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You can run RTEMS in the open source processor emulator QEMU. The QEMU emulator runs on a number of hosts including Linux and Windows. It also supports a number of processors.

QEMU On Windows

Install the RTEMS MinGW Tools for Windows. Download the processor emulator you wish to use and install. For example we will install the i368 emulator to c:\rtems\i386\qemu-0.8.2-windows. Download the boot floppy image to c:\rtems\i386. To run QEMU create the batch file c:\rtems\i386\qemu.bat with:

rem @echo off
set QEMU=C:\rtems\i386\qemu-0.8.2-windows
%QEMU%\qemu -L %QEMU% -m 128 -boot a -fda c:\rtems\i386\rtems-boot.img -hda fat:%1

You can download this batch file from

The boot floppy will look in the root directory the QEMU C: drive for a file called rtems-grub.cfg. The QEMU C: is a directory on your hard disk passed to QEMU on its command line and can be seen in the qemu.bat file we created as the -hda fat:%1 option. The command line option we pass to qemu.bat is a directory that is the root directory the QEMU C: drive. For example download and build RTEMS for the i386 target as described on the MinGW Tools for Windows page. Then install into the c:\rtems\src\i386\i386-rtems4.7\c\pc586\testsuites\samples directory. Open the RTEMS (i386) command window by running c:\rtems\i386\rtems-cmd.bat then enter:

RTEMS(i386) C:\rtems\i386>qemu c:\rtems\src\i386\i386-rtems4.7\c\pc586\testsuites\samples

The QEMU windows will open and GRUB will provide you with a menu of the RTEMS sample applications. Select an application and watch it run.

QEMU PC (i386) Emulator

The RTEMS PC BSP runs in the QEMU emulator (it also runs in the VirtualBox emulator?). The QEMU emulator needs a boot image that will load RTEMS and we use GRUB for this. The simplest may to do this is to create a floppy disk image and tell QEMU to boot from a floppy disk.

Making a Boot Floppy Disk

We run RTEMS in QEMU by first booting from a floppy disk image containing the GRUB boot loader. You can use the image we have made by downloading it from the RTEMS ftp server. The script builds the floppy disk, the grub source as per GPL, and the boot image.

Instructions on how to built the image and were run on a Fedora Core 5 Linux host can be found at

Now we need to setup some disk drivers for Qemu:

 $ dd if=/dev/zero of=hda.img count=1000
 $ ls -las hda.img

The first creates a disk and second shows you the details of the disk you just created.

The default grub.cfg file will attempt to read a GRUB configuration file called rtems-grub.cfg from the root directory of the hard-disk. This lets you create a specific GRUB configuration for you testing without needing to rebuild the floppy image. An example configuration that lets you run the RTEMS Samples is:

# RTEMS Grub configuration for the Samples

set default=0
menuentry "RTEMS - Hello" {
 set root=(hd1,1)
 multiboot (hd1,1)/hello/hello.exe

menuentry "RTEMS - Ticker" {
 set root=(hd1,1)
 multiboot (hd1,1)/ticker/ticker.exe

menuentry "RTEMS - Minimum" {
 set root=(hd1,1)
 multiboot (hd1,1)/minimum/minimum.exe

menuentry "RTEMS - Paranoia" {
 set root=(hd1,1)
 multiboot (hd1,1)/paranoia/paranoia.exe

menuentry "RTEMS - File IO" {
 set root=(hd1,1)
 multiboot (hd1,1)/fileio/fileio.exe

menuentry "RTEMS - Unlimited" {
 set root=(hd1,1)
 multiboot (hd1,1)/unlimited/unlimited.exe

menuentry "RTEMS - Loopback" {
 set root=(hd1,1)
 multiboot (hd1,1)/loopback/loopback.exe

menuentry "RTEMS - PPPD" {
 set root=(hd1,1)
 multiboot (hd1,1)/pppd/pppd.exe

menuentry "RTEMS - Base Mulitprocessor" {
 set root=(hd1,1)
 multiboot (hd1,1)/base_mp/base_mp.exe

menuentry "RTEMS - Base Single Processor" {
 set root=(hd1,1)
 multiboot (hd1,1)/base_sp/base_sp.exe

menuentry "RTEMS - C++ Static Constructors" {
 set root=(hd1,1)
 multiboot (hd1,1)/cdtest/cdtest.exe

menuentry "RTEMS - C++ IO Streams" {
 set root=(hd1,1)
 multiboot (hd1,1)/iostream/iostream.exe

To start Qemu 0.9.1 you have to use this script:

$ qemu -m 128 \
       -boot a \
       -fda <path to rtems-boot.img> \
       -hda <path to hda.img> \
       -hdb fat:. \
       -serial stdio \
       -no-reboot \

List of the options explained:

-m 128
sets the memory size to 128M
-boot a
boot from the floppy disk a (best option)
-fda <path to rtems-boot.img>
defines the second best floppy disk image
-hda <path to hda.img>
defines the first hard disk image
-hdb fat
says hdb is fat disk and . is the directory to make the root of that disk and -hdb is the mapping to the local disk so we can read the RTEMS executable into memory
-serial stdio
directs the serial port to the stdio of qemu
tells Qemu to not reboot if the code running wants to. Stops an error looping over and over.
tells Qemu to start a GDB server

"Couldn't open file" GRUB Error Message

If GRUB gives this error while trying to load a menu entry, press 'c' on the keyboard at the GRUB menu screen to open the GRUB command prompt.

