The iPhone Becomes a Gaming Platform: Enter the PowerVR SGX

Now that we’re familiar with the 3GS’ CPU, it’s time to talk about the GPU: the PowerVR SGX.

Those familiar with graphics evolution in the PC space may remember Imagination Technologies and its PowerVR brand by their most popular desktop graphics card: STMicro’s Kyro and Kyro II. The Kyro series used the PowerVR3 chips and while STMicro ultimately failed to cement itself as a NVIDIA competitor in the desktop, the PowerVR technology lived on in ultra-mobile devices.

The SGX is on Imagination Technologies’ fifth generation of its PowerVR architecture, and just like the Kyro cards we loved, the SGX uses a tile based renderer. The idea behind a tile or deferred renderer is to render only what the camera sees, not wasting clocks and memory bandwidth on determining the color of pixels hidden by another object in the scene. Tile based renderers get their name from dividing the screen up into smaller blocks, or tiles, and working on each one independently. The smaller the tile, the easier it is to work on the tile on-chip without going to main memory. This approach is particularly important in the mobile space because there simply isn’t much available bandwidth or power. These chips consume milliwatts, efficiency is key.

The MBX-Lite used in the original iPhone was also a tile based architecture, the SGX is just better.

Also built on a 65nm process the PowerVR SGX is a fully programmable core, much like our desktop DX8/DX9 GPUs. While the MBX only supported OpenGL ES 1.0, you get 2.0 support from the SGX. The architecture also looks much more like a modern GPU:

Pixel, vertex and geometry instructions are executed by a programmable shader engine, which Imagination calls its Universal Scalable Shader Engine (USSE). The “coprocessor” hardware at the end of the pipeline is most likely fixed-function or scalar hardware that’s aids the engine.

The SGX ranges from the PowerVR SGX 520 which only has one USSE pipe to the high end SGX 543MP16 which has 64 USSE2 pipes (4 USSE2 pipes per core x 16 cores). The iPhone 3GS, I believe, uses the 520 - the lowest end of the new product offering.

A single USSE pipe can execute, in a single clock, a two-component vector operation or a 2 or 4-way SIMD operation for scalars. The USSE2 pipes are upgraded that handle single clock 3 or 4 component vector operations, have wider SIMD and can co-issue vector and scalar ops. The USSE2 pipes are definitely heavier and have some added benefits for OpenCL. For the 3GS, all we have to worry about is the single USSE configuration.

In its lowest end configuration with only one USSE pipe running at 200MHz, the SGX can push through 7M triangles per second and render 250M pixels per second. That’s 7x the geometry throughput of the iPhone 3G and 2.5x the fill rate. Even if the SGX ran at half that speed, we’d still be at 3.5x the geometry performance of the iPhone 3G and a 25% increase in fill rate. Given the 65nm manufacturing process, I’d expect higher clock speeds than what was possible on the MBX-Lite. Also note that these fill rates take into account the efficiency of the SGX’s tile based rendering engine.