The Emotion Engine and Vector Unit Design
2002
Kim L. Vu
Kim L. Vu
2002
The Emotion Engine and Vector Unit Design
The Sony Emotion Engine’s architecture is structured around the ability to achieve “emotion
synthesis,” or in simpler terms, high quality image expression, through the integration of wide
internal buses and multiple arithmetic operating units. While most PCs are based on a 64-bit data
structure, the Emotion Engine (EE) doubles its registers and data busses to 128-bits allowing for
large amounts of multimedia data to be processed and transported at the highest possible speeds.
For further implementation, ten floating-point multiplier-accumulators (FMACS), four floatingpoint dividers (FDIV) and an image-processing unit (IPU) are used to speed up calculations.
Before venturing forth into a detailed analysis of the EE, let us briefly analyze the basic
interactions of the EE with its surrounding components. The following figure is an overview of
the Sony Play Station 2 architecture.
The I/O Processor sends controller input to the Emotion Engine where the state of the game
world is updated. To do this, the EE produces a set of rendering commands called display lists
and sends it to the Graphics Synthesizer where basic elementary shapes are drawn into complex
3D graphics. Sound is omitted in this document.
The Emotion Engine is a 2-instruction, super-scalar MIPS III architecture CPU. It has three
coprocessors, one of which is a floating-point unit (FPU) and two of which are vector units
(VU). Other components include a 10-channel Direct Memory Access Controller (DMAC), a
Kim L. Vu
2002
Graphic Interface Unit (GIF), a MPEG2 decoder as an IPU and two interfaces, one for the
RDRAM and the other, for the I/O. A 128b single instruction multiple data (SIMD) set and a
scratchpad RAM (SPRAM) are added as multimedia functional enhancements. The following
diagram displays these components.
Lighting calculations and perspective transformations are the keys in achieving emotion
synthesis. Designed for this purpose, Sony cleverly juxtaposes traditional CPU structure with
Digital Signal Processing (DSP). In order for the EE to manage the large amounts of data
required by multi-media applications, information is processed in digital electronic format in
repetitive, loop-based calculations or otherwise known as, DSP. In this scheme, large data
streams are being acted upon by relatively few repetitive instructions. Consequently, the EE is
equipped with small caches and large buses. These small caches have very low latency and can
be accessed in one cycle allowing rapid data access. Likewise, the widened data busses result in
higher-bandwidth and faster information processing.
MIPS III Core
The CPU core of the Emotion Engine, as mentioned earlier, is based on a RISC ISA. However,
to take advantage of the FP SIMD capabilities, the EE veers from MIPS III ISA by creating an
entirely new set of 128-bit integer instructions to replace the conventional 64-bit integer
instruction set. To place this new set into practice, the registers in the EE are expanded to 128bits and the two ALUs are locked together to operate as a single unit. Used in parallel, the two-
Kim L. Vu
2002
integer units can perform four 32-bit, eight 16-bit, or sixteen 8-bit integer arithmetic operations
each cycle. The CPU core supports the following instruction types:
-
MUL/DIV instructions
3-opo MUL/MADD instructions
Arithmetic ADD/SUB instructions
Pack and extend instructions
Min/Max instructions
Absolute instructions
Shift instructions
Logical instructions
Compare instructions
Quadword load/store instructions
Other miscellaneous instructios
The core comprises of a 16K instruction cache, an 8K data cache, a 16K SPRAM, 32 128-bit
general purpose registers and 5 logical pipes. The logical pipes are: two 64-bit iALUs, 128-bit
Load/Store unit, 2 coprocessors and a branch execution unit. The data cache is a two-way set
associative cache that supports non-blocking access. The instruction cache is also two-way set
associative and includes two bits of branch history tag (BHT) and three instruction steering bits.
Both caches are virtually indexed and physically tagged. The SPRAM can be accessed through
the same pipeline stage as the data cache, hence a flag is added into the MIPS-standard TLB to
differentiate between the SPRAM and the cache address space. The CPU uses standard MIPS III
Load/Store instructions to access the SPRAM once it is loaded by the DMAC.
The CPU also has a 6-stage pipeline (PC|Fetch|Register|Exec|Cache Access|Write Back) and a
speculative execution capability centered upon a 64-entry branch target-address cache (BTAC)
and a BHT integrated into the instruction cache. Observe that this simple, two-branch prediction
mechanism adequately suffices for the EE because of the small misprediction penalty inherent in
a short-staged pipeline design. In particular, the EE yields a misprediction penalty of only 3cycles.
