Xenon (CPU)

Xenon is the CPU that 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