Intro to Computer Systems

Chapter 2: System Basics

System Architecture

Regardless of the computing platform, there is a common design philosophy in how the internal components of a desktop computer are designed to work together. These core components are put together on a circuit board known as the motherboard, (also known in some references as a logic board or planar board).

A modern computer motherboard.
A computer motherboard: Photo: Gigabyte Co.

The Motherboard

The motherboard (aka logic board) is the circuit board on which the core internals of the computer are placed. The picture below has been highlighted and annotated to show where each of the main components are, which we'll explain shortly.

The core internals of the computer system are laid out on the motherboard.
The core internals of the computer system are laid out on the motherboard. Source Image: Gigabyte Co.

Each motherboard is unique in its exact layout, however there are standards in their physical design (such as size, mounting holes, where ports will be) to ensure that motherboards consistently fit into computer cases. Two such standards exist for modern desktop PCs; ATX and BTX. "Small form factor" computers tend to have unique layout standards.

System Design

The core components can be diagramatically reperesented as such; the modern system design (with a greater level of component integration) is that on the left.

The core internals of the computer system shown in a simplified diagram, showing how they are connected. Modern, highly-integrated system to the left, legacy discrete north/southbridge system to the right.
The core internals of the computer system shown in a simplified diagram, showing how they are connected.
Modern, highly-integrated system to the left, legacy discrete north/southbridge system to the right.

The CPU is the processing core of the computer. As it needs to interact with a large number of different interfaces (computer memory, disk drives, displays, and so on) it requires a supporting chipset to be able to connect with these devices in a cohesive, efficient manner.

Chipsets

Legacy systems (as depicted on the right side of the diagram above) often broke this task up into two discrete chips, known as a northbridge and southbridge. The northbridge connected directly to the CPU, and provides it with high-speed access to the computer memory and graphics card, two devices that the CPU needs the fastest communication with.

The reason why the chipset was split in this way is to improve performance: by placing the most speed-critical components "closest" to the CPU, they can be accessed more quickly (a metric known as latency -- how long it takes to access a certain peripheral device).

The southbridge connected to the northbridge, and provides all other functionality that is not so time critical (relatively speaking; we're talking in terms of nanosecond delays here). The southbridge chip provided connections to other peripherals both inside and outside the computer case, including:

Increased Integration: Modern Chipset Architecture

In electronic systems, it makes good sense to integrate devices (i.e. put all the functions onto a single chip) in order to boost performance and decrease latency. When the northbridge-southbridge model was used, it was not realistically possible to host so many features on a single silicon die without serious manufacturing and heat dissipation issues.

As IC manufacturing processes improved, it gradually became possible to integrate more features onto fewer chips.

Some systems, notably those for embedded systems and small mobile computers such as tablets and smartphones, have the entire computer system - CPU, memory controller, graphics controller, I/O, and even system memory - on a single physical chip. These are called SoC (System on a Chip) integrated circuits.

Actual System Designs

The system diagrams below are of actual computer systems. Note the high level of integration on the newer Intel X79 chipset (pictured on the left), in comparison to the older PowerPC G5-based system diagram on the right.

An Intel 925X motherboard system diagram.   The Apple iMac G5 system diagram.
The system diagrams for an Intel X79-based motherboard (left) and Apple's legacy iMac G5 computer (right).
Click the images for a larger view. Source: Intel Corporation, Apple Computer