Then type:

grub> ls

which should give you a list of the devices currently on the system.

Try the following for every device until you find the application you want:

grub> ls DEVICE/

Now edit the entry that gave you the error (press 'e' at the GRUB menu screen) and correct the path.

Running Other Images

Instead of adding menu entries to the rtems-grub.cfg file, you can instead boot an RTEMS image directly from the GRUB command prompt.

Run QEMU as before (or qemu.bat batch file as mentioned above (if you are running windows)). At the GRUB menu screen, press 'c' on the keyboard to switch from the menu to the GRUB command prompt. At the prompt type:

grub> set root=(hd1,1)/
grub> multiboot (hd1,1)/myimage.exe

where myimage.exe should be replaced with the name of the RTEMS image that you want to run. GRUB supports filename completion via the TAB key, so you can just type "multiboot (hd1,1)/" and then press the TAB key to see a list of all files and directories.

Then just type

grub> boot

to load and run your image.

Using the rtems-testing Module

The automated testing infrastructure provided by the rtems-testing (formerly gcc-testing) module can be used to run applications in QEMU.

The first step is to check out rtems-testing from Git. Instructions can be found on the RTEMS GIT Repository? page.

git clone git://

Create two directories, ~/qemu and ~/qemu/hd.

mkdir ~/qemu
mkdir ~/qemu/hd

There is a qemu-support directory in rtems-testing with a boot floppy image. Copy rtems-testing/qemu-support/pc386_fda.bz2 to ~/qemu.

cp ./qemu-support/pc386_fda.bz2 ~/qemu

Unzip (bunzip or bzip2) pc386_fda.bz2.

bunzip ~/qemu/pc386_fda*

Run make at the root of the rtems-testing module. This should create a pc386 script in ./sim-scripts. To use these scripts for pc386, you have to build RTEMS with USE_COM1_AS_CONSOLE=1 and BSP_PRESS_KEY_FOR_RESET=0 on the configure command line. For example, if rtems is checked out at /home/user/rtems/rtems and you build in /home/user/rtems/b-rtems, then from b-rtems you will configure rtems as:

../rtems/configure --target=i386-rtems4.10 \
   --prefix=$INSTALL --disable-multiprocessing \
   --disable-cxx --disable-rdbg \
   --enable-maintainer-mode --enable-tests \
   --enable-networking --enable-posix \
   --disable-itron --disable-deprecated \
   --disable-ada --disable-expada \
   --enable-rtemsbsp=pc386 \

If you run:

./sim-script/pc386 -i PATH_TO_MY_EXE

The pc386 script will copy your executable to ~/qemu/hd/test.exe, which is what pc386_fda boots. You should see the result in your console. Scripts to run other simulators are in the sim-scripts directory.

NOTE: If you have a means to pass command line arguments to the RTEMS executable (e.g., this works when booting with grub) then there is no need to specify the serial console at 'configure'-time. You simply add the command line argument


Booting an RTEMS executable directly with qemu

Qemu has a native multiboot implementation that is smart enough to load an ELF .exe into memory and transfer control to it. This makes it unnecessary to prepare a full floppy or harddisk image complete with MBR and boot loader. To use it, provide the -kernel option:

qemu-system-i386 -kernel hello/hello.exe

Running RTEMS TCP/IP Applications

You can run RTEMS applications that use TCP/IP on QEMU. It requires getting a few very picky things just right so hang on and this section should help. The RTEMS application running on QEMU is executing on a virtual machine and thus QEMU must provide a simulation of a real NIC whose I/O is relayed or tunneled through a virtual interface to the real world. For most testing purposes, it is sufficient to have the connection work between just your QEMU session and the host computer running QEMU. AFAIK, for this to work, you must execute qemu as root. In all other cases of running QEMU with RTEMS, you can be a regular user. So the lesson here is only turn on TCP/IP support in QEMU when you really need it.

This is the script I use to start QEMU 0.9.1 with networking enabled:

qemu -cpu 486 -m 8 -boot a \
    -fda /home/joel/qemu/pc386_fda \
    -hda fat:/home/joel/qemu/hd \
    -net nic,macaddr=00:80:7F:22:61:77,model=ne2k_isa \
    -net tap,script=/etc/qemu-ifup  \
    -serial stdio --no-reboot

This says you are simulating an i486 CPU, booting from the floppy image /home/joel/qemu/pc386_fda and using the directory /home/joel/qemu/hd as the HDA presented to the QEMU session as a FAT filesystem. In my case, the floppy has GRUB on and always boots quickly to run the file test.exe from HDA. This is done to assist in automating test execution. All the script has to do it copy an executable to /home/joel/qemu/hd/test.exe and it will quickly run.

