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PCIe Bifurcation: Why It's Rare in Modern Computers, Its Benefits, and When It Can Be Used

Andrew Smalley

• 3 min read
PCIe Bifurcation: Why It's Rare in Modern Computers, Its Benefits, and When It Can Be Used

Introduction

What Is PCIe Bifurcation?

PCIe (Peripheral Component Interconnect Express) bifurcation is a technique that splits a single PCIe slot into multiple independent lanes, allowing various devices to share the bandwidth of a single PCIe slot. This approach was historically helpful in expanding connectivity options without requiring additional PCIe slots on a motherboard.

Why PCIe Bifurcation Is Rare in Modern Computers

  • Motherboard and CPU Limitations
    • Modern consumer-grade motherboards and CPUs often have a fixed lane allocation controlled by the chipset and BIOS.
    • Most consumer motherboards do not expose bifurcation settings in BIOS, restricting the ability to split lanes manually.
  • Chipset and CPU Constraints
    • Intel's consumer chipsets (e.g., Z690, Z790) often limit bifurcation support on non-HEDT (High-End Desktop) platforms.
    • AMD's mainstream AM4 and AM5 chipsets (e.g., B550, X570) generally do not support bifurcation unless explicitly enabled by the motherboard vendor.
    • HEDT platforms like Intel's X299 or AMD's TRX40 offer better bifurcation support due to their higher PCIe lane count.
  • Rise of Dedicated Multi-Device Solutions
    • Multi-GPU setups have mainly been abandoned in favour of single powerful GPUs.
    • NVMe storage solutions now utilize dedicated M.2 slots or PCIe switch chips instead of bifurcating a single slot.
  • PCIe Switch Chips Replacing Bifurcation
    • Higher-end systems that need multiple devices in a single slot use PCIe switch chips, which dynamically manage bandwidth.
    • This approach eliminates the need for BIOS-level bifurcation support.
  • Standardization of PCIe Slot Usage
    • Most users only install one GPU, making x16 slots the de facto standard.
    • Workstations and servers that need multiple devices usually use riser cards or backplanes designed for their specific workloads.

Benefits of PCIe Bifurcation

  • Maximizing PCIe Slot Usage
    • Ideal for systems with limited PCIe slots but an abundance of available lanes.
    • It enables the addition of multiple NVMe drives or expansion cards without needing a dedicated PCIe switch.
  • Cost Savings in Workstation and Server Builds
    • Allows for cheaper multi-device setups in cases where a motherboard does not include enough dedicated slots.
    • Helps in reducing reliance on expensive enterprise-grade backplanes.
  • Flexibility for Custom Builds
    • Enthusiasts and professionals benefit from configuring PCIe lane allocation to best fit their use case (e.g., running multiple NVMe drives on a single PCIe slot).

When PCIe Bifurcation Can Be Used

  • Server and Workstation Boards
    • Many high-end motherboards explicitly support PCIe bifurcation in BIOS.
    • Enterprise motherboards designed for multiple NVMe storage solutions often enable x16 slots to split into x4/x4/x4/x4 or x8/x8 configurations.
  • Specialized Storage and Accelerator Cards
    • NVMe RAID cards often rely on bifurcation to support multiple drives.
    • Some AI and FPGA accelerators require specific PCIe lane configurations that benefit from bifurcation.
  • Enthusiast Builds with BIOS Modifications
    • Some consumer motherboards have hidden bifurcation options that can be enabled via modding.
    • BIOS modifications (if supported by the chipset) allow enabling x8/x8 or x4/x4/x4/x4 configurations where the manufacturer has disabled them.

How to Mod a BIOS to Enable PCIe Bifurcation

  • Checking for Existing Bifurcation Support
    • Some motherboards have hidden options that are accessible through UEFI shell tools or modding software.
    • Vendors like ASUS and Gigabyte sometimes provide beta BIOS updates, enabling bifurcation options.
  • Extracting and Modifying the BIOS
    • Tools Required:
    • Steps:
      • Extract the BIOS firmware using UEFITool.
      • Open the firmware in AMIBCP to check for hidden PCIe bifurcation settings.
      • Modify the relevant settings to enable x8/x8 or x4/x4/x4/x4 modes.
      • Save and repackage the modified BIOS.
      • If necessary, flash the modified BIOS using a USB BIOS flasher or SPI programmer.

Editing the UEFI Setup Variables

If AMIBCP doesn't expose bifurcation options, you can modify UEFI variables directly using GRUB or an EFI shell.

Steps:

      • Boot into an EFI shell.
      • Use setup_var commands to change hidden settings (e.g., setup_var 0x5C3 0x1 to enable bifurcation).
      • Restart and verify changes in the BIOS.

Using PCIe Adapters with Built-in Switch Chips

    • If BIOS modding is not an option, a PCIe expansion card with an integrated switch can achieve similar results without BIOS modifications.

Conclusion

PCIe bifurcation was once a valuable technique for maximizing PCIe slot efficiency but has become less relevant due to chipset improvements and dedicated multi-device solutions. However, for specific use cases such as NVMe expansion and specialized computing workloads, enabling PCIe bifurcation via BIOS modifications or using dedicated switch-based adapters can still provide significant benefits. While consumer-grade motherboards rarely support bifurcation out of the box, workstations and servers continue to leverage this feature for high-performance configurations. With the right tools and knowledge, BIOS modifications can unlock bifurcation on select motherboards, offering greater flexibility for custom PC builds.