Cerny said, pointing to the 8GB of GDDR5 RAM that's fully addressable by both the CPU and GPU. "If [a PC] had 8 gigabytes of memory on it, the CPU or GPU could only share about 1 percent of that memory on any given frame. That's simply a limit imposed by the speed of the PCIe. So, yes, there is substantial benefit to having a unified architecture on PS4, and it's a very straightforward benefit that you get even on your first day of coding with the system."
According to Cerny, PS4 addresses the hiccups that can come from the communication between CPU, GPU, and RAM in a traditional PC. "A typical PC GPU has two buses," Cerny told Gamasutra in a very detailed technical write-up. "There's a bus the GPU uses to access VRAM, and there is a second bus that goes over the PCI Express that the GPU uses to access system memory. But whichever bus is used, the internal caches of the GPU become a significant barrier to CPU/GPU communication--any time the GPU wants to read information the CPU wrote, or the GPU wants to write information so that the CPU can see it, time-consuming flushes of the GPU internal caches are required."
PS4 addresses these concerns by adding another bus to the GPU "that allows it to read directly from system memory or write directly to system memory, bypassing its own L1 and L2 caches." The end result is that it removes synchronization issues between the CPU and GPU. "We can pass almost 20 gigabytes a second down that bus," Cerny said, pointing out that it's "larger than the PCIe on most PCs!"