This RTEMS BSP is built with the console redirected to COM1 so -serial stdio ensures that the COM1 output goes to stdout. This makes it easier to redirect the output.

The QEMU session is configured to have a single NIC which is a Novell NE2000 ISA NIC with the specified MAC address. This matches the configuration used by the file network-demos/networkconfig-qemu.h from RTEMS. The network uses a GNU/Linux tap interface per the QEMU documentation. You may have to do a modprobe tap command to get this to work and you will definitely have to have the script /etc/qemu-ifup with the following contents.

/sbin/ifconfig $TUN_DEV $TUN_HOST
exit 0

This matches the defaults as presented in the QEMU manual. The single network interface available to the RTEMS application will have the IP address All of the RTEMS network demos can run in this environment given proper configuration.

"isa irq 9 already assigned"

With a more recent QEMU (0.13), I needed to add the "-M isapc" flag to make this error message go away.

--User:SimonPerreault? Mon Nov 29 16:43:38 EST 2010

Setting the IRQ number

By default, QEMU uses IRQ 9, while RTEMS uses IRQ 5. I just hard-coded the IRQ in my struct rtems_bsdnet_ifconfig, along with the IO address (0x300) while I am at it.

static struct rtems_bsdnet_ifconfig netdriver_config = {
        (char *)BSP_NE2000_NETWORK_DRIVER_NAME,

--User:SimonPerreault? Mon Nov 29 16:43:38 EST 2010

Alternatively, you can simply add


to the grub command line to let RTEMS' ne driver know it should use interrupt 9.

Running CAN Applications

See the QEMU with CAN Emulation? page for more details.

Debugging with QEMU

You can connect a debugger to QEMU and debug your application. If you start QEMU and load your application then switch consoles by pressing Ctl-Alt-2 to the QEMU command console then enter the command gdbserver. After this start gdb for your target with the ELF image for the execute. Once gdb has started enter the remote target command for localhost:1234. On Windows with and the i386 target the commands are:

 RTEMS(i386) C:\rtems\src\i386\i386-rtems4.7\c\pc586\testsuites\samples\capture>i386-rtems4.7-gdb capture.elf
 GNU gdb 6.5
 Copyright (C) 2006 Free Software Foundation, Inc.
 GDB is free software, covered by the GNU General Public License, and you are
 welcome to change it and/or distribute copies of it under certain conditions.
 Type "show copying" to see the conditions.
 There is absolutely no warranty for GDB.  Type "show warranty" for details.
 This GDB was configured as "--host=i686-pc-mingw32 --target=i386-rtems4.7"...
 (gdb) target remote localhost:1234
 Remote debugging using localhost:1234
 0x00115c11 in _CPU_Thread_Idle_body ()
     at c:/rtems/src/rtems-
 92          asm volatile ("hlt");
 (gdb) l _Exception_Handler
 108             frstor    (eax)                    /* restore FP context */
 109             ret
 111             PUBLIC (_Exception_Handler)
 112     SYM (_Exception_Handler):
 113             pusha                              /* Push general purpose registers */
 114             pushl   esp                        /* Push exception frame address */
 115             movl    _currentExcHandler, eax    /* Call function storead in _currentExcHandler */
 116             call    * eax
 117             addl    $4, esp

Do not forget to build RTEMS with the --enable-rtems-debug configure option to get debug information built into RTEMS.


As of 10 March 2007, RTEMS does not support USB. One of the issues in RTEMS supporting USB is finding a target BSP where it is easy to debug the USB stack initially. QEMU appears to be a viable candidate for this task. QEMU is supposed to support USB emulation and there are reports on the Internet that it does work. My experimentation was not so lucky.

Since my effort was all about making sure I knew how to setup and use QEMU's emulated USB, I used a Knoppix LiveCD image. I thought that if I could get Knoppix to access a USB mouse and a USB flash drive, I would be happy in the capabilities in QEMU.

The USB mouse emulation worked well enough that Knoppix recognised it automatically and it worked.

I could never figure out a way to get the USB flash drive recognised. There are comments that there might be a patch needed but I wasn't very confident.

I saw a report that someone got a USB GPS working with Windows XP running under QEMU ( so there must be a magic combination of things that actually make this work

So that's my report. Maybe someone can follow up and make more work.

--Dr. Joel? 13:25, 10 March 2007 (CST)

Emulating Other Platforms with QEMU

PowerPC PReP Emulation

See Developer/Simulators/QEMU/Preparation.

Motorola