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An in-depth look at the Xbox One’s innards: SRAM is the reason for the small GPU

For months, Sony and Microsoft fanboys have lined up to hurl insults at each other over which console would pack the best hardware, hit higher performance targets, or prove a better design for the next generation. With the two consoles now out in the US (and the Xbox One launched, and PS4 due to come out this Friday in the UK), the game-to-game comparisons have mostly come out a wash, with a slight edge for the PlayStation 4. But there have still been questions about the underlying chip design – which architecture is more efficient, and what unique sauce went into each console?

Fortunately, the fine folks at Chipworks have completed their teardown of the Xbox One and given us an answer to that question – and a few puzzles to go with it.

The Xbox One die is 363 square millimetres, up from the PS4′s 348 sq mm. The 5 per cent additional space, despite having the smaller GPU core, is mostly due to RAM. The Xbox One contains a whopping 47MB of on-die RAM, and that pushes the die size up considerably. It’s also why Microsoft didn’t have room on the APU for a larger GPU. (See the Xbox One APU die shot, by Chipworks, below).

There are some interesting differences to explore. First, consider the Xbox One’s Jaguar CPU blocks. Like the PS4, it has two quad-core chips – but the Xbox One has a bit of circuitry hanging off the CPU that the PS4 lacks. Here’s a comparison of the Xbox One (on the left) and PS4 CPU islands. We had to rotate the blocks to line them up identically, which is why the label is reversed in the image below.

See the block in red? The PS4 doesn’t seem to have an equivalent. What it actually does is unclear. It’s a bit large to be the built-in audio or the IOMMU that HSA theoretically requires. There’s nothing analogous on any of the Kabini floor plans we’ve ever seen.

(Note that it’s also possible that this is a Photoshop artifact or deliberate obfuscation. Companies often mask details on die shots).

Now, over to the GPU. Like the Sony PS4, the Xbox One contains more Compute Units than are actually active on the console. The chip has 14 CUs, 12 of which are turned on, while the PS4 has 18 active CUs out of 20 on-die. These are disabled to improve yield. Whether Sony or Microsoft might one day choose to enable the CUs in future console versions is unknown – typically console manufacturers don’t update core specs post-launch, but consoles have been trending towards greater upgradeability over the past two generations. It’s not impossible that this could change.

The other mystery? The Xbox One GPU cores are physically shorter than the PS4′s equivalents. I don’t mean the GPU block, which is obviously smaller – one GPU Compute Unit on the PS4 diagram is 50 pixels wide and 395 pixels tall. On the Xbox One, each Compute Unit is 42 pixels wide and 347 pixels tall. It looks as though Microsoft may have picked a tighter arrangement for its GPU core, again possibly to save the maximum amount of space and make room for as much SRAM on die as possible.

Speaking of SRAM, the arrangement of the Xbox One’s was a considerable puzzle when Microsoft unveiled the console architecture. According to the company, the Xbox One doesn’t really have a 32MB contiguous cache, but four 8MB cache blocks instead. There are two blocks of cache to the right of the GPU and a smaller block to the left. This smaller block is possibly used for cross-CPU communication.

That leaves about 10MB of cache missing. If the SRAM block between the two CPUs is that large, it’s far more dense than the SRAM to the right of the GPU.

Chipworks also tore into the Xbox One controller, but it’s not that interesting. It has an ultra-low power Freescale microcrontroller and a Cortex-M0+ core. A custom Microsoft Wi-Fi chip handles communication with the mother ship. The chip count here is kept minimal to speed manufacturing and lower cost.

A teardown of Kinect should be up and available in the not-too-distant future. (The PS4 APU die shot is below, for comparison with the Xbox One).

Different designs lead to similar places

After looking at both the Xbox One and PS4, I think we see companies arriving at the same destination via rather different approaches. Both manufacturers chose architecture they felt would allow them to work most effectively. Microsoft invested more silicon in large, low latency caches, while Sony sank more money into raw bandwidth. As far as performance is concerned, this could well end up a tie – the Xbox One should be able to access data more quickly, while the PS4 can stream sustained data far more effectively. Since game developers can leverage both of those features, the final result could be a wash.

Both companies also picked designs that should be relatively easy to migrate to new process nodes. As 20nm technology comes online, we’ll probably see refreshes in 12-18 months. It won’t surprise me if the first SSD designs start to pop up then, too – there’s too much potential upside in a premium SKU with solid state storage for either company to ignore the possibility.

For more on Microsoft's console, see our closer look at the cost of building the Xbox One. You might also want to have a look at our review of the console, and our piece comparing the Xbox One and PlayStation 4 in terms of hardware, launch games, and media features.

Image Credits: Chipworks