Graphic cards contain a lot of very fast RAM, typically between 64 and 512 MB. With Linux, it's possible to use it as swap space, or even as RAM disk.
To use the graphic (and possibly any other) card's memory, it needs to be mapped by the kernel. By default only the system RAM is considered as usable RAM, to access memory from the PCI address space one needs a driver which registers them. In 2.6 kernels this driver is called Memory Technology Device (MTD). You need to enable this and some other key features in your kernel: Template:Kernel You can compile them as modules or built-in drivers, it works either way.
Finding the memory
Now let's search for the video memory in the PCI space. The easiest way is to use Template:Codeline(Template:Package). Start with looking up your graphics card: Template:Root An example output would be:
02:00.0 VGA compatible controller: ATI Technologies Inc R300 AD [Radeon 9500 Pro]
That's the graphics card. now we need to use Template:Codeline again to check the card for more details. To to this we tell Template:Codeline to be verbose when checking the PCI slot 02:00.0, as the previous lspci output indicated the graphics card sits there: Template:Root
0000:02:00.0 VGA compatible controller: ATI Technologies Inc R300 AD [Radeon 9500 Pro] (prog-if 00 [VGA]) Subsystem: PC Partner Limited: Unknown device 7c07 Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping+ SERR- FastB2B- Status: Cap+ 66Mhz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- Latency: 32 (2000ns min), cache line size 08 Interrupt: pin A routed to IRQ 255 Region 0: Memory at d8000000 (32-bit, prefetchable) [size=128M] Region 1: I/O ports at b000 [size=256] Region 2: Memory at e9000000 (32-bit, non-prefetchable) [size=64K] Expansion ROM at e8000000 [disabled] [size=128K] Capabilities:  AGP version 3.0 Status: RQ=256 Iso- ArqSz=0 Cal=0 SBA+ ITACoh- GART64- HTrans- 64bit- FW+ AGP3+ Rate=x4,x8 Command: RQ=1 ArqSz=0 Cal=0 SBA+ AGP- GART64- 64bit- FW- Rate=<none> Capabilities:  Power Management version 2 Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-) Status: D0 PME-Enable- DSel=0 DScale=0 PME-
What we need to look for is the prefetchable memory region, Region 0. The start of the memory space is 0xd8000000, a 32 bit hex value.
Mapping the memory
To actually access the memory from PCI space, you need to tell the MTD driver where it starts and how much of it you want to allocate. Template:Code
Creating the mtd devices
After you have the support in the kernel and all the required data, you need way to access the memory. Time for creating the devices!
If you have udev just modprobe mtdchar and modprobe mtdblock and hald will create device files for you automatically. Template:Code
Now, you can either use it as a RAM Disk, or as swap. Either way, you need to create a FS on it, I'll go with the swap way. Template:Code
This will create the swap structure in the video card's RAM, and also activate it as system swap. To specify the priority (which swap source will be used first in the swap list), use swapon's switch, -p X where X is a number between 0..32767, the higher the number, the higher the priority. Template:Code
Without specifying priority, the next lowest will be used. Also, this can be added to the fstab, so you won't need to manually add this after every boot: Template:Code
In the options field, the pri= is the same as swapon's -p switch. Please read the troubleshooting section, before closing this page.
It's nice to have fast swap or RAM disk on your home computer but be warned, if a binary driver is loaded for X, it may freeze the whole system or create graphical glitches. Usually there is no way to tell the driver how much memory could be used, so it won't know the upper limit. However, the VESA driver can be used because it provides the possibility to set the video RAM size.
So, Direct Rendering or fast swap. Your choice.
Unlike motherboard RAM and hard drives, there aren't any known video cards that have ECC memory. This may not be a big deal for graphics rendering, but you definitely don't want to put critical data in it or use this feature on servers.
- Q: mkswap/mkfs.whatever succeeds, but I can't swapon/mount it! Also it reads 0xff bytes! What to do now?
- A: You have to set the BIOS setting, AGP Aperture Size to at least or equal to your video RAM. Most people say it's better to set it to the half of your system RAM/video RAM size, but it does nothing just limits the BIOS' capability to map from PCI address space.
- A: It is also possible that you are using wrong address/offset values. Double-check that you are using the 'prefetchable' space.
- A: Make sure the videocard that contains the memory has a driver loaded.
- Q: I have shut down my computer and during the next boot I see it couldn't add /dev/mtdblock0 as swap! What did go wrong?
- A: It's trivial but at first time everyone is surprised: RAM will lose its contents if not powered. Solution: mkswap again!
- Q: The kernel tells me: slram: ioremap failed. What is this?
- A: You forgot to add + before the actual value in the third parameter, so it's not considered as offset code but an actual PCI address. Prefix with +, or calculate the exact ending location.
- Q: There is no /dev/mtd0 and/or /dev/mtdblock0 device.
- A: Read the 'mknod magic' box.
- Q: I have enabled Highmem support in the kernel and now slram complains something about vmalloc in dmesg. I can't access the device.
- A: Try appending vmalloc=nM to the kernel parameters, with n something like 256 or higher if needed. The default is 128
- Q: Does this work with onboard display with side-port memory?
- A: Recent AMD/ATI Integrated Graphics Processor (IGP) can be equipped with dedicated memory, the so-called side-port memory. The IGP, together with the side-port memory, is disabled by default when you are using an independent graphics card. You can enable the IGP by enabling the SurroundView in the BIOS, this will also enable the side-port memory. Then you can configure it as a MTD.
Credits goes to
This article was plagi^W inspired by the article written by Michal Schulz, available here: .