BIOS settings for DMA on the game PC
How virtualisation, Above 4G Decoding, Resizable BAR, CSM, Secure Boot and PCIe generation on the gaming PC decide whether a DMA stack initialises, and what each one can break.
Short answer
Six firmware settings decide whether a DMA stack initialises: virtualisation (VT-x/SVM and VT-d/IOMMU), Above 4G Decoding, Resizable BAR, CSM, Secure Boot and PCIe slot generation. Change one at a time, write down each change, and trust your board's own manual over any list.
On this page10
- Before you change anything in the BIOS
- Which settings matter and what each one risks
- Is virtualisation one setting or two?
- What Above 4G Decoding and Resizable BAR change
- Can turning off CSM stop Windows from booting?
- PCIe generation and bifurcation
- How to change firmware settings one at a time
- Problems caused by firmware settings
- Common questions about BIOS settings
- Sources
Warning
Before you change anything in the BIOS
Firmware menus are not standardised, so the same setting goes by different names across vendors and options hide behind other options. The right value also differs between boards, chipsets and firmware revisions, so learn what each setting does and what it breaks instead of applying a checklist blind. Your board's manual and the instructions issued with your firmware outrank any generic advice, ours included, so follow them wherever we differ. Changing these settings can break the game or Windows, so write down the current value of anything you intend to touch before you open the menu. That note is what gets you back to a working machine.
Which settings matter and what each one risks
The recommended values are typical starting points, not universal answers. Read the risk column first, because it is the part people skip and then regret.
| Setting | What it does | Typical starting point | Risk if you change it |
|---|---|---|---|
| Intel VT-x / AMD SVM | CPU virtualisation extensions. Lets hypervisors and virtualisation-based OS features run | Usually on. A few stacks specifically want it off — follow the note issued with your software | Turning it off breaks Hyper-V, WSL, Android emulators, sandboxes and virtualisation-based Windows security features |
| Intel VT-d / AMD IOMMU | Device-level address translation for PCIe devices. Governs how a device reaches system memory and is the basis of DMA remapping | Follow your firmware provider's instruction for your exact board. This is the setting most often named in an initialisation failure | Changing it can break unrelated device passthrough, and on some platforms it interacts with Windows Core Isolation and memory integrity |
| Above 4G Decoding | Lets firmware map device memory windows above the 4 GB address boundary | On, on any modern UEFI system. It must be on before Resizable BAR appears at all | Boards still booting in legacy mode may hide the option or fail to POST after it is enabled |
| Resizable BAR / Smart Access Memory | Lets a device expose a large memory window instead of the classic small aperture | On for a modern GPU. For a memory-access stack, test it rather than assuming — some setups behave differently with it enabled | It changes the PCIe memory map, so a previously working stack can stop working after a toggle. Record the state before and after |
| CSM / Legacy boot | A compatibility layer that boots in BIOS mode instead of UEFI | Off. UEFI boot is the assumption behind Above 4G Decoding, Resizable BAR and Secure Boot | Turning CSM off on a Windows install that was made in legacy MBR mode makes Windows unbootable until the disk is properly converted |
| Secure Boot | Verifies each boot component against keys the firmware trusts | On, unless something specific requires otherwise. Several current anti-cheats require it | Turning it off can lock you out of the game entirely. Turning it on over an unsigned boot path stops Windows booting |
| PCIe slot generation (Auto / Gen3 / Gen4) | Pins the link speed a slot is allowed to negotiate | Auto first. Forcing a lower generation is a legitimate fix for a slot that will not train reliably with an add-in card | Forcing a lower generation reduces bandwidth for anything else sharing those lanes, a GPU included |
| PCIe bifurcation | Splits a slot's lanes into smaller groups so several devices can share it | Leave at Auto unless a riser or carrier board specifically documents a required split | A wrong split can make everything in that slot disappear, including the graphics card |
| XMP / EXPO memory profile | Applies a faster memory timing profile than the module's stock setting | Leave it exactly as it was while you are diagnosing anything | A marginal memory profile produces random read failures and DTB errors that look identical to a bad memory map |
The dependency order explains most missing options: turn CSM off, then turn Above 4G Decoding on, and the Resizable BAR toggle becomes visible.
Is virtualisation one setting or two?
People say virtualisation as if it were one switch, but there are two, and they do different things. VT-x on Intel and SVM on AMD are CPU extensions that let a hypervisor run, which is what Hyper-V, WSL and virtual machines need. VT-d on Intel and IOMMU on AMD work on the device side, translating and constraining the addresses a PCIe device may reach. A DMA card is a PCIe device that reads memory, so the device-side setting is the one named in provider instructions and in initialisation failures. Do not assume the two move together, or that values that work on one board carry over to another.
What Above 4G Decoding and Resizable BAR change
Above 4G Decoding lets the firmware place device memory windows above the 4 GB boundary, which any modern system with a lot of RAM and a modern GPU needs. Resizable BAR builds on it, letting a device advertise a much larger memory window than the traditional small aperture. Both settings change the shape of the PCIe memory map, which is exactly what a memory-access stack cares about. This is why toggling Resizable BAR sometimes fixes an initialisation problem and sometimes creates one, so test it as a single, recorded change. If the stack works with it off, leaving it off is a legitimate setup.
