MEG B550 Unify and Unify-X offer four M.2 slots and three of them support PCIe 4.0 x4 to max out the performance PCIe 4.0 SSDs.

The first M.2 slot (labeled as M2_1) is connected to CPU and supports the bandwidth of PCIe 4.0 x4. The second (M2_2) and third slot (M2_3) can either connect to CPU or chipset, depending on your demand. The fourth slot (M2_4) is from chipset and supports up to PCIe 3.0 x4 since B550 chipset only support PCIe 3.0.

This article will explain how to switch the M2_2 and M2_3 slots between CPU mode and Chipset mode, and also the lane sharing for PCIe, M.2 slots and SATA ports

PCIe and M2. Slots on MEG B550 Unify & Unify-X

MEG B550 Unify and B550 Unify-X offers 4 PCIe slots. Here is the configuration:
  • PCI_E1 is connected to CPU and supports PCIe 4.0 x16.
  • PCI_E2 & PCI_3 are connected to chipset and support PCI 3.0 x1.
  • PCI_E4 is connected to chipset and supports up to PCIe 3.0 x4.

B550 Unify
(Source: user’s manual)

There are four M.2 slots available on the motherboard:
  • M2_1 is connected to CPU and supports PCIe 4.0 x4.
  • M2_2 and M2_3 support either PCIe 4.0 x4 from CPU or PCIe 3.0 x2 from chipset.
  • M2_4 slot is from chipset and supports PCIe 3.0 x4.

B550 Unify

CPU Mode and Chipset Mode for M2_2 & M2_3 Slots

MSI offers two bandwidth options for M2_2 and M2_3 slots: CPU mode, and Chipset mode. The stock setting is “Chipset mode” which means they utilize the PCIe 3.0 lanes from the B550 chipset. You can switch to “CPU” mode to connect these two slots to the CPU and benefit from the PCIe 4.0 x4 bandwidth.

1. CPU Mode – if you switch M2_2 and M2_3 lane source to CPU mode, the slots are running at PCIe 4.0 x4. However, if M2_2 or M2_3 is populated, it’ll use x8 lanes from the CPU and therefore a graphics card installed in PCI_E1 is running at PCIe 4.0 x8.

Note 1: PCI_E1 must be configured to “x8+x4+x4” to switch to CPU mode. MSI BIOS will automatically do this configuration when CPU mode is enabled.
Note 2: if CPU mode is used for M2_2 and M2_3, the graphics card in PCI_E1 is running at PCIe 4.0 x8.

2. Chipset Mode – this is the stock setting. Both M2_2 and M2_3 are connected to the B550 chipset and support PCIe 3.0 x2 only. This means your PCIe 4.0 SSD will be limited to the bandwidth of PCIe 3.0x2.

B550 Unify

You can switch between CPU mode and Chipset mode in BIOS.

17938MB/s Read with 3x Samsung 980 PRO SSDs

If M2_2 and M2_3 are configured to CPU mode, there will be 3 PCIe 4.0 x4 slots for best M.2 performance. It can do an amazing 17938MB/s sequential read with three Samsung 980 PRO NVMe SSDs configured in RAID0.
B550 Unify

PCIe/M.2/SATA Lane Sharing

It’s always confusing what’s the PCIe lane sharing for PCIe, M.2 slots and SATA ports. Here is the lane sharing for B550 Unify and Unify-X. M2_4 shares PCIe 3.0 x4 lanes with PCI_E4. In Chipset Mode, M2_2 shares the PCIe lanes with PCI_E2 and PCI_E3.

As this table suggests, for Chipset mode, when M2_2 is populated, PCI_E2 and PCI_E3 are not available. While M2_4 is used, PCI_E4 is not available.
B550 Unify
(Source: user’s manual)

When it comes to SATA ports, M.2_3 shares the chipset PCIe lanes with SATA5 & SATA6. If M.2_3 is populated with an NVMe SSD, both SATA5 & SATA6 will not be available to use. If it’s populated with a SATA SSD, SATA 5 will not be available.
Note: For more detail, please refer to user’s manual.
B550 Unify
(Source: user’s manual).