ARM Port

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ReactOS 32-bit ARM port was created by ReactOS Portable Systems Group back in 2008. Originally it was designed for ARMv5 and ARMv4, but these cores have an MMU that is incompatible with ReactOS/Windows multiprocessor requirements, and lacks instructions needed for synchronization done in user-mode, so currently the port targets ARMv7 compatible cores.

It should be possible to port to ARMv6 and possibly older ARM architecture revisions, but currently it's postponed and the system will be operable only in single-processor mode in these cases.

Since the GCC compiler has dropped the support for ARM PE executables, and the project had to upgrade GCC version, the port become unmaintained somewhere back in 2015 (see #Known facts section).

However in November 2018 Microsoft has added official ARM and ARM64 target support into Visual Studio 15.9, so now it is possible to bring the port back to life!

Follow the discussion in this thread:

Known facts

  • 15 Oct 2009: Last commit by ReactOS Portable Systems Group. [3]
  • 22 Mar 2012: GCC 4.7.0 has dropped support for ARM PE executables. [4] [5]
  • 22 Jul 2012: Timo Kreuzer has added support for MSVC ARM build. [6]
  • 15 May 2015: Beginning with r67740 ReactOS builds for ARM, too. Timo Kreuzer made some effort to get it build and succeeded. [7]
  • 15 Nov 2018: Microsoft announced official ARM target support for Visual Studio 2017. [8]
  • 19 Nov 2018: Support for VS2017 (15.9) compiler was added. [9]
  • 31 May 2019: A big streak of Pull Requests from woachk related to ARM port, totally not a coincidence. [10] [11]
  • 17 Aug 2019: Configure option WITH_HOST_TOOLS added to allow cross-compiling to ARM. [12]
  • 20 Aug 2019: Fixed some ARM compilation problems. [13][14]
  • 26 Apr 2020: ReactOS Applications Manager was able to start on Windows 10 for ARM32. [15]
  • 22 Oct 2020: Building host-tools for the host architecture has been broken, so building ARM binaries on x86 and x64 hosts is not possible. [16]
  • 11 Apr 2021: A new Pull Request with cross-compilation fix has been opened, so building for ARM hopefully will be fixed [17]
  • 24 May 2021: ARM cross-compilation has been fixed and simplified. GitHub ARM builders were added (Visual Studio 2017 and 2019), for now only rapps is built. Also it is possible to run ReactOS Calculator on Windows 10 for ARM32. [18][19]
  • 28 May 2021: A Jira ticket was created to track problems and progress on ARM port. [20]


Visual Studio

This is recommended build method. Please refer to these articles:

Building steps

Visual Studio ARM cross-compilation tools

You will need latest RosBE and Visual Studio 2017 or 2019 installed. It would work with VS community edition as well.

  • Make sure you have selected "Add BIN folder to PATH variable" when installing RosBE (see Visual Studio for details)
    • If you have RosBE installed but did not selected this option earlier, you need to add C:\RosBE\Bin (or your installation directory) to the PATH environment variable
  • Start VS command prompt for ARM cross-compilation
    • Normally you should have "x64_arm Cross Tools Command Prompt for VS" shortcut in your Start Menu within Visual Studio section
    • If you don't have it, make sure you have selected ARM cross-compilation tools in VS installer
    • If the tools are installed but there is still no shortcut, you can create it manually:
      %comspec% /k "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsamd64_arm.bat" (replace with your VS installation path)
  • Make sure to set BISON_PKGDATADIR and M4 variables as described in Visual Studio
  • Run configure.cmd
  • Type cd output-VS-arm and run ninja rapps or ninja calc, or just ninja -k 1 to build everything (in latter case there will be lots of build errors to fix)

Debugging steps

The steps below are describing how to debug ReactOS applications built for ARM architecture on Windows platform. It's important to ensure our applications are running there without problems.

