VRAM Configuration Guide¶
The BC-250 shares the same 16GB of memory between the CPU and GPU. This is called "Unified Memory Architecture" or UMA.
However, all PC software still expects CPU and GPU to have separate pools of memory, therefore the system splits usage of the memory into RAM and VRAM.
Changing the VRAM Split¶
The stock 3.00 BIOS splits the memory into 8GB RAM and 8GB VRAM.
Control of the RAM/VRAM split is ordinarily not exposed to the user. However, there are ways to modify it.
Memcfg Utility¶
You can set the VRAM split from a running Linux system by running a program: fanoush/bc250_memcfg.
This is the preferred option because it does not involve reflashing BIOS, and therefore it is safer for novice users.
git clone https://github.com/fanoush/bc250_memcfg
cd bc250_memcfg
make
sudo ./bc250memcfg UMA_SIZE 512
Reboot to apply the change.
Even though this is a Linux program, the change is stored into CMOS, so it only needs to be ran once and not at every boot.
Where the setting lives, and when it gets lost¶
bc250memcfg writes the split into extended CMOS (battery-backed RTC RAM, via I/O ports
0x72/0x73, block 0x90–0xAB plus a checksum), not into the SPI flash chip that holds the
BIOS. That has practical consequences, verified on a real board:
- It survives a BIOS reflash. A 6 GB split stayed intact through a full BIOS flash with no re-run of memcfg needed.
- It is lost on a CMOS clear or battery pull. Since a CMOS clear is the standard step after flashing a BIOS, check the split afterwards and re-apply if it reads small.
- Normal power-cycling is safe. Powering off or unplugging the PSU is not a CMOS clear; the RTC battery keeps the setting.
Checking the active split:
cat /sys/class/drm/card*/device/mem_info_vram_total
# 6442450944 = 6 GB split, for example
free -h
# System RAM should be 16GB minus the split (~9.4 GB with a 6 GB split).
# This confirms the split is live this boot rather than a stale value
A note on modified BIOS¶
You may have read about needing to flash the BIOS to unlock the VRAM split option in the BIOS setup. This was previously required to get the option, but it is now obsolete.
Because of the above utility, there is no longer any need to flash BIOS to change VRAM settings.
There may be no VRAM menu item at all
Some video guides show a VRAM/UMA selector in the BIOS setup screens. On at least one common community 3.00-based modded BIOS, a menu-by-menu walk of the entire setup found no UMA or frame-buffer size option anywhere. If your BIOS doesn't have it either, that is normal. Use memcfg and don't go looking for the menu.
Dynamic VRAM¶
It is commonly stated that 512MB VRAM split is a "dynamic allocation" mode, where the system will automatically reassign RAM to the VRAM pool as needed, but then will return that VRAM back to RAM when the game is closed.
In truth, ALL VRAM split settings are dynamic! There is nothing special about the 512MB mode except that it has a very small MINIMUM VRAM allocation.
What the VRAM split setting above is actually doing is setting the MINIMUM amount of memory that is used as VRAM. This means that with a 512MB VRAM split, when a game is not running, almost the whole 16GB of system memory is available as regular RAM, while with an 8GB split nearly half of the system memory will never be usable as regular RAM. But, both modes will dynamically use more than 512MB and 8GB of VRAM respectively.
So what is the maximum VRAM?
By default, Linux will use up to half of the RAM capacity (that is not being dedicated to VRAM) as dynamic VRAM. This is used in addition to the VRAM split selected above. When this total is exceeded, the display driver will crash.
With this knowledge, here is the default maximum VRAM usage before crash for each VRAM split: - 512 MB split means 8.25 GB max VRAM - 4 GB split means 10 GB max VRAM - 6 GB split means 11 GB max VRAM - 8 GB split means 12 GB max VRAM
Solving Crashes with 512MB VRAM split¶
Because the default max VRAM usage with 512MB VRAM split is 8.25 GB, the system can tip over the limit when the game is set up to use 8GB or more of VRAM. This is the primary reason for games crashing when using the 512MB VRAM split, while working fine with an 8GB VRAM split.
To solve this while staying with the 512MB VRAM split, there is a Linux kernel boot parameter that will override the dynamic VRAM limit with any amount.
ttm.pages_limit=value- where
valueis replaced by a number using the following formula: - ttm.pages_limit =
(Desired max dynamic VRAM in GB) * 1024 * 1024 / 4
- where
For example, to extend the max dynamic VRAM to 12GB, use ttm.pages_limit=3014656.
This has the same maximum VRAM usage as the 8GB VRAM split, while returning nearly all of the VRAM
back to regular RAM when the game is closed.
Applying the kernel parameter¶
Applying the kernel boot param differs depending on your distro:
- On Bazzite:
- sudo rpm-ostree kargs --delete=ttm.pages_limit --append=ttm.pages_limit=3014656 --reboot
- On CachyOS:
- CachyOS can have one of several different bootloaders depending on your installation settings.
- See CachyOS Boot Manager Configuration
- On others:
- Determine which bootloader your distro uses and consult its documentation.
A second crash mechanism the kernel parameter does not fix¶
There is a separate way the 512MB split kills game sessions, even when total VRAM usage is nowhere near the dynamic ceiling: scanout framebuffers must live in the real (minimum) VRAM carve; dynamically borrowed GTT pages don't qualify.
Root-caused from the journal on a real crash (Cyberpunk 2077 under gamescope, 512MB split): the game's vkd3d device filled the 512MB device-local heap, the compositor's next framebuffer could not be pinned, and the whole session died. The journal signature to look for:
amdgpu: pin failed
[drm] Failed to pin framebuffer with error -12
gamescope: drm: fatal flip error, aborting
followed by gamescope aborting (SIGABRT). Notably there is no GPU hang and no OOM-kill: the system is fine, only the display path ran out of pinnable memory.
For this failure ttm.pages_limit does not help, because it only raises the dynamic ceiling.
The fix is a larger minimum split. 6 GB (UMA_SIZE 6144) resolved it and the crash never
returned.
What VRAM Split is Best?¶
Using 256MB or 512MB with the above ttm.pages_limit kernel parameter ensures that your system
has as much RAM available as possible while avoiding crashes in games, while still being able to
allocate as much VRAM as needed.
However, there are reports that higher VRAM splits perform a little bit better due to not needing to reallocate memory as often. There is not conclusive data on this so it is up to you to experiment.
No matter what your VRAM split is, it is a good idea to set up your game settings to keep total VRAM usage on the lower side, such as 8GB or less.
With a low VRAM split, a game using less than 8GB of VRAM gives memory back to the regular RAM pool, which lets a game needing lots of RAM stay out of swap more. This helps with performance and stuttering.
With an 8GB VRAM split, memory is wasted if your game uses less than 8GB of VRAM, because the 8GB split makes that VRAM unusable as regular RAM.