PS4 architect discusses console's custom AMD processor

Since February, we've known that Sony's upcoming PlayStation 4 console will have a "supercharged PC architecture" based on a custom AMD processor with GCN-derived graphics and eight Jaguar CPU cores. The chip's GPU and CPU elements will share 8GB of GDDR5 memory over an interface with a claimed 176GB/s of bandwidth, and the graphics component promises comparable horsepower to the Radeon HD 7850. Gamasutra recently discussed the PS4's underlying hardware with lead architect Mark Cerny, providing new insight on the guts of the console.

Cerny says the PS4's custom silicon incorporates not only the CPU and GPU, but also a "large number of other units." The chip has a dedicated audio unit to perform processing for voice chat and multiple audio streams. It also has a hardware block designed explicitly for zlib decompression. The main processor is backed by a secondary chip that enables an ultra-low-power mode for background downloading. In that mode, the CPU and GPU shut down, leaving only the auxiliary chip, system memory, networking, and storage active.

A 256-bit interface links the console's processor to its shared memory pool. According to Cerny, Sony considered a 128-bit implementation paired with on-chip eDRAM but deemed that solution too complex for developers to exploit. Sony has also taken steps to make it easier for developers to use the graphics component for general-purpose computing tasks. Cerny identifies three custom features dedicated to that mission:

  • An additional bus has been grafted to the GPU, providing a direct link to system memory that bypasses the GPU's caches. This dedicated bus offers "almost 20GB/s" of bandwidth, according to Cerny.
  • The GPU's L2 cache has been enhanced to better support simultaneous use by graphics and compute workloads. Compute-related cache lines are marked as "volatile" and can be written or invalidated selectively.
  • The number of "sources" for GPU compute commands has been increased dramatically. The GCN architecture supports one graphics source and two compute sources, according to Cerny, but the PS4 boosts the number of compute command sources to 64.

If developers take advantage of the PS4's apparently robust support for mixed GPU workloads, we could see more compute tasks being offloaded to the GPU in PC games. Let's hope developers don't rely too much on Sony's customizations, though.

Interestingly, Cerny has little to say about the PS4's Jaguar-based CPU. All the focus on general-purpose GPU computing suggests the console's CPU component is relatively weak, which isn't a big surprise. Jaguar will replace AMD's low-power Bobcat CPU architecture, and it's designed primarily with mobile systems like tablets in mind.

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