Vector Unit Design Strategy
The VUs operate in two modes: coprocessor mode for use as MIPS COP2 and VLIW for use as a
stand-alone processor. This arrangement gives the vector units the flexibility to optimize its
performance with the CPU core. The coprocessor mode is available only in the VU0 and while
the VLIW mode is available in both vector units, the VUO operates mainly in coprocessor mode.
In the VLIW mode, each 64-bit instruction format is split into an upper instruction and a lower
instruction. Its instruction set comprises of 164 instructions; 95 of these are the upper
instructions and 69 of these are the lower instructions. The coprocessor mode has 130
instructions that include all of the COP2 instructions and most of the upper and lower VLIW
instructions.
To help the vector units handle DMA access to and from memories, interfaces are employed.
The VU0 has a coprocessor interface and a vector unit interface (VIF0). Along with its own
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2002
vector unit interface (VIF1), VU1 also collaborates with a GIF. These interfaces in conjunction
with EE’s memory double-buffering techniques are the means of continuous data processing.
VU Instruction Format
As noted above, the vector units are two-instruction, SIMD/VLIW processors. To accommodate
the VLIW 64-bit bundle, the vector processing units (VPU) are divided into an upper, “SIMD”
unit and a lower execution unit. Hence the term “SIMD/VLIW processor.” The upper and lower
execution units have the following functionality.
Upper Instructions
- 4 parallel FP ADD/SUB
- 4 parallel FP MUL
- 4 parallel FP ADD/MSUB
- 4 parallel MAX/MIN
- Outer product calculation
- Clipping Detection
Lower Instructions
- FP DIV/SQRT/RSQRT
- Load/Store 128b data
- EFU
- Jump/Branch
- Random number generator
Note that because the VUO operates mainly in coprocessor mode, which utilizes only 32-bit
instructions, the lower opcode is often a NOP. This permits the VU0 to execute 4 FP SIMD
operations every clock cycle using the 4 FMACS available to it. However, the VU0 can step out
of coprocessor mode and into VLIW mode simply by taking the 64-bit instruction bundle,
splitting it into two 32-bit COP2 instructions and running them in parallel.
VU0
The VU0 chiefly acts as a coprocessor for the MIPS III core and thus for the most part, appears
to the programmer as another logical pipe. Like the VU1, it has 5 computational units available
to it: 4 FMACS, 1 FDIV, 1 LSU, 1 ALU and a random number generator. Of these units, the
FMACS and the FDIV plays the most important role. The FMAC can perform two types of
calculations, both utilizing one cycle. These calculations are the min/max and the floating-point
accumulator calculations. The FDIV on the other hand can perform three types of calculations:
floating-point divide, square root, and inverse square root. These calculations take 7, 7, and 13
cycles respectively.
VU1
As a stand-alone processor, VU1 is mainly responsible for simple 3D graphic calculations. These
calculations require the transference of large amounts of data. For this reason, VU1 is equipped
with four times more memory than VU0. In addition, it has an elementary function unit (EFU)
added to the lower execution unit that uses one FMAC and one FDIV to perform basic
calculations such as sin and exp. This EFU is architecturally analogous to the FPU coprocessor.
To transfer the display list to the GIF, it has three optional data paths. The VU1 can work on the
data list in its 16k data RAM by means of the direct 128-bit bus from the data RAM to the GIF
or it can use the other two transfer paths, mainly via the VIF data path or the main data bus. For
a visual of this, please refer ahead to figure 4. One other difference betwixt the VU0 and the
VU1 is VU1’s dependency on its VIF. Whereas, in the VU0 the VIF plays no crucial role, in the
VU1 it decodes and parses the commands within the 3D display list checking that the commands
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2002
have transferred to the correct area. By this work, the VIF frees the VU1 from CPU dependency
sanctioning it to perform as pure SIMD/VLIW processor.
The main internal blocks of the VU1 are:
1) Upper execution unit with four parallel FMACs
2) Lower execution unit with FDIV, load/store, integer ALU, and a branch
3) 32, 128-bit floating registers
4) 16, 16-bit integer registers
Although microarchitecturally identical, the vector units are designed to fit certain roles and have
been customized in advanced to fill these roles under a logical unit design, or simply, a “team”
design. Two main “teams” each housing an individual vector unit (VU) are formed in advance
by Toshiba which I have called “Team 1” and “Team 2.” They can be seen above in Figure 2.
Team 1: FPU, core CPU, VU0
As a side note, since the FPU is an uncomplicated structure, it was not discussed in length prior
to this. However, it is adequate enough for our purposes to know that the structure of the FPU is
actually a reversion back to early RISC ISA’s when the FPU was a separate unit from the cpu.