  • You will need Windows on ARM environment (or just WoA) and WinDbg debugger
    • Most devices/computers with Windows 10 ARM64 would work
    • Windows RT devices are okay in case you are able to run non-signed code there
  • Get WinDbg debugger for ARM architecture (available in Windows 10 SDK)
    • Once installed, it's usually located in C:\Program Files (x86)\Windows Kits\10\Debuggers\arm
  • Set debug symbols path to C:\path\to\reactos\output-VS-arm\msvc_pdb (see WinDBG#Symbols for details)
  • Build wanted application for ARM and start it with WinDbg in WoA environment
  • Debugging experience should be the same as usual at this point, if you are not familiar with it, see WinDBG

General TODO list

  • [✓] Make configure.cmd detect the Visual Studio 15.9 environment
  • [✓] Make sure it detects and builds both x86 and x64 ports successfully
  • [✓] Add ARM target support to the configure.cmd build script
  • [✓] Create MSVC ARM builder in our GitHub CI so we notice when stuff is broken
  • Make ReactOS applications work on ARM Windows without crashes (RT, RT 8.1, 10 ARM32 PE, 10 ARM32 build 15035)
    • See existing tickets in this Jira epic: CORE-17604
    • Fix remaining problems in our CRT and SDK: sync with newer versions of upstream components
  • Fix the ARM port build so it can generate bootcd-arm.iso and livecd-arm.iso
    • Get rid of boot sector, it's not supported on ARM
      • Issue with bootcd - boot sectors should be built only for x86/x64 BIOS platform, see message in chat
    • Choose your way of working on ARM port boot process:
      • 1st way: Revive ARM low-level bootloader (ARMLLB) that will bootstrap FreeLoader on ARM
        • Issue with freeldr - boot\freeldr\freeldr\ui\ui.c(532): error C2061
      • 2nd way: Make it booting from UEFI
        • Quibble bootloader by Mark Harmstone might help
        • Revive our bootmgr / rosload
        • Make FreeLoader compatible with UEFI (freeldr.efi)
      • 3rd way: Work on both UEFI and FreeLoader via ARMLLB or U-Boot (requires more effort)
  • Once the whole tree is built without issues, we can think about updating this TODO list

These pages may be helpful to accomplish the tasks above:

Don't be in hurry and propose a Pull Request for the each fix you have done into the ReactOS repository.

RosBE (outdated)

This build method has not been maintained for some years and may not work.

Obtain the React OS Build Environment for ARM (RosBE-ARM). The latest version is 1.0 and available from:

Launch the RosBE-ARM command prompt, and type makex on the command line. After some minutes, you should have the whole tree built.


Making a RAM disk

Currently, the ARM port will only boot from a RAM disk, either in NOR/NAND flash on a real device, or an emulated one like QEMU. A RAM disk is a simple flat image file that is loaded into memory.

To generate one of these images, you may use a tool such as qemu-img, the Mac OS Disk Image utility, or any other tool capable of generating a raw image file that corresponds to a hard disk image. You should generate a RAM disk of 32MB or less.

You then need a tool (on Windows), or the proper set of commands, to mount this virtual file/image as an actual hard drive on your machine. This step is required as the ARM port does not have an "installer" of any sorts, and cannot partition your disk, so you must do so yourself.

On a system such as Linux, you can use a local loopback mount, and then launch mkfs_fat on the mounted device. You can use either FAT16 or FAT32, as both have been tested with success. On OS X, the Disk Utility will automatically ask you to partition/format the image. Make sure you select "MS-DOS" partitioning when asked. On Windows, you may need to obtain a tool that can mount such images, such as vdkmount. Alternatively, you may use this image as an x86 ReactOS hard disk in QEMU, and then launch the x86 ReactOS setup program. Continue through the installer pages until you reach the formatting step, and allow ReactOS setup to format the disk. After that, you should be able to quit QEMU and have a formatted image file.

