B550 Mobo
B550 Mobo
With a B550 motherboard, having a GPU and an M.2 slot still allows for a PCIe 4 connection.
Yes, it will support PCIe 4.0 regardless of the number of devices connected
Zen2 and Zen3 offer 24 PCIe Gen 4.0 lanes: 16+4+4. Typically, 16 lanes are dedicated to the GPU, 4 for the main M.2 slot, and another 4 for the chipset downlink. Because B550 doesn’t support Gen 4.0, the downlink will be reduced to Gen 3.0 on a B550 board (similar to inserting a Gen 3.0 GPU into a Gen 4.0 slot with a compatible CPU). You can still utilize PCIe splitting, which divides lanes for a single slot. Some B550 boards achieve this to provide more than one Gen 4.0 M.2 port. In such cases, the originally allocated 16 lanes (often 8+4+4) are divided, likely resulting in two Gen 4.0 M.2 slots. Whether this adjustment is handled automatically in BIOS or by the board itself probably depends on the specific model. The limited downlink bandwidth of just four lanes also accounts for why extra M.2 ports and certain SATA connections share the same bandwidth, regardless of the motherboard type.