Xenon (CPU)

Xbox 360 Xenon CPU

Xenon is the CPU that is used in the Xbox 360. The processor, internally codenamed Waternoose by IBM and XCPU by Microsoft. "The basic design is a 64-bit PowerPC architecture, with the complete PowerPC ISA available."

It is different from the Intel Pentium III in the original Xbox: the Xenon CPU is composed of three symmetrical cores on a single die, aach core has two symmetric hardware threads (SMT), for a total of six hardware threads available to games. Each individual core also includes 32 KiB of L1 instruction cache and 32 KiB of L1 data cache.

Specifications

• The package is a 2-2-2 FC-PBGA, measuring 31mm by 31mm
• 168 mm2 die size; one cpu core is about 28mm2
• Support logic for Array and Logic Built in Selftest
• 165 Million transistors
• Three symmetrical cores running at 3.2 GHz each = 9.6Ghz throughput.
  • 32 KiB of L1 instruction cache and 32 KiB of L1 data cache per core
  • 64 bit CPU cores based on the IBM 970 "G5" design
  • Each core is two way SMT-capable and clocked at 3.2Ghz
  • 2× (128×128 bit) register files for each core
  • Two hardware threads per core; six hardware threads total
  • VMX-128 vector unit per core; three total
  • 128 VMX-128 registers per hardware thread
• 1 MB L2 cache
  • (lockable by the GPU) running at half-speed (1.6 GHz) with a 256-bit bus
  • 51.2 gigabytes per second of L2 memory bandwidth (256 bit × 1600 MHz)
• 21.6 GB/s front side bus (aggregated 10.8 GB/s upstream and downstream)
• 115 GFLOPS theoretical peak performance
• CPU Integer Perf (VPR): 1089 (363 each)
• Dot product performance: 9.6 billion per second
• IBM eFuse technology
• IBM's PowerPC instruction set architecture
• Restricted to In-order code execution
• Big endian architecture

Linux Support

• We have full SMP support, so we can use all three cores.
• However, we currently need to disable the secondary threads because of a yet-to-be analyzed stability issue.
• The CPU is quite slow on general purpose code. Due to the non out-of-order execution core, it heavily relies on the compiler to do proper optimizations. GCC currently doesn't know how to do this, resulting in running but very inefficient code.
• The Cell people worked on PPU-GCC, from which the Xenon will benefit as well, as the PPC cores is quite similiar to the Cell's PPU. All PPU-GCC work have been merged on mainline GCC 4.4

External Links

• Application-customized CPU design: The Microsoft Xbox 360 CPU story Jeffrey Brown, Distinguished Engineer, IBM
• "Power" Architecture Technology at IBM
• SoC drawer: The resource view at IBM (mentions Xbox CPU design. Gets nitty gritty on resources and scheduling, i/o interconnects, & on chip memory)
• Just like being there: Papers from the Fall Processor Forum 2005: Application-customized CPU design at IBM (Explore the customized IBM PowerPC processor designed for the Microsoft XBox 360)
• Xenon hardware overview by Pete Isensee, Development Lead, Xbox Advanced Technology Group, written some time before 23 June 2004
• Ars Technica explains the Xenon CPU
• Xbox360 CPU at Wikipedia
• PowerPC 970 at Wikipedia
• PowerPC at Wikipedia
• POWER4 at Wikipedia
• CPU Chip for Xbox 360 Manufacturing Overview, FSB, PPC Core, VMX & FPU, & L2 Cache block diagrams from IBM