Since Microsoft's announced Xbox 360 would support backwards compatibility with "top selling Xbox titles" at the Electronic Entertainment Expo, the company has kept quiet about the details of the feature. A Bit-Tech interview with ATI's European Developer Relations Manager Richard Huddy has revealed more about Microsoft's progress.
The original Xbox was built upon an Intel CPU and Nvidia GPU, whereas Xbox 360 utilizes an IBM CPU and ATI GPU. Games on the Xbox taking advantage of Nvidia-specific coding would have trouble being emulated on the Xbox 360 hardware.

"Emulating the CPU isn't really a difficult task. They have three 3GHz cores, so emulating one 733MHz chip is pretty easy," said Huddy. "The real bottlenecks in the emulation are GPU calls - calls made specifically by games to the Nvidia hardware in a certain way. General GPU instructions are easy to convert - an instruction to draw a triangle in a certain way will be pretty generic. However, it's the odd cases, the proprietary routines, that will cause hassle."

Also, Huddy confirms rumors that backwards compatibility has not been a part of Microsoft's plans for very long. It reportedly wasn't in the original design specification, as the company didn't want to compromise their next-generation hardware catering to their previous architecture. While that doesn't appear to be happen, the rapid inclusion of backwards compatibility seems to have complicated matters.

Instead, emulation has been coded at the CPU level, and Huddy believes emulation profiles will be stored on the Xbox 360's hard drive. "I imagine that a certain number will ship with the system," he said. "They already have the infrastructure to distribute more profiles via Live, and more and more can be made available online periodically."

Microsoft and Nvidia could not be reached for comment as this went to press. Stay tuned to 1UP.com for more.

Whilst ostensibly Xenos has been hailed as a Shader Model 3.0 part, its capabilities don't fall directly inline with it and exceed it in some areas giving this more than a whiff of WGF2.0 (Windows Graphics Foundation 2.0 - the new name for DirectX Next / DirectX 10) about it.

As each of the three shader ALU arrays is a separate array there is no dependency between one another so what programs are being executed on them at any one point in time is completely independent - at a snapshot in time they could, potentially, all be vertex processing, all be pixel processing or there can be a mixture of both vertex processing and pixel processing occurring on the three different 16 ALU arrays. The arrows on the diagram above indicates that there is some dependency from one of the shader arrays to another, almost as though they are pipelined; this is in fact not the case and each ALU array is working independently of the other and the data is not pipelined between them.

The combination of the shader array and tessellation unit can now make the, oft spoken of but rarely seen, capability of displacement mapping an attainable method to use as this truly becomes a single pass algorithm for Xenos. A simple primitive can be sent to the tessellation unit which is then subdivided into a vertex mesh and then that can be applied to a vertex shader program that does displacement map lookups via the vertex fetch texture units and then the geometry mesh altered according to the sampled values from the texture sampler. Alternatively, if the screen-space projection of the input primitive to the tessellation unit is calculated prior to tessellation then the per-edge tessellation level can be figured out dependant on that projection such that displacement mapping with correct, dynamic level of detail can be achieved.

As the CPU is going to be using Xenos to handle all its memory transfers, the connection between the two has 10.8GB/s of bandwidth both upstream and downstream simultaneously. Additionally the Xenos graphics processor is able to directly lock the cache of the CPU in order to retrieve data directly from it without it having to go to system memory beforehand. The purpose of this is that one (or more, if wanted) of the three CPU cores could be generating very high levels of geometry that the developer doesn't want to, or can't, preserve in the memory footprints available on the system when in use. High-resolution dynamic geometry such as grass, leaves, hair, particles, water droplets and explosion effects are all examples of one type of scenario that the cache locking may be used in.

The ROP's can handle several different formats, including a special FP10 mode. FP10 is a floating point precision mode in the format of 10-10-10-2 (bits for Red, Green, Blue, Alpha). The 10 bit colour storage has a 3 bit exponent and 7 bit mantissa, with an available range of -32.0 to 32.0. Whilst this mode does have some limitations it can offer HDR effects but at the same cost in performance and size as standard 32-bit (8-8-8-8) integer formats which will probably result in this format being used quite frequently on XBOX 360 titles. Other formats such as INT16 and FP16 are also available, but they obviously have space implications. Like the resolution of the MSAA samples, there is a conversion step to change the front buffer format to a displayable 8-8-8-8 format when moving the completed frame buffer portion from the eDRAM memory out to system RAM.

