How to fix Error 791 on Windows 11

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Windows 11 Error 791: Device Resource Conflict with MCFG Table

Why This Error Happens

Error 791 occurs when a hardware device’s requested memory resources overlap with address ranges reserved in the MCFG (Memory Mapped Configuration Space Base Address Table), which is defined by the system BIOS/UEFI firmware for PCIe configuration space. This typically happens after a BIOS update, when installing new hardware (such as GPUs, NVMe drives, or expansion cards), or when outdated drivers request memory address ranges that no longer align with firmware-reserved space. Since MCFG is a fixed ACPI table generated by firmware, Windows cannot resolve the conflict on its own without adjusting device configuration or firmware settings.

Step-by-Step Resolution Guide

  1. Update Your BIOS/UEFI Firmware
    Visit your motherboard or laptop manufacturer’s support page, download the latest BIOS version, and flash it using their update utility (e.g., ASUS EZ Flash, MSI Live Update). This often corrects malformed or outdated MCFG table entries.

  2. Update the Conflicting Device Driver

  • Open Device Manager (Win + X → Device Manager).
  • Locate the device throwing the error (usually flagged with a yellow warning icon).
  • Right-click → Update driver → Search automatically for drivers.
  • If unresolved, manually download the latest driver from the manufacturer’s website.

  1. Disable Memory Remapping / Resizable BAR in BIOS
    Restart into BIOS/UEFI setup (usually Del or F2 during boot). Navigate to Advanced → PCI Configuration and try toggling Above 4G Decoding or Resizable BAR Support off, then back on after saving. This forces the system to recalculate MCFG memory mapping.

  2. Reset BIOS Resource Allocation
    In BIOS, look for PCI Express Configuration settings and set resource allocation to Auto (if manually configured). Save and exit (F10), then reboot fully (not a fast restart) to allow Windows to re-enumerate PCI devices.

  3. Physically Reseat or Relocate the Hardware
    If the error points to a specific expansion card (GPU, sound card, capture card), power down, unplug, and reseat the card—or move it to a different PCIe slot—to force Windows to reassign non-conflicting memory addresses.

Alternative or Verification Steps

  • Verify the fix: Open Event Viewer (eventvwr.msc) → Windows Logs → System, and confirm Error 791 no longer appears after a fresh reboot. Also check Device Manager for the absence of yellow warning icons.
  • Alternative fix: If the conflict persists, use msinfo32 (System Information → Hardware Resources → Conflicts/Sharing) to identify the exact address range in conflict, then manually disable unused onboard devices (e.g., legacy serial/parallel ports) in BIOS to free up MCFG-reserved space.