AMD exposes Bulldozer and Bobcat cores

With Fusion's grand entrance rapidly approaching, AMD has finally decided to stop being all secretive and start dishing out the details.

The company is revealing all the juicy gossip about its forthcoming Bobcat and Bulldozer cores at the Hot Chips symposium today, but it also kindly shared the information with us beforehand.

As you may already know, Fusion sees an entirely new approach to building CPUs, where a processor is constructed from modules. These modules can be anything from a CPU core, to a video decoding unit to an array of GPU stream processors. Bulldozer and Bobcat are specifically the CPU core modules, but these also show a very different design from AMD's previous CPU cores.

Let's start with Bulldozer, which as its name suggests, is the heavyweight core for servers and desktops. This 32nm silicon-on-insulator (SOI) core sees AMD finally taking on Intel's Hyper-Threading technology, but with a very different attitude.

AMD Bulldozer

Rather than executing two threads on a single core, where the threads will be jostling for resources, Bulldozer instead splits the load over two integer units. According to AMD, this is based on the principle that '80 per cent' of compute tasks use integer (whole number) calculations.

Each integer unit has its own allocation of Level 1 cache, but shares a block of Level 2 cache and a large floating point (for calculations with decimal points) unit with the other integer blocks. In addition to this, all the Bulldozer modules in a Fusion chip will have access to a shared pool of Level 3 cache at the chip level, as well as the memory controller.

Basically, you get nearly two whole processing cores inside a single Bulldozer module, which should enable it to crunch through two threads much more convincingly than an Intel core with Hyper-Threading. Whether it works in practice remains to be seen, but it's an efficient piece of design in theory.

According to AMD, Bulldozer dynamically switches between its shared and dedicated components in order to maximise the performance per watt, making it much more efficient in terms of both speed and power consumption. Several Bulldozer modules can then be chained together in a single CPU package, meaning a quad-core CPU can effectively execute eight simultaneous threads.

AMD Bulldozer

Bobcat on the next page...

Next we come to Bobcat, which is a completely different design again. After watching Intel's Atom CPU hoover up business in the netbook market, AMD has now engineered a module specifically for netbooks and thin and light laptops.

You don't get the two integer units as you do in Bulldozer; instead you get a pretty serious floating point unit and a single integer unit. In this case, the integer block has its own allocation of Level 1 data cache, but the Level 1 instruction cache is shared with the floating point unit. A pool of Level 2 cache is also shared between the two blocks.

AMD Bobcat

Perhaps more interestingly, though, Bobcat is able to perform out-of order execution, much like VIA's Nano CPU, meaning it can rearrange instructions to maximise performance. This usually comes at a cost of die space, which is why Intel decided to revert to in-order execution with its Atom CPUs. As such, this could potentially make Bobcat a much quicker mobile core than Atom.

Of course, it can only do this if it gets the power consumption down as well, but AMD claims that a Bobcat core can happily run while consuming less than a single Watt. In addition to out-of order execution, AMD also says that the 64-bit Bobcat core features an advanced branch predictor, a high-performance floating point unit and support for SSE 3.

AMD Ontario

As with Bulldozer, Bobcat is just one component of a new Fusion CPU, and AMD provided a snapshot of its forthcoming Ontario package to show where it fits in. Along with the CPU cores, an Ontario CPU also features a 'dedicated SIMD array' (GPU), a unified video decoder and an integrated memory controller.

It's also worth noting that not all Fusion CPUs will be based on the new processor cores. AMD points out that while its forthcoming Llano chips will feature an integrated DirectX 11 GPU, the CPU cores themselves will be based on the company's older K8 architecture.

AMD says both new cores will be released in 2011.