In this construct, the FPU is simply a coprocessor that uses one FMAC and one FDIV to execute
32-bit MIPS coprocessor instructions.
The first team handles the physics, program control, AI and behavior calculations. In order for
Team 1 to handle these complex calculations, each member in the team is in constant
communication with the other members. Two main items make this constant interaction
feasible. The first item is the CPU’s 2 separate, 128-bit exclusive coprocessor data bus to the
FPU and the VUO. These data busses allow the CPU to transfer data to the other members
without reliance on the main bus and the manipulation of VU0’s registers by CPU instructions.
The second important component is the CPU’s scratch pad RAM (SPRAM). The SPRAM is
accessible in a single cycle and available to the DMAC while the processor is busy executing
instructions and accessing the data cache. Because of its ease of access, the SPRAM acts as a
shared workspace for the CPU and the VU0. Before data is sent to the 128-bit internal bus, it is
first work on in the SPRAM by the CPU and the VU0.
Kim L. Vu
2002
Team 2: VU1 and GIF
Team 2’s main responsibility is simple geometric calculations. Unlike Team 1, the members in
this team are somewhat independent of each other; no member controls the other member but yet
act as equal partners. However, like the members in Team 1, to ease communication VU1 and
GIF also share a dedicated 128-bit bus. The VU1 processes the calculations and sends this
information as a display list to the GIF where it is in turn sent to the graphic synthesizer. The
graphic synthesizer then draws the basic shapes, turning them into complex graphics.
Team Interoperation
Although, Toshiba has pre-designed these two teams for specific jobs, the actual allocation of
work among these components is quite flexible and is in the end up to the individual game
programmer. For example in the following figure labeled “Serial Connection,” Team 1 can be
programmed to act as a preprocessor for VU1.
In this organization, the team processes
conditional branches in the code,
loads data from main memory and updates
the state of the game world. This
information is then passed on to VU1
where it is used to compose a display list.
Kim L. Vu
2002
The next figure labeled “paralled connection”
depicts another example where Team 1 and
Team 2 are programmed to work in
parallel to simultaneously create display
lists.
This design flexibility allows
programmers to tailor the process of
3D creation and rendering
to adapt to the specific needs of an application.
Conclusion
The Emotion Engine is an important entry into the game console world. Its unique architectural
approach has permitted a design that is now one of the most powerful media engines on the
market. As one of the first processors to utilize a 128-bit architecture with a VLIW/SIMD
design and vector units, it has “uped the ante” on emotion synthesis and real-time interactive 3D
graphics thereby elevating the “gaming” world into another new level. Some have even argued
that the Emotion Engine has transcended the “gaming” world and is now a threatening contender
in the PC market. It will be interesting to see where the future lies and whether or when PCs and
gaming consoles will truly integrate and become one market and what role will the EE
architecture play in it.
Acronyms
ALU
DSP
EE
EFU
FDIV
FMAC
GIF
IPU
SPRAM
VPU
VU
Arithmetic Logic Units
Digital Signal Processing
Emotion Engine
Elementary Functional Unit
Floating-Point Divider
Floating-Point Multiply Accumulator
Graphics InterFace
Image Processing Unit
Scratch-Pad RAM
Vector Processing Unit
Vector Unit
Kim L. Vu
2002
References
[1] Atsushi Kunimatsu, et. al., “Vector Unit Architecture for Emotion Synthesis”, IEEE Micro,
Vol. 20, No. 2, March/April 2000, pp. 40-47
[2] K. Kutaragie et. al., “A Microprocessor with 128b CPU, 10 Floating-Point MACS, 4
Floating-Point Dividers, and MPEG2 Decoder,” ISSCC (Int’l Solid-States Circuit Conf.) Digest
Tech. Papers, IEEE Press, Piscatawey, New Jersey, Feb. 1999, pp. 256-257
[3] F. Micheal Raam, et. al., “A High-Bandwidth Superscalar Microprocessor for Multimedia
Applications,” ISSCC Digest Tech. Papers, IEEE Press, Feb. 1999, pp. 258-259
[4] Sound and Vision: A Technical Overview of the Emotion Engine by John Stokes, Ars
Technica
- http://arstechnica.com/reviews/1q00/playstation2/ee-1.html
[5] The Playstation2 vs. the PC: A System-level Comparison of Two 3D Platforms by John
Stokes, Ars Technica
- http://arstechnica.com/cpu/2q00/ps2/ps2vspc-1.html
[6] Emotion Engine Caching, Arspentia
- http://www.arspentia.org/lgl/eecaching
Kim L. Vu
2002