Memory Subsystem

With the same underlying CPU and GPU architectures, porting games between the two should be much easier than ever before. Making the situation even better is the fact that both systems ship with 8GB of total system memory and Blu-ray disc support. Game developers can look forward to the same amount of storage per disc, and relatively similar amounts of storage in main memory. That’s the good news.

The bad news is the two wildly different approaches to memory subsystems. Sony’s approach with the PS4 SoC was to use a 256-bit wide GDDR5 memory interface running somewhere around a 5.5GHz datarate, delivering peak memory bandwidth of 176GB/s. That’s roughly the amount of memory bandwidth we’ve come to expect from a $300 GPU, and great news for the console.

Xbox One Motherboard, courtesy Wired

Die size dictates memory interface width, so the 256-bit interface remains but Microsoft chose to go for DDR3 memory instead. A look at Wired’s excellent high-res teardown photo of the motherboard reveals Micron DDR3-2133 DRAM on board (16 x 16-bit DDR3 devices to be exact). A little math gives us 68.3GB/s of bandwidth to system memory.

To make up for the gap, Microsoft added embedded SRAM on die (not eDRAM, less area efficient but lower latency and doesn't need refreshing). All information points to 32MB of 6T-SRAM, or roughly 1.6 billion transistors for this memory. It’s not immediately clear whether or not this is a true cache or software managed memory. I’d hope for the former but it’s quite possible that it isn’t. At 32MB the ESRAM is more than enough for frame buffer storage, indicating that Microsoft expects developers to use it to offload requests from the system memory bus. Game console makers (Microsoft included) have often used large high speed memories to get around memory bandwidth limitations, so this is no different. Although 32MB doesn’t sound like much, if it is indeed used as a cache (with the frame buffer kept in main memory) it’s actually enough to have a substantial hit rate in current workloads (although there’s not much room for growth).

Vgleaks has a wealth of info, likely supplied from game developers with direct access to Xbox One specs, that looks to be very accurate at this point. According to their data, there’s roughly 50GB/s of bandwidth in each direction to the SoC’s embedded SRAM (102GB/s total bandwidth). The combination of the two plus the CPU-GPU connection at 30GB/s is how Microsoft arrives at its 200GB/s bandwidth figure, although in reality that’s not how any of this works. If it’s used as a cache, the embedded SRAM should significantly cut down on GPU memory bandwidth requests which will give the GPU much more bandwidth than the 256-bit DDR3-2133 memory interface would otherwise imply. Depending on how the eSRAM is managed, it’s very possible that the Xbox One could have comparable effective memory bandwidth to the PlayStation 4. If the eSRAM isn’t managed as a cache however, this all gets much more complicated.

Microsoft Xbox One vs. Sony PlayStation 4 Memory Subsystem Comparison
  Xbox 360 Xbox One PlayStation 4
Embedded Memory 10MB eDRAM 32MB eSRAM -
Embedded Memory Bandwidth 32GB/s 102GB/s -
System Memory 512MB 1400MHz GDDR3 8GB 2133MHz DDR3 8GB 5500MHz GDDR5
System Memory Bus 128-bits 256-bits 256-bits
System Memory Bandwidth 22.4 GB/s 68.3 GB/s 176.0 GB/s

There are merits to both approaches. Sony has the most present-day-GPU-centric approach to its memory subsystem: give the GPU a wide and fast GDDR5 interface and call it a day. It’s well understood and simple to manage. The downsides? High speed GDDR5 isn’t the most power efficient, and Sony is now married to a more costly memory technology for the life of the PlayStation 4.

Microsoft’s approach leaves some questions about implementation, and is potentially more complex to deal with depending on that implementation. Microsoft specifically called out its 8GB of memory as being “power friendly”, a nod to the lower power operation of DDR3-2133 compared to 5.5GHz GDDR5 used in the PS4. There are also cost benefits. DDR3 is presently cheaper than GDDR5 and that gap should remain over time (although 2133MHz DDR3 is by no means the cheapest available). The 32MB of embedded SRAM is costly, but SRAM scales well with smaller processes. Microsoft probably figures it can significantly cut down the die area of the eSRAM at 20nm and by 14/16nm it shouldn’t be a problem at all.

Even if Microsoft can’t deliver the same effective memory bandwidth as Sony, it also has fewer GPU execution resources - it’s entirely possible that the Xbox One’s memory bandwidth demands will be inherently lower to begin with.

CPU & GPU Hardware Analyzed Power/Thermals, OS, Kinect & TV
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  • THizzle7XU - Wednesday, May 22, 2013 - link

    Ignite is only the name of their next gen game engine. Reply
  • cbrownx88 - Wednesday, May 22, 2013 - link

    Negative - they have an exclusive "launch on xbox FIRST" deal... No way EA would allow those franchises to be exclusive Reply
  • Friendly0Fire - Wednesday, May 22, 2013 - link

    Ghosts only has timed exclusivity on DLC. The game will see simultaneous release on all platforms. Reply
  • lmcd - Wednesday, May 22, 2013 - link

    What are you smoking? It's news but not surprising that EA is dumping the Wii U, but the PS4? That would be insane. And in fact, such a thing has not happened.

    Same with COD:G. AQll of the ones you have mentioned are cross-platform.
    Reply
  • blacks329 - Wednesday, May 22, 2013 - link

    Here is what is exclusive regarding EA Sports titles and CoD Ghosts.

    X1 will receive the Ultimate Teams feature exclusively. They haven't mentioned what that is. But the games themselves will release simultaneously on 360, PS3, X1, PS4 (assuming the last two arrive before the sports title is released).

    X1 will receive the CoD Ghosts DLC pack first, before it is released on PS4. Similarly to how it has been with every release of COD this gen for the past few years. CoD Ghosts will release simultaneously on 360, PS3, X1 and PS4.
    Reply
  • Inteli - Wednesday, May 22, 2013 - link

    I'm personally waiting to see someone strip Xbox OS from this and stick plain-old Windows on it. This might make a good HTPC without having to deal with any sort of specialized OS Reply
  • geniekid - Wednesday, May 22, 2013 - link

    That was one of the main draws of the PS3 until they removed the ability to run Linux on it. Reply
  • lmcd - Wednesday, May 22, 2013 - link

    Already has the Metro part of plain Windows. Reply
  • epobirs - Saturday, May 25, 2013 - link

    Why bother? You can build a mini-ITX HTPC using an AMD APU right now for less than the XO will likely cost at launch. I'm building a mini-ITX system right now that is thus far not breaking the bank. (To be fair, I got the Core i7-3770K for an unusually low price but I'd have settled for a low-power A10 model for about the same price and lower cost on the motherboard.) In fact, by taking it slow and gathering parts as deals come along, it has totaled remarkably little so far.

    I suspect Newegg has my phone bugged. Whenever I mention in a conversation not being able to find a good price on a particular needed item, I seem to get an email within hours with a sale on that item.
    Reply
  • Chad Boga - Wednesday, May 22, 2013 - link

    Why didn't Microsoft simply match the PS4's GPU capabilities? If they had, surely they then could have finished off Sony for good.

    Now Sony has a chance to once again become the premier gaming console.

    Microsoft has got so much wrong in the last decade and it looks like this is just a continuation of these stuff ups.
    Reply

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