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P Apr 3, 2013 07:37 AM
Future Macbooks and RAM limits - rumor discussion
So AMD is making rumors with their integrated graphics again, and we begin to see where the PS3 design comes from. Long story short, AMD's is rumored to be preparing laptop "APUs" - basically CPUs with integrated graphics - that use GDDR5 memory, just like the PS4. For those not familiar with the memory type, it can be summarized as memory with significantly higher max bandwidth per pin than DDR3, but with lower max RAM and no ability to upgrade the RAM.

Doing the math, it seems that the max would be 4 GB of GDDR5 RAM, shared between CPU and GPU. This chip would then have the graphics power - both shader and memory bandwidth - of a Radeon 7750 (ie a bit better than the discrete 650M in current MBPs), and a CPU that AMD promises will really be good this time, promise, but in all likelihood somewhere between 2010 and 2011 MBPs (between Arrandale and Sandy Bridge).

My question, then: Would this be an attractive choice for a future Macbook? Only 4 GB of RAM and a slightly weaker CPU in return for a GPU that can comfortably run a Retina display without discrete graphics.
ajprice Apr 3, 2013 10:21 AM
Is 4GB the economic limit to get the PS4 out at a price point, or a technical limit of the system that cannot ever ever be raised? Macbook Airs and Pros now can be specced with between 4 and 16GB, there will have to be some creative advertising to get the message over that 4GB shared is better (if it is better).

Initial reaction is that I'm not convinced.
P Apr 3, 2013 10:55 AM
It's a technical limit that comes from current state of the art of making semiconductors. Basically each GDDR5 channel can fit at most 2 chips - or at least that's the most anyone has ever shipped, not entirely certain if that's in the specs or just that the timings don't work with more. Current GDDR5 chips are 32-bits wide and AMD is supposedly is planning for a 128-bit wide memory bus like most desktop and laptop CPUs have today, meaning that you can fit 4 32-bit channels and a total of 8 chips. Currently the biggest anyone has made a GDDR5 chip is 2 Gbit, but 4 Gbit chips are on the way - Samsung claims to be in serial production of them. 4 Gbit * 8 = 4 GB.

The PS4 will use a 256-bit bus, which doubles the total memory capacity to 8 GB. At the announcement, there was some discussion about whether Sony and AMD had done something special to be able to fit more than 2 chips on each channel, but now that Samsung claims to be making 4 Gbit GDDR5 chips, it's almost certain that Sony is simply using these chips.

The question isn't if 4 GB will be OK for ever and ever, because we will have DDR4 (and GDDR6) here in a year or two to change the equation again, and further more we have on package caches coming to relieve the bandwidth crunch for integrated graphics, but whether it will be OK in 2013. I've been going back and forth on this. On the one hand, a CPU like this with a decently specced modern GPU, four reasonably powerful OoOE cores and lots of memory bandwidth looks to be perfectly balanced between allround power and good battery life and the 4 GB RAM in my MBA isn't bothering me any. On the other hand, GDDR5 memory isn't exactly cheap, so we're running the risk of a rather expensive laptop with this potentially fatal flaw.
mduell Apr 3, 2013 01:27 PM
It may be a good idea for a gaming box, but a terrible idea for a general purpose computer.

I don't have any issues with Intel HD 4000 for non-gaming use in my RMBP, and Haswell will be even faster for games.
OreoCookie Apr 3, 2013 09:50 PM
I think the better solution is to introduce another level into the memory hierarchy such as the inclusion of stacked eDRAM with some Haswell cpus and some of nVidia's future GPUs. 4 GB is nowhere near enough for future cpus/gpus/apus. While the amounts are much, much smaller (64 MB for Haswell), they'd relieve the main memory bus a bit. Personally, I expect that Intel puts more attention to the power consumption of RAM and prioritizes that over an increase in bandwidth for the next one or two generations.
P Apr 4, 2013 04:05 AM
On-package RAM is a great solution (I go on about it in the gaming forums sometimes), and 64MB will be enough for the frame buffer (and likely Z-buffer) for most resolutions, which reduces the bandwidth requirements. MS is doing the same for the next Xbox, and since AMD makes that chip, they likely have something similar in the works. We don't know what the performance will be in practice, though. This is another solution, and likely a cheaper one (especially if you don't go insane with the top speed GDDR5), and it's an interesting option - if true, of course.
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