Emotion Engine

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Emotion Engine

• A look at the microprocessor at the center of the
  PlayStation2 gaming console

Charles Aldrich
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The PlayStation2

• Unlike many of the devices we have looked at, the
  PS2 seeks to do a specific task very well, namely
  3D Gaming
• As a result the construction of the platform is
  different from most computers
• The EmotionEngine is also quite different as a
  result of this

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The PlayStation2

• EmotionEngine is specifically designed to handle
  several tasks (in order of importance)
• Geometry Calculations
• Behaviour/World Simulation
• System Functions

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Creating Graphics

• The EmotionEngine is not responsible for creating
  and displaying graphics, rather it is responsible
  for passing a list of rendering commands to the
  graphics synthesizer
• The graphics synthesizer, in the PS2 is a souped
  up video accelerator

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EmotionEngine basic architecture

• MIPS III core
• A Vector Unit (actually made up of 2, VU0 and
  VU1)
• Floating Point
• Image Processing Unit (does mpg2)
• Graphics Interface Unit
• RDRAM interface
• I/O interface
• DMA Controller (10 channel)

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Image courtesy of arstechnica.com
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Architecture, cont.

• All components are connected via a 128-bit
  internal bus
• Toshiba, who developed the EmotionEngine (and
  licensed it to Sony) decided, early on, that the
  EmotionEngine was not going to be a general
  purpose microprocessor rather it was only going
  to be designed to run 3D games
• This allowed the design team to determine the
  roles of the various internal devices during
  design

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Architecture, cont.

• The idea was to set up the various components to
  work in groups to accomplish set tasks
• CPU FPU basic program control, housekeeping,
  etc
• CPU FPU VU0 behavior, physics calculations,
  etc
• VU1 simple geometry calculations that produce
  display lists which are sent directly to the
  Graphics Synthesizer
• IPU image decompression
• Next well look at how each of these groups work
  together

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CPU/FPU/VU0 group

• Both the FPU and the VU0 have a dedicated 128-bit
  bus, so they do not need to use the shared system
  bus
• The dedicated bus allows the VU0 to function as a
  MIPS III coprocessor
• The VU0 and the CPU share the ScratchPadRam (16k
  of very fast RAM)
• The SPRAM functions as a workspace where both the
  CPU and the VU0 work together on a piece of data
  before sending it out over the system bus

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VU1/Graphics Synthesizer

• Similar to the VU0 and the CPU, the VU1 has a
  dedicated 128-bit bus connection to the Graphics
  Interface
• Do not work as closely as the VU0 and CPU,
  neither controls the other

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MIPS III Core details

• The EmotionEngine uses a specialized version of
  the MIPS architecture
• 32 128-bit registers
• 2 64-bit integer ALUs
• 128-bit load/store unit
• Branch execution unit
• FPU Coprocessor
• Vector Coprocessor

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MIPS III Core details, cont.

• Cache
• 16k instruction
• 8k data
• Capable of speculative execution
• 2 part branch-prediction mechanism
• Fairly simple

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MIPS III Core details, cont.

• Pipeline
• Relatively short, 6 stage pipeline
• 1. PC Select
• 2. Instruction fetch
• 3. Instruction decode and register read
• 4. Execute
• 5. Cache access
• 6. Write back
• FPU coprocessor is fairly standard

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SIMD

• The emotion engine takes advantage of SIMD
  (Single Instruction stream, Multiple Data stream)
  

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SIMD, cont.

• As the picture shows, parallel streams of data
  all have the same instructions done to it
• The EmotionEngine supports (using both ALUs)
  128-bit integer SIMD in several configurations
• 16 8-bit ops/cycle
• 8 16-bit ops/cycle
• 4 32-bit ops/cycle

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Visual Power

• The EmotionEngine is best suited for precision
  graphics, and the power of the processor comes
  from the Vector Processing units
• As weve discussed already, there are two vector
  units
• The two are similar on an architectural level,
  but very different on a functional level

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VU0

• 128-bit SIMD/VLIW design
• Works as a coprocessor with the CPU
• Has its own set of 32 128-bit FPRs (floating
  point registers)
• Also has 16 16-bit integer registers for integer
  operations

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VU1

• Has all the architectural features of VU0
• Also has some additions which pertain to geometry
  functions in coordination with the Graphics
  Synthesizer
• This includes the addition of the EFU (Elementary
  Functional Unit) which does basic math essential
  to geometric processing

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Basic Stats

• Clock Speed - 300 Mhz
• Transistors - 10.5 Million
• Memory - 32MB (Direct RDRAM)
• Memory Bandwidth - 3.5GB/sec

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Performance Numbers

• Floating Point - 6.2 Gigaflops
• Perspective Transformation - 66 Million
  Polygons/sec
• Lighting - 38 Million Polygons/sec
• Fog - 36 Million Polygons/sec
• Curved Surface Generation (Bezier) - 16 Million
  Polygons/sec

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Conclusion

• The EmotionEngine is clearly a very capable
  graphics processor, but its true power is most
  evident when placed in the context of the PS2
  which was designed to take advantage of the
  processors specific architecture