Inside AMD64 Architecture

Introduction

In this tutorial we will explain you how AMD64 architecture – which is used by Athlon 64, Athlon 64 X2, Athlon 64 FX, Opteron, Turion 64 and some Sempron models – works in an easy to follow language. You will learn exactly how this architecture – also known as K8 or hammer – works so you will be able to compare it more precisely to competitor architectures from Intel.

In order to continue you need to have read our tutorial “How a CPU Works”. In this tutorial we explain the basics about how a CPU works. In the present tutorial we are assuming that you have already read it, so if you didn’t, please take a moment to read it before continuing, otherwise you may find yourself a little bit lost. Actually we can consider the present tutorial as a sequel to our How a CPU Works tutorial. You may also be interested in reading tutorials about other CPUs, so you can compare AMD architecture to Intel’s: Inside Pentium 4 Architecture, Inside Pentium M Architecture and Inside Intel Core Microarchitecture.

The main difference between AMD64 architecture and the design of other CPUs – including previous CPUs from AMD, like Athlon XP and the original Athlon – is that the memory controller is embedded in the CPU, and not on the north bridge chip (the main chip on the motherboard chipset). So, on motherboards targeted to CPUs based on AMD64 architecture the “north bridge” chip is just a bridge between the CPU and the graphics bus of choice (AGP or PCI Express) and the south bridge chip. Since this “north bridge” is simpler to be made, some manufacturers have single-chip chipset models for AMD64 CPUs.

Since the memory controller is embedded in the CPU, the memory capacity – including memory types supported and support for dual channel – is defined by the CPU and not by the north bride (i.e., by the motherboard), as it happens with CPUs based on other architectures. A side effect of this architecture is that motherboards for AMD64 CPUs don’t have noticeable performance difference between them, as all of them use the very same memory controller (the one inside the CPU). This statement is only true for motherboards without on-board video, as motherboards with embedded graphics have an on-board video controller, which is outside the CPU, so its performance varies depending on the video controller used.

In Figure 1, you can see the architecture used by other CPUs, while in Figure 2 you can see the architecture used by AMD64 CPUs.

Figure 1: Architecture used by other CPUs.

Figure 2: Architecture used by AMD64 CPUs.

We can say that the north bridge chip is embedded inside the CPU. On the motherboard you will find a bridge chip, which will make the proper interface between the HyperTransport bus (i.e., the CPU), the video card bus (AGP or PCI Express x16) and the south bridge. Sometimes the chipset manufacturer builds this bridge chip and the south bridge in just one chip. This is what is called “single-chip solution”.

The memory controlled embedded in the AMD64 processors can drive up to four memory modules per channel. So on a dual-channel system it can drive eight memory modules. The number of sockets available on the motherboard is limited by the motherboard manufacturer design.