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# PCIe Bifurcation: Why It's Rare in Modern Computers, Its Benefits, and When It Can Be Used
- URL: https://articles.akadata.co.uk/pcie-bifurcation-why-its-rare-in-modern-computers-its-benefits-and-when-it-can-be-used/
- Published: 2025-03-10T01:38:20.000Z
- Updated: 2025-03-10T04:07:15.000Z
- Author: AKADATA

### **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](https://www.intel.com/content/www/us/en/products/sku/218833/intel-z690-chipset/specifications.html?ref=articles.akadata.co.uk), [Z790](https://www.intel.com/content/www/us/en/products/sku/229721/intel-z790-chipset/specifications.html?ref=articles.akadata.co.uk)) often limit bifurcation support on non-HEDT (High-End Desktop) platforms.
  - AMD's mainstream [AM4 ](https://www.amd.com/en/products/processors/chipsets/am4.html?ref=articles.akadata.co.uk)and [AM5 ](https://www.amd.com/en/products/processors/chipsets/am5.html?ref=articles.akadata.co.uk)chipsets (e.g., [B550](https://www.asus.com/microsite/motherboard/AMD-B550/?ref=articles.akadata.co.uk), [X570](https://www.anandtech.com/show/14161/the-amd-x570-motherboard-overview?ref=articles.akadata.co.uk)) generally do not support bifurcation unless explicitly enabled by the motherboard vendor.
  - [HEDT ](https://www.cpushack.com/2022/07/11/the-history-of-the-hedt-x86-pc-part-1/?ref=articles.akadata.co.uk)platforms like Intel's [X299 ](https://www.intel.com/content/www/us/en/products/sku/122941/intel-x299-chipset/specifications.html?ref=articles.akadata.co.uk)or AMD's [TRX40 ](https://www.amd.com/en/products/processors/chipsets/strx40.html?ref=articles.akadata.co.uk)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 ](https://www.asus.com/?ref=articles.akadata.co.uk)and [Gigabyte ](https://www.gigabyte.com/?ref=articles.akadata.co.uk)sometimes provide beta BIOS updates, enabling bifurcation options.
- **Extracting and Modifying the BIOS**
  - **Tools Required:**
    - [AMIBCP ](https://archive.org/details/bnobtc%5Fbntbtc)([AMI BIOS Configuration Program](https://archive.org/details/amibcp-4.55.0070)) for AMI BIOS-based motherboards.
    - [UEFITool ](https://github.com/LongSoft/UEFITool?ref=articles.akadata.co.uk)for extracting BIOS images.
    - Hex editors ([HxD ](https://mh-nexus.de/en/hxd/?ref=articles.akadata.co.uk)or similar) will manually adjust values if needed.
  - **Steps:**
    - Extract the BIOS firmware using UEFITool.
    - Open the firmware in [AMIBCP ](https://archive.org/details/bnobtc%5Fbntbtc)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 ](https://archive.org/details/amibcp-4.55.0070)doesn't expose bifurcation options, you can modify UEFI variables directly using GRUB or an EFI shell.

**Steps:**

    - Boot into an [EFI shell](https://github.com/pbatard/UEFI-Shell/releases?ref=articles.akadata.co.uk).
    - 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.