There’s a new budget AMD AM5 motherboard chipset in the works, which I’d usually applaud in these times of overpriced components and high living costs, but there’s one aspect of it that caught my eye, and it’s a big red flag if you’re building a new PC, particularly for gaming. The new AMD B840 chipset only supports PCIe 3.0 for both storage and graphics cards, like it’s traveled in time from 2016, and this will seriously impact performance.
It was easy to miss this announcement in the flurry of new information AMD just released about its new Ryzen 9000 CPUs, but it’s right there at the bottom of the table announcing the new lineup of motherboard chipsets. That limit to the aging PCIe 3.0 interface doesn’t only mean you won’t be able to run the best graphics cards at full speed on this new chipset, but you won’t be able to get the most out of the best SSDs either.
I’ll start with graphics cards, because this is potentially a big issue for gaming PCs. If you’re buying a budget motherboard, you’ll probably be pairing it with a budget GPU, and that’s a problem, because today’s budget GPUs use the PCIe 4.0 interface. You’ll still be able to plug a PCIe 4.0 graphics card into a PCIe 3.0 slot, of course, but it will be slower.
The problem here is that budget gaming GPUs don’t use 16 PCIe lanes. Instead, they use eight lanes, as with the RTX 4060, or in some cases just four lanes, as with the Radeon RX 6500 XT. That’s fine on a PCIe 4.0 motherboard, but if you plug an 8x PCIe 4.0 GPU into a PCIe 3.0 motherboard you only get half the bandwidth, leading to lower frame rates. The situation is only going to become worse once we start getting the first PCIe 5.0 graphics cards in the near future.
Then there’s storage. OK, so you’re unlikely to be pairing a budget motherboard with a top-end PCIe 5.0 SSD such as the Crucial T705, but PCIe 4.0 SSDs, such as the Samsung 980 Pro and WD Black SN850X, can be found for very reasonable prices now. The top speeds of these drives will drop sharply if they’re plugged into PCIe 3.0 M.2 ports, though. The WD Black SN850X can hit a top sequential read speed of 6,973MB/s in our tests, for example, but you can’t get more than 4,000MB/s out of a 4x PCIe 3.0 slot, and in reality, the speed will be even lower.
There are other restrictions on this chipset too. It only supports USB 3.2 Gen 2, rather than Gen 2×2, for example, giving you a maximum speed of 10Gbps. That’s poor when 20Gbps speeds are supported by the B850 chipset, and the top two X870 and X870E chipsets make USB 4 support mandatory. You can’t overclock your CPU on the B840 chipset either, though this isn’t such a massive issue these days, when CPU boost clocks are usually high enough. On the plus side, at least the B840 chipset allows you to overclock your memory.
AMD says that the competition for B840 is Intel’s B760 chipset, which doesn’t allow CPU overclocking either, but at least the budget Intel offering supports PCIe 4.0. B840 looks much more like a competitor to Intel’s low-end H-series chipsets. The new motherboards based on this chipset had better be cheap.
Basically, a B840 motherboard will be fine if you just want to run a work PC based on one of AMD’s new Zen 5 CPUs, and you’re not bothered by storage performance, but if you’re building a gaming rig with one of AMD’s new motherboards, you’ll need a B850 motherboard at the bare minimum, and preferably an X870 board, which supports PCIe 5.0 on both your SSD and graphics card.
I honestly have no idea what AMD was thinking with releasing this chipset in 2024, but it’s definitely one for gamers to avoid. That’s a shame because the new AMD Zen 5 CPUs look like they’re going to offer a solid step up in performance from its current Zen 4 lineup.
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