S.T.A.G.E. said:
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The actual problem with GDDR5 is that it is designed for larger blocks of data. The other problem is you can't simply update the memory, you have to clear it and rewrite it. This is great when you're dealing with graphics, because the GPU takes the original data and if it needs to replace it so what if you lose the original data. You have the updated, possibly final form of the data you want and it generally takes up the entire block.
The problem with GDDR5 is when you're dealing with the OS, and why I believe the PS4 does hard drive caching (virtual memory). The OS and applications tend to use small blocks of data, and they often tend to append those blocks of data. So, instead of replacing them, they simple add to them or subtract from them.
This post, for example, is an addition of memory (data) to the original block I created when I opened the tab. As I type, this block gets seemlessly updated. In GDDR memory that doesn't happen. Every letter I write would require the memory be cleared, and data be reloaded from a cache into memory. Not an impossible task, especially when you consider the speed of GDDR5 memory but every letter typed requires access the cache and that takes away a clock cycle that could be used for something else.
Sony wasn't the first company to use unified memory in a console. Microsoft was. Sony wasn't the first company to use GDDR memory for their unified memory either. Microsoft was. There was a reason why Microsoft abandoned GDDR memory. In fact, on this front Sony is the LAST company to do both. Even Nintendo came before them on unified memory.
