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What is Above 4G Decoding – your BIOS's dark horse. Should it be enabled?

Rinocrosserв 16:25 (08/02/21)

Above 4G Decoding is your BIOS's dark horse. What is it and should it be enabled?

In modern motherboards BIOS, you can find the Above 4G Decoding checkbox, and now the Resizable Bar has appeared next to it. And the Resizable Bar was already well explained by AMD – the processor can directly communicate with the card through a couple of big steps, instead of a million tiny ones.

But what then is Above 4G Decoding and is it equal to Resizable Bar? No article on the internet explains the term precisely, but I will try to make the best explanation possible.

What can this switch do and why does a video card need so much memory?

You may have noticed that a video card looks like a separate computer inside a computer. These devices have their own BIOS, a separate set of memory, their own power circuits and, of course, their own processor, even it called GPU, not CPU. It is easy to guess that they have similar and different features with the central processor, but they need to communicate: it is the central processor that commands the video card.

Therefore, for interaction, the processor often places copies or a cache of data in RAM to work with a video card. Then it transfers such information to the card via the PCI bus. Usually, the area for storing such information was limited to the first 4 Gigabytes of RAM.

And since modern software, operating systems and processors began to work with 64-bits, then much more space must be managed. And after 32-bit, where there was a limit of only 4 Gigabytes of data, now hardware and software can handle much larger values at the same time (in theory, up to 16 Exabytes, in existing hardware implementations there are only 256 Terabytes).

Should I enable Above 4G Decoding?

Judging by the reviews, it is not worth fixing what is not broken: some users have problems when this option is enabled. However, owners of cards with more than 4GB of memory, or with more than 16GB of RAM in the system, can try this function for better performance of computing tasks. Before that, many recommend updating the drivers and BIOS.

For example, scientific computing on Tesla cards, rendering large scenes for 3D artists, or mining modern cryptocurrencies will require more than 4 gigabytes of data at a time to be used effectively.

Above 4G Decoding is your BIOS's dark horse. What is it and should it be enabled?

Previously, the task was solved by a queue of intermediate operations in the drivers and the system so it never exceeds the allocated 4 Gigabytes, but this reduces performance, and for some tasks, it is completely unacceptable (owners of Tesla cards or several video capture cards have faced problems for a long time).

For example, to work with the RGB24 signal on the Avermedia website it is explicitly stated that the signal does not fit into the standard 32 bits.

Crypto-miners are advised to enable the checkbox from the moment it appears in the BIOS for proper scaling between two or more cards, because even now the 4GB limit is simply unacceptable for mining ETH.

It is worth considering that the consumption of RAM may increase at some moments, so it is rather dangerous to carry out this trick on computers with 8-12GB in modern realities.

Is Above 4G Decoding Needed For Gaming?

The Above 4G Decoding checkbox is definitely required in order for the Resizable Bar to work for Radeon 6000 (RDNA2) and NVIDIA RTX 3000 (Ampere) users. Moreover, both functions can already be enabled on modern motherboards with a fresh BIOS.

What's the difference between these two technologies?

Above 4G Decoding affects the amount of RAM allocated at a time for the processor to work with video memory. And this explains some increase in the consumption of RAM. Previously, you had to huddle in a 4GB block, while with the Above 4G, you can use the whole area.

Resizable Bar allows the graphics API to pass large blocks of memory to the graphics card. Previously, they communicated in chunks of 256 megabytes, but now you can send a lot at once and from any part of the memory.

Sounds like a pretty handy combination. By the way, I was pushed to the sinful land of BIOS tuning due to investigating the mining boom. Well, at least some benefit from these good guys... miners!

What hardware currently supports Above 4G Decoding?

Above 4G Decoding can be currently enabled on motherboards with support for AMD Ryzen 5000 processors (for example, AMD B550, X570 chipsets), and on other platforms, this feature is determined by the manufacturer.

If you do not find such a function, then you may need to update the BIOS (if support is declared in the description of the updates or from the manufacturer).

For Intel users, the feature is widely available on the Z490 chipsets, but it is also optionally found on the Z390 and even some server solutions, including relatively old ones, because the option itself was incorporated into the PCI-E standard more than 10 years ago.