RSX Reality Synthesizer

The Reality Synthesizer (RSX) is a proprietary graphics processing unit (GPU) developed jointly by Nvidia and Sony for the PlayStation 3 video game console. Based on Nvidia's GeForce 7 series, specifically the 7800 GTX, the RSX utilizes a hybrid design incorporating elements of the G70 and G71 (previously known as NV47) architecture. It features separate vertex and pixel shader pipelines and supports advanced graphics rendering features such as high dynamic range, anti-aliasing, and S3 texture compression, with a theoretical floating-point performance of 192 GFLOPS.

The RSX includes 256 MB of GDDR3 SDRAM, clocked at 650 MHz with an effective transmission rate of 1300 MT/s. It can also access up to 224 MB of the console’s XDR DRAM main memory through the Cell Broadband Engine, the PlayStation 3's CPU, allowing for a combined maximum of 480 MB of usable memory.

While the RSX handles the majority of graphics processing tasks, the Cell processor assists with graphics-related computations, offering a complementary role in rendering workloads.

Specifications

RSX memory map

Although the RSX has 256 MB of GDDR3 RAM, not all of it is usable. The last 4 MB is reserved for keeping track of the RSX internal state and issued commands. The 4 MB of GPU Data contains RAMIN, RAMHT, RAMFC, DMA Objects, Graphic Objects, and the Graphic Context. The following is a breakdown of the address within 256 MB of the RSX.

Besides local GDDR3 memory, main XDR memory can be accessed by RSX too, which is limited to either:

• 0 MB – 256 MB (0x00000000 – 0x0FFFFFFF)
• -or-
• 0 MB – 512 MB (0x00000000 – 0x1FFFFFFF)

Speed, bandwidth and latency

System bandwidth (theoretical maximum):

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 (rather than calculated 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.

• Cell to/from 256 MB XDR : 25.6 GB/s
• Cell to RSX (IOIFO): 20 GB/s (practical : 15.8 GB/s @ packetsize 128B)
• Cell from RSX (IOIFI) : 15 GB/s (practical : 11.9 GB/s @ packetsize 128B)
• RSX to/from 256 MB GDDR3 : 20.8 GB/s (@ 650 MHz)

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 basically 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. (PSGL is actually implemented on top of LibGCM). This is done by setting up commands (via FIFO Context) 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.[7] 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.[7]

Other RSX features/differences include:

• More shader instructions
• Extra texture lookup logic (helps RSX transport data from XDR)
• Fast vector normalize

Press releases

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 Jensen Huang stated during Sony's pre-show press conference at E3 2005 that the RSX is twice as powerful as the GeForce 6800 Ultra.[2]

Bumpgate

The RSX GPU in early models of the PlayStation 3 was initially fabricated using a 90 nm process and was affected by reliability issues related to its packaging and thermal behavior—a problem commonly referred to as "Bumpgate". The high operating temperatures of the chip could weaken the solder joints in the ball grid array (BGA) connecting the die to the interposer, leading to degraded performance or complete hardware failure over time.

Several factors contributed to these failures:

These issues primarily affected launch and early production units. Later revisions of the RSX transitioned to smaller process nodes—65 nm in 2008,[4] 40 nm in 2010,[5] and 28 nm in 2013—which reduced power consumption and, in turn, heat generation, helping to mitigate the thermal stress that contributed to earlier failures.

• Mismatched coefficients of thermal expansion between materials in the die and interposer caused differential expansion during thermal cycles, placing mechanical stress on the BGA.

See also

• Xenos – GPU used in the Xbox 360
• Cell Broadband Engine – CPU used in the PlayStation 3