboard as well. This is one of the reasons that socket 1366 with triple channel memory has more pins (1,366 pins) than Sandy Bridge (1,155 pins) which only supports dual channel. Sandy Bridge E (socket 2011) has a massive pin count of 2,011. Although officially, socket 2011 supports PCI 2.x tests have recently confirmed that it runs successfully on 3.0 speeds with the new HD7970 graphics card from AMD.



Next Page