12 June 2005 - Microsoft has announced that it is planning on offering a subscription download music service in an attempt to claw back some of the ground lost between it and Apple’s iTunes service.

Microsoft, which launched its MSN Music download service last year to rival Apple's offering, said on Friday that "offering a subscription service was an important step in helping people find and discover great music online."

The subscription service is expected to be run similar to way to Napster or RealMedia’s current offering. It would allow people to download unlimited tracks on a monthly basis for one set fee to be played on a number of different media devices including perhaps we think the Xbox 360.

Reports that Microsoft was getting closer to launching such a service, hit share prices of Napster by three percent and Apple by five percent.

The chaps over at Eurogamer have got a great interview with Paul Sams, VP of Business Operations at Blizzard. Paul goes into some great detail about the future of World of Warcraft, which will surely be of interest to the thousands of addicts that I know will be reading this!

WoW has "Far surpassed our expectations," he unsurprisingly comments. With 1.5m+ paying players right now, WoW has had an unbelievable start to its life. However, Paul does admit that "Our first weekend numbers were so gigantic... I don't want to say that it took us by surprise, but in some respects it really did... We were able to respond and provide additional hardware very quickly." I think some of the first American players might dispute that!

Sams states that Blizzard will "Most certainly" be working on paid expansion content in the near-term future.

As for next-gen, Blizzard is "Exploring any variety of possibilities," and says it has been approached by Microsoft to put WoW on the Xbox 360 - but is hesitant because it would require a keyboard and mouse to be sold or attached separately.

Of course, every one is dying to know about Diablo 3 - is it in the works? "I think that there are definitely some new project announcements coming in the future..." With all the speculation that music for the title has already been recorded, we certainly hope that the beloved RPG makes a comeback.

Microsoft has signed a deal with NVIDIA to license the company's technology in order to enable backwards compatibility in the Xbox 360, which uses an ATI graphics chipset that isn't natively compatible with Xbox titles.

The question of how to get Xbox 360 - which uses a radically different architecture to the Xbox - to play Xbox titles has been a major problem for Microsoft, and it's rumoured that backwards compatibility wasn't actually part of the original specification of the console.

Indeed, it wasn't even confirmed that the system would be able to play Xbox games until E3 this year, when Microsoft announced that "best-selling" Xbox titles would work on the Xbox 360 - a curious piece of double-speak which the software giant refused to clarify fully.

Following the announcement, sources close to the company indicated that a form of recompilation (known as "transcompilation") would be required to make Xbox games work on the 360, with the resulting patched executables being shipped on the system's hard drive for certain popular games, and patched versions of other games gradually being added over the Xbox Live network.

Speaking to GamesIndustry.biz at the time, Microsoft Xbox PR manager Michael Wolf denied that games would need to be recompiled, and assured us that the company had no intention of charging for updates to the backward compatibility service. However, he refused to answer further questions about how the process will work.

Now a report on US website 1up confirms that Microsoft has in fact licensed parts of NVIDIA's technology from the Xbox to allow it to emulate the chipset in Xbox 360, a deal which will see Microsoft continuing to make payments to NVIDIA for several years.

The company apparently plans to emulate the Xbox' Intel CPU on the 360's multiple IBM PowerPC cores, and will now be able to emulate the NVIDIA chip to some extent on the ATI graphics chip in the 360 - but it would appear that many games will still not work without some modification.

Patches for some popular games - or "emulation profiles" - will ship on the Xbox hard drive, and those games will as a result work out of the box. Microsoft is expected to work to get emulation working for other games over time, and will distribute further emulation profiles over the Xbox Live service.

What's not clear is how users without Xbox Live access - around 90 per cent of users on the current generation Xbox, for reference - will be able to update those profiles, and whether Microsoft will ever get the full range of Xbox software working on the new console.

Wolf declined to answer the first of those questions when we put it to him last month, but was more positive on the second issue, telling us that "our goal is to make ALL Xbox games play on Xbox 360, and at launch we'll have a selection of the top selling that will be tested and confirmed to work."