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May 2nd, 2013, 06:49
Originally Posted by Dr. A View Post
An article on AMD's tech here which is probably used for PS4 and possibly for the next XBox.
Yeah it's the memory that's unified more truly unified, and this is accomplished while reducing bottle-necking by using 2 dedicated to memory and one bus (entirely on-die) between the gpu and cpu. Now the cpu-RAM bus and the gpu-RAM bus are conceptually familiar - similar to how a CPU accesses system RAM and GPU accesses VRAM on a normal gaming PC. The innovation is really the on-die passing of pointers between the cpu and gpu.

In a system with discrete VRAM and system RAM, the memory adressed by each component is obviously physically seperate. In those systems information that needs to be from system ram to the GPU has to be written by the CPU and then read across the PCIE bus to the GPU and written into VRAM. Normally systems with shared VRAM/system RAM do not entirely get around this bottleneck as they are just stripped down approximations of the above setup and do not have low-latency direct busses between the cpu and gpu. Even though the CPU and GPU write the information to shared memory, past systems did not allow them to affectively adress the same areas AT THE SAME TIME. So even with the dynamically shared RAM in the Xbox and staticly split RAM of the PS3, data written to memory by the CPU to be processed by the GPU still had to be "sent" to the GPU and written back into memory allocated for its use.

The xbox360 did speed this up compared to bargain pc implementations of shared vram because it has a fsb replacement bus on-die. That differs from what is being done by the PS4 because it was still performing the same process as described previously - passing all the data from CPU adressed memory to GPU adressed memory. It was just doing it on-die which is faster than what a cheap laptop would do but it wasn't a new bus - just some functions of the same familiar architecture shrunk down and put on-die. So yeah the innovation here is not just that they have a specialized bus for communicating between the GPU and CPU - as well as dedicated pipes between each and the shared RAM - but more importantly that GPU and CPU communications is made much more efficient by being able to point to data in memory rather than reading it all back to the other.

compared to

Not the best images to compare, but that might give a better idea of how the paths/chokepoints have changed. Considering my shitty and inaccurate description of the process, the pictures might be better than more words.

Last edited by jhwisner; May 2nd, 2013 at 07:18.
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