Warning
Can turning off CSM stop Windows from booting?
CSM is the most dangerous of these settings and the least understood. If Windows was installed while CSM was on, the disk is very likely MBR and the boot path is legacy. Disabling CSM in that state removes the path the machine boots by, so the next power-on reaches the firmware and stops. To recover, re-enable CSM and boot normally, and only then decide whether to convert the disk to GPT with Microsoft's own tooling. Do that conversion deliberately and with a backup, not in the middle of troubleshooting a card. Before you touch this setting, check which mode you are in: System Information reports BIOS Mode as UEFI or Legacy.
PCIe generation and bifurcation
These two settings explain a card that is invisible rather than misbehaving. When a slot will not train reliably with an add-in board, pinning it one generation lower often makes it stable, which is a legitimate fix. Bifurcation is different: it splits a slot's lanes for multi-device carriers, and a wrong split makes the devices in that slot vanish outright. Leave bifurcation on Auto unless the riser's or adapter's own documentation tells you which split it needs. If a card is not enumerated at all, reseat it before you change either setting.
How to change firmware settings one at a time
The method matters more than the individual settings, because most long troubleshooting sessions start with someone who changed six things and cannot get back.
- Before the first change, photograph or write down every menu page you intend to touch, with its current values
- Confirm your boot mode by running msinfo32 in Windows and reading BIOS Mode, so you know whether Windows depends on CSM to boot
- Change exactly one setting, save, and boot all the way into Windows
- Re-run your card test and record the result against that one change
- If the change made things worse, put it back before you try the next one
- Follow the dependency order: CSM off first, then Above 4G Decoding, then Resizable BAR
- Leave memory profiles, fan curves and overclocks completely alone while you diagnose
- When you reach a working configuration, write it down somewhere you will still find it after the next firmware update
Problems caused by firmware settings
Each of these patterns has its cause in firmware rather than software, and for all of them, reseating the card comes before anything else.
| Symptom | Likely cause | Fix |
|---|---|---|
| Card is not enumerated at all — no device on either machine | Not fully seated, or the slot is disabled or wrongly bifurcated in firmware | Reseat the card and confirm the retention clip closes. Then check that the slot is enabled with bifurcation on Auto before you try a different slot |
| Worked before a board firmware update, fails after | The update reset settings to optimised defaults | Compare the menu against your written record and restore the values you had changed, which is exactly why you keep that record |
| Windows will not boot after disabling CSM | The install is in legacy MBR mode, so removing the legacy boot path removes the only path | Re-enable CSM to get back into Windows, then either convert the disk to GPT deliberately and with a backup, or leave CSM on |
| Stack stops initialising immediately after enabling Resizable BAR | The PCIe memory map changed underneath it | Turn Resizable BAR back off and confirm the stack recovers, then write down that working state as your configuration |
| Random read failures and DTB errors on a machine that used to be stable | A marginal memory profile, or a firmware update that re-applied one | Set memory back to stock speed, retest, and regenerate the memory map before blaming the card or the firmware |
| A setting the manual documents is not in your menu | Vendors hide options behind their dependencies or behind an advanced mode | Follow the dependency chain (CSM off, then Above 4G Decoding on), then look for an advanced or expert mode toggle on the menu's front page |
Common questions about BIOS settings
Which single setting matters most?
There is no universal answer. The chain of CSM, Above 4G Decoding and Resizable BAR controls access to the largest number of other options. The device-side virtualisation setting, VT-d or IOMMU, is the one provider instructions name most often, so follow the notes issued for your board.
Should I just load optimised defaults and start again?
Sometimes that is the fastest reset from a confused state, as long as you write your current settings down first. Afterwards, expect to redo the memory profile, boot order and fan curves.
Is updating the board firmware a good idea?
It is ordinary maintenance and it does sometimes fix PCIe enumeration problems, but it also resets your settings and can change which options exist. Do it as a deliberate step, not in the middle of diagnosing something else.
Do I change any of this on the second PC?
No. The second PC does not host the card, so its firmware has nothing to do with card initialisation. What matters on the second PC is its USB and power settings, which are covered in a separate guide.
Does turning virtualisation off make anything safer?
No. It changes what software can run on the machine, not whether anything is detected, so do not treat firmware toggles as a safety measure.
A forum post contradicts my firmware provider. Which should I follow?
Follow your board's documentation and the instruction issued for your exact firmware and board revision. A generic checklist was written for somebody else's hardware, so applying it can break a machine that was working.
Sources
Apart from the linked sources, the material comes from bench work, where the team recorded what each firmware setting did on their own boards. Your board's manual still takes priority over it.
- 01Kernel DMA Protection (Microsoft Learn)
what the IOMMU and DMA remapping control about a PCIe device's reach into memory
learn.microsoft.com
- 02TPM 2.0 and Secure Boot requirement (Activision Support)
the claim that current anti-cheats require Secure Boot to be enabled
support.activision.com
Read next
- Secure Boot: what it does and what turning it off costs
- Windows versions and builds: what breaks and when
- Installing a DMA card in the game PC
- How to generate a memory map (MMAP)
- DMA troubleshooting: fixes for common issues
- How a wrong system clock breaks TLS and licence checks
- Setting up Windows on the second PC for reliable reads
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