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Memory Bus Width

The memory bus width refers to the number of bits that can be transferred between the memory and the processor in a single cycle. It essentially defines how much data moves in parallel. For example, a 128-bit bus can transfer 128 bits (16 bytes) per cycle, while a 256-bit bus can transfer 256 bits (32 bytes) per cycle. A wider bus increases the amount of data that can flow at once, which directly impacts memory bandwidth—the total data transfer rate. This is critical for workloads that are bandwidth-intensive, such as AI model inference or high-resolution gaming, because a wider bus reduces bottlenecks and improves overall system performance. However, increasing bus width also adds complexity, cost, and power consumption, so it’s a trade-off between performance and efficiency.

  • Bus width = number of bits transferred per cycle.
  • 128-bit bus = 16 bytes per cycle.
  • 256-bit bus = 32 bytes per cycle.
Bandwidth = Data Rate (MT/s) × Bus Width (Bytes)

Example with LPDDR5X-8000:

  • 128-bit bus:
    8000 × 16 = 128GB/s
  • 256-bit bus:
    8000 × 32 = 256GB/s

Impact on Workloads

AI:

  • Bandwidth-bound → wider bus = faster tensor fetches.
  • Reduces stalls in compute units.
  • Significant performance boost for inference/training.

Gaming:

  • GPU-heavy → benefits from higher bandwidth for textures/frame buffers.
  • Unified memory (CPU + GPU) → less contention, smoother frame pacing.
  • Gains noticeable at 4K/8K or when AI tasks run alongside gaming.

Other Benefits

  • Lower latency under load: Less queuing when multiple cores request memory.
  • Better scaling: Supports heterogeneous compute (CPU + GPU + AI accelerators).

Trade-offs

  • Power & cost: More pins, PCB complexity, higher energy per transfer.
  • Diminishing returns: If compute units can’t saturate 128 GB/s, extra bandwidth won’t help much.