ARM9

ARM9 is a group of older 32-bit RISC ARM processor cores licensed by ARM Holdings.

Overview

With this design generation, ARM moved from a von Neumann architecture (Princeton architecture) to a Harvard architecture with separate instruction and data buses (and caches), significantly increasing its potential speed. Most silicon chips integrating these cores will package them as modified Harvard architecture chips, combining the two address buses on the other side of separated CPU caches and tightly coupled memories.

There are two subfamilies, implementing different ARM architecture versions.

Differences from ARM7 cores

Key improvements over ARM7 cores, enabled by spending more transistors, include:

  • Decreased heat production and lower overheating risk.
  • Clock frequency improvements. Shifting from a three-stage pipeline to a five-stage one lets the clock speed be approximately doubled, on the same silicon fabrication process.
  • Cycle count improvements. Many unmodified ARM7 binaries were measured as taking about 30% fewer cycles to execute on ARM9 cores. Key improvements include:
    • Faster loads and stores; many instructions now cost just one cycle. This is helped by both the modified Harvard architecture (reducing bus and cache contention) and the new pipeline stages.
    • Exposing pipeline interlocks, enabling compiler optimizations to reduce blockage between stages.
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