RSX Reality Synthesizer


The RSX 'Reality Synthesizer' is a proprietary graphics processing unit codeveloped by Nvidia and Sony for the PlayStation 3 game console. It is a GPU based on the Nvidia 7800GTX graphics processor and, according to Nvidia, is a G70/G71 hybrid architecture with some modifications. The RSX has separate vertex and pixel shader pipelines. The GPU makes use of 256 MB GDDR3 RAM clocked at 650 MHz with an effective transmission rate of 1.3 GHz and up to 224 MB of the 3.2 GHz XDR main memory via the CPU.
Although it carries the majority of the graphics processing, the Cell Broadband Engine, the console's CPU, is also used complementarily for some graphics-related computational loads of the console.

Specifications

Unless otherwise noted, the following specifications are based on a press release by Sony at the E3 2005 conference, slides from the same conference, and slides from a Sony presentation at the 2006 Game Developer's Conference.
Other features: Support for Bilinear, trilinear, anisotropic, quincunx texture filtering, quincunx antialiasing, up to 4xMSAA, SSAA, Alpha to Coverage and Alphakill.

Model numbers

90nm:
65nm:
40nm:
Although the RSX has 256MB of GDDR3 RAM, not all of it is usable. The last 4MB is reserved for keeping track of the RSX internal state and issued commands. The 4MB of GPU Data contains RAMIN, RAMHT, RAMFC, DMA Objects, Graphic Objects, and the Graphic Context. The following is a breakdown of the address within 256MB of the RSX.
Address RangeSizeComment
0000000-FBFFFFF252 MBFramebuffer
FC00000-FFFFFFF4 MBGPU Data
FF80000-FFFFFFF512KBRAMIN: Instance Memory
FF90000-FF93FFF16KBRAMHT: Hash Table
FFA0000-FFA0FFF4KBRAMFC: FIFO Context
FFC0000-FFCFFFF64KBDMA Objects
FFD0000-FFDFFFF64KBGraphic Objects
FFE0000-FFFFFFF128KBGRAPH: Graphic Context

Besides local GDDR3 memory, main XDR memory can be accessed by RSX too, which is limited to either:
System bandwidth :
Because of the aforementioned layout of the communication path between the different chips, and the latency and bandwidth differences between the various components, there are different access speeds depending on the direction of the access in relation to the source and destination. The following is a chart showing the speed of reads and writes to the GDDR3 and XDR memory from the viewpoint of the Cell and RSX. Note that these are measured speeds and they should be worse if RSX and GDDR3 access are involved because these figures were measured when the RSX was clocked at 550Mhz and the GDDR3 memory was clocked at 700Mhz. The shipped PS3 has the RSX clocked in at 500Mhz. In addition, the GDDR3 memory was also clocked lower at 650Mhz.

Speed table

Because of the very slow Cell Read speed from the 256MB GDDR3 memory, it is more efficient for the Cell to work in XDR and then have the RSX pull data from XDR and write to GDDR3 for output to the HDMI display. This is why extra texture lookup instructions were included in the RSX to allow loading data from XDR memory.

RSX libraries

The RSX is dedicated to 3D graphics, and developers are able to use different API libraries to access its features. The easiest way is to use high level PSGL, which is basicially OpenGL|ES with programmable pipeline added in, however this is unpopular due to the performance overhead on a relatively weak console CPU.
At a lower level developers can use LibGCM, which is an API that builds RSX command buffers at a lower level.. This is done by setting up commands and DMA Objects and issuing them to the RSX via DMA calls.

Differences with the G70 architecture

The RSX 'Reality Synthesizer' is based on the G70 architecture, but features a few changes to the core. The biggest difference between the two chips is the way the memory bandwidth works. The G70 only supports rendering to local memory, while the RSX is able to render to both system and local memory. Since rendering from system memory has a much higher latency compared to rendering from local memory, the chip's architecture had to be modified to avoid a performance penalty. This was achieved by enlarging the chip size to accommodate larger buffers and caches in order to keep the graphics pipeline full. The result was that the RSX only has 60% of the local memory bandwidth of the G70, making it necessary for developers to use the system memory in order to achieve performance targets.
DifferenceRSXnVidia 7800GTX
GDDR3 Memory bus128bit256bit
ROPs816
Post Transform and Lighting Cache63 max vertices45 max vertices
Total Texture Cache Per Quad of Pixel Pipes 96kB48kB
CPU interfaceFlexIOPCI-Express 16x
Technology40 nm/65 nm/90 nm110 nm

Other RSX features/differences include:
Sony staff were quoted in PlayStation Magazine saying that the "RSX shares a lot of inner workings with NVIDIA 7800 which is based on G70 architecture." Since the G70 is capable of carrying out 136 shader operations per clock cycle, the RSX was expected to feature the same number of parallel pixel and vertex shader pipelines as the G70, which contains 24 pixel and 8 vertex pipelines.
Nvidia CEO Jen-Hsun Huang stated during Sony's pre-show press conference at E3 2005 that the RSX is twice as powerful as the GeForce 6800 Ultra.