Hardware

ReBAR, Above 4G Decoding and CSM

Last updated: 5 November 2026

What Above 4G Decoding, Resizable BAR and CSM do, when they matter for a mining rig, how to set them safely, and how to check the gain.

What the three BIOS settings do

All three settings live in the motherboard BIOS. Two of them help a rig use many GPUs, and the third gets in their way.

Above 4G Decoding lets the system map devices above the 4 GB address boundary. This makes room for many GPUs and their large memory windows.
Resizable BAR (ReBAR) lets the CPU see a GPU's entire memory as one window, instead of small slices.
CSM (Compatibility Support Module) is legacy BIOS boot emulation. It conflicts with the first two settings.

Above 4G Decoding and ReBAR work in modern UEFI boot mode with CSM turned off.

The GPU benchmark table ranking cards by mining income — Multi-GPU rigs are where these BIOS settings matter most.

When do they matter for mining?

One or two GPUs in a normal PC usually need nothing changed. The settings start to matter in two cases.

A rig carries several GPUs. Without Above 4G Decoding, the board may refuse to boot or drop cards.
An algorithm or miner streams large working sets. Here ReBAR can add a few percent.

Memory-heavy GPU mining is the kind of load that benefits. Treat the exact gain as something to measure, not assume.

Qubitcoin's qhash is not such a load. Its simulated circuit is small, so on Qubitcoin these settings matter mainly for rigs with many GPUs.

How to set them

Update the BIOS first. Older boards gained ReBAR through an update.
Turn CSM off.
Turn Above 4G Decoding on.
Turn Resizable BAR on. Some boards call this setting Auto.

The OS must be installed in UEFI mode, or it will not boot with CSM off. A very old Windows install in legacy mode is the one common thing this breaks. The fix is to reinstall Windows in UEFI mode.

A BIOS screen with an update chip at the top and three switches, each with a green tick: legacy boot, a crossed-out floppy disk, is off; Above 4G Decoding, several graphics cards, is on; Resizable BAR, a whole memory chip in one frame, is on. — Update the BIOS, then turn CSM off and Above 4G Decoding and Resizable BAR on.

How to check it worked

On Windows, GPU-Z shows whether ReBAR is active. On Linux, the kernel log and vendor tools show it.

Then benchmark your miner before and after the change. Use a long window rather than the first minute.

Keep the change only if the number says so. Copying BIOS settings without measuring is how rigs end up unstable for zero gain.

Ready to put this into practice?

The How to Mine guide picks your closest server and builds your exact miner command.

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