IMPORTANT: Usually, your raw image file will look like a real "disk". It will have an MBR sector at 0x00000000, followed by a boot sector for your first partition somewhere else on the disk. Sometimes this is at sector 63, 2, 1, 60, etc. You will need to know the offset of where the partition starts. Often this will be 0x7E00 (32256), although on Mac OS X .dmg images it will be 0x200. This offset is easy to find, either with a tool that understands your image file when mounting it, or by using a hex editor and locating the boot sector (look for "FAT").


Now that you have a mountable, formatted, FAT image file, go ahead and mount it, and set your ROS_INSTALL environment variable to the \reactos folder on the virtual drive. For example, if you mapped it to X:\, this would be ROS_INSTALL=X:\reactos.

Now issue a makex install from the RosBE command line. This will copy all the files to your new RAM disk.

The last thing you'll need is a freeldr.ini. You can write one from scratch, or simply take the usual one that the x86 ReactOS setup program installs on your x86 ReactOS partition/image file. You'll want to keep the ReactOS entry that's marked as "(RAMDisk)".

Preparing to boot

Now that you have a RAM disk, you are ready to build the final components. The ARM port basically uses an LLB (Low-Level Bootloader) to initialize the hardware, and then pass control to FreeLDR, the ReactOS Boot Loader. FreeLDR will then use the RAM disk to search for freeldr.ini, present the OS options, etc, and finally load the kernel.

Therefore, there are three components that the system needs: the LLB, the loader, and the RAM disk. QEMU has command-line options and support for only loading two image files, and so the LLB and the loader must be fused together into one binary file. Additionally, the RAM disk image file you created must be slightly modified for technical reasons beyond the scope of this document.

Thankfully, a tool exists to perform these operations: nandflash. You simply need to run nandflash (from your output-arm/tools/nandflash directory) with the "1" option to instruct it to generate the necessary files. The other use of nandflash (without the parameter) is to generate OMAP3 ROM NAND flash files, but that usage is beyond the scope of this document.

Note that nandflash expects your image file to have been called ramdisk.img, so make sure you rename it to match this. After running nandflash, you will have two new files: ramdisk.bin, which is a modified version of your ramdisk image, and reactos.bin, which is the fused LLB and loader together.


You can now instruct QEMU to boot ReactOS for ARM. You will need to obtain the ARM version of QEMU (qemu-system-arm) or build it yourself. Currently, testing is done with the "Versatile/PB" platform, but with a modified CPU target being the Cortex A8, an ARMv7-a core, which is being used internally for BeagleBoard and ZoomII-MDK work. To specify these two options, use -M versatilepb -cpu cortex-a8 on your command line.

You now need to specify the two files that nandflash created for you. You may do this with the following two options: -kernel reactos.bin -initrd ramdisk.bin.

Finally, you need to tell the LLB that we are booting from a RAM disk, and you need to input the boot sector offset we talked about earlier. You can do this with the following options: -append boot-device=RAMDISK,rdoffset=0x200. In general, -append is used to send command-line parameters to the LLB, and you can specify additional options by comma-separating them.

Here is an example final command line for QEMU:

qemu-system-arm -M versatilepb -cpu cortex-a8 -kernel reactos.bin -initrd ramdisk.bin -append boot-device=RAMDISK,rdoffset=0x200

Past this point, the experience should be familiar. Note that the only thing you can do for now is select the OS from the boot selection menu, and see it load a couple of drivers and hang somewhere in the kernel.

Compatible devices list

See this forum post:



See Also

ReactOS ports‎
In active development Intel IA-32 (x86) | AMD64 (x64)
In slow development ARM32 (armv7) | NEC PC-98 (x86) | Xbox (x86)
Unmaintained PowerPC (ppc) | OLPC (x86)
Missing ports that Windows has ARM64 (aarch64) | Intel IA-64 (Itanium) | SGI 320/540 (x86) | DEC Alpha (axp) | MIPS
Ideas for new ports FM Towns (x86) | IBM PS/2 MCA (x86) | PlayStation 4 (x64)