PCIe Max Lanes On MSI B350 Tomahawk Help

RadeonJR2600 · January 4, 2019

Okay, so I picked up a MSI B350 Tomahawk for my first computer build in some time. I also picked up an AMD Ryzen 5 2400G (with Vega graphics).

Planing on in the future getting a "Okay" graphics card for light gaming.

The board has an M.2 slot and four Sata slots.

The Ryzen 5 2400G has 12 PCIe lanes, in a 1x8 + 1x4 or a 2x4 + 1x4 configuration.

A few questions...

1. what is the meaning of 1x8 + 1x4

2. does each HDD or an M.2 use it's own PCIe lane?

3. does the graphics in the APU use a PCIe lane?

Thank you in advance.

EspinalAndres · January 4, 2019

1. what is the meaning of 1x8 + 1x4, I don´t know.

2. HDDs don't use lines. M.2 Mvme uses PCI lanes from the chipset depending on the ssd (x2 or x4)

3. does the graphics in the APU use a PCIe lane? Not from the chipset.

FloRolf · January 4, 2019

Hey there,

please make sure the B350 board has a proper BIOS out of the Box so it supports the Ryzen. If it doesnt then you either have to get a new Board or another, 1st gen CPU in order to flash the bios.

Onto your questions:

1. 1x8x + 1x4x means you can run 1Nvme Drive (4x PCIe Lanes) and 1 GPU (at 8x PCIe Lanes speed). Whilste GPUs usually want 16x, it will not really matter a lot. I've personally ran a GTX1080 at 8x without any issues.

2. No, only M.2 does. Some Boards will use PCIe Lanes to get another 2 SATA ports and then, once you put in like 6 SATA Drives, it will disable the M.2 Slot. Also nothing to worry about.

3. i'm not entirely sure how it's connected but it will not use any of the 12 PCIe Lanes.

Zagna · January 4, 2019

3. Infinity Fabric connects the CCX, GPU, memory controller and PCIe complex all together.

mariushm · January 4, 2019

There's 1x8 going to the pci-e slot "optimized" for graphics.

Normal Ryzen processors have x16 lanes for graphics, which can be divided by motherboard into 2x8 if you have the right chipset (x370 or x470)

Ryzen processors with integrated graphic use x8 of those x16 lanes to connect the integrated graphics part to the cpu part inside the processor, so there's only 8 lanes out of those 16 lanes going outside the cpu to the slot "optimized" for graphics.

I don't know if these 8 lanes can be further divided into 2 x4 slots, it may be possible if you use a board with x370 or x470 chipsets, the lower b350 and b450 should not support this.

Besides this, there's 8 more lanes coming from cpu - 4 lanes go to the m.2 connector, or the motherboard maker may split them to various things (pci-e x1, onboard 10gbps lan, extra usb 3.1 gen 2 controllers)

The last 4 lanes go directly to chipset, and that's how chipset communicates with the cpu and transfers data from sata, usb, onboard devices, other pci-e slots.

The chipset can create a bunch of pci-e lanes, which are used to either connect onboard devices ( network chip, sound) or create extra pci-e slots.

If my memory is correct, b350 chipset creates 6 additional pci-e v2.0 lanes and typically, 1 lane is used for gigabit lan, 1 for audio and the four remaining usually go to a pci-e x4 slot or to a couple of pci-e x1 slots.

edit:

does each HDD or an M.2 use it's own PCIe lane?

On AM4 boards, the Ryzen processors have those 4 pci-e lanes "dedicated" to M.2 connector (like I said, they can be used for other purposes, but the cheapest and easiest way is to use them for m.2 connector).

This means that at least ONE m.2 connector can be used with nvme SSD drives, and depending on the SSD controller used, you then are able to achive up to 4 GB/s in either direction. There are some cheaper SSD controllers that use only 2 pci-e lanes, even if the m.2 connector has 4 lanes wired to it, and these SSD drives will only transfer at up to 2 GB/s in either direction.

Some motherboards have more than one m.2 connector. The second m.2 connector will either have only SATA wired to it (so you won't be able to use nvme drives with it, if those drives support only nvme), or the m.2 connector may have 1, 2 or 4 pci-e lanes routed from the chipset to it. On AM4, these lanes would be pci-e 2.0 so at reduced speed (500 MB/s per lane) compared to pci-e 3.0 lanes (~970MB/s per lane).

As for HDD each using its own pci-e lane.... it's a yes and no.

Think of the chipset as a big network switch, which has a 10 gbps port, and 16 or 24 1 gbps ports. The switch can pass data packets between those 16 or 24 ports, and can take data packets from all 1gbps ports and push them to the outside world through that 10gbps port , but obviously it can't pack 16gbps or 24 gbps into 10 gbps, so if all ports want to send data at the same time, each port gets slower speed.

Just the same , the chipset is internally like a network switch... it has a 4 GB/s connection going to the CPU (the outside world) which is like the 10gbps port on the switch, and internally it creates multiple "1 gbps ports" to which it connects various things : let's say every 2 sata connectors get a 1 GB/s connection, so if you have 6 sata ports on the motherboard 3 1GB/s ports on the chipset are used, every two usb 3.1 10gbps use a 1 GB/s port, every two or 4 usb 3.0 ports at a time use a 1 GB/s connection and last but not least, those 6 pci-e 2.0 lanes may be combined in one or two 1 GB/s "ports" and so on.