How AMD's Fusion A8-3850 APU Changes Personal Computing

With the introduction of the A-series APU also known by it's codename Llano, AMD brought their Fusion technology to mainstream notebooks and desktops. In this review we will take a closer look at the desktop version of Llano. First we will go into detail about the technical side of things, then we will continue with various benchmarks and measurements and evaluate the APU as a whole.

Llano architecture

Llano marks the introduction of a Fusion product for mainstream PCs and notebooks. The first Fusion designs codenamed Ontario and Zacate targeted ultra-mobile and embedded usage models. Thus, AMD came up with a dedicated low power architecture, in some ways similar to Intel and their Atom line of CPUs. For the mainstream market this architecture would not really be suited for performance reasons, so AMD simply took their K10.5 architecture, which is used up to this point in their mainstream and high-end offerings for desktops and servers.

For the GPU part of Llano, AMD took the Redwood design, which was employed in the Radeon 5550, 5570 and 5670 products, all of which were launched in early 2010. As a descendant of the Evergreen-generation, the GPU features AMDs VLIW5 architecture. However, it was updated with the most recent UVD3 video decoder, adding support for 3D Bluray, MPEG2 and MPEG4 ASP (DivX / Xvid) bitstream decode. AMD also implemented more aggressive power gating than in the original Redwood design to optimize power consumption. Instead of the usual memory controller in the GPU, AMD designed special interfaces to the northbridge and the coherent request queues.

The CPU part did only receive minor optimizations. AMD claims a 6% IPC increase over previous K10.5 CPUs, which can mostly be attributed to the larger L2 cache, which is now 1MB per core. That is up 50% from the Propus core used in Athlon II X4 models. The DDR3 memory controller got updates as well and now supports speeds up to DDR3-1866. This speed is only available if you use no more than two DIMMs. DIMMs with capacities up to 16GB are supported, so Llano tops out at a maximum configuration of 64GB RAM.

We earlier mentioned that the GPU has two different interfaces to other parts of the chip. The direct link to the northbridge is used to access the UMA memory, that is dedicated to the GPU at boot time. The size of this UMA memory can be configured in BIOS, with options being 256MB, 512MB and 1GB. The other interface is used to access general RAM and the address space of other devices. Basically all communication from the rest of the system to the GPU is handled this way.

Platform-wise there are a few changes too. Ever since the introduction of the K8-based Athlon64 CPU, AMD employed a HyperTransport link to a northbridge providing a PCI-Express interface to dedicated graphics. This northbridge was either accompanied by a southbridge providing storage and other connectivity or it was directly integrated into the northbridge as seen on some NVIDIA chipsets. With the integration of most of the traditional northbridge features into the CPU this kind of setup no longer makes too much sense for notebook and desktop platforms.

Instead of a HyperTransport interface, Llano directly integrates a PCIe interface. 16 lanes of it are reserved for a discrete graphics card and the remaining four are used to connect to the Fusion controller hub. This connection is actually called Unified Media Interface, but the technology behind it is basically PCIe. This is remarkably similar to what Intel did with their Sandy Bridge generation. For the controller hub, there are two options available, the A75 and the A55. The A55 only provides SATA 3Gb/s support, while the A75 allows for 6Gb/s over the six ports it features. On top of that it integrates 4 USB 3.0 ports, something AMD is very proud of and even Intel has to acknowledge. The chipsets are codenamed Hudson D3 (A75) and Hudson D2 (A55) respectively which are manufactured at a 65nm process at TSMC.

The Llano chip is manufactured using Globalfoundries' 32nm SOI process in Dresden, Germany. There are a few remarkable things about that. Llano is actually the first design that entered volume production on this process and started shipping in the second quarter of this year. Also it is the first time AMD managed to manufacture a relatively current GPU architecture on a CPU manufacturing process, all integrated into a single die. The Bobcat-based Fusion products launched earlier this year were all manufactured on bulk silicon. The Llano die measures 228mm² and consists of roughly 1 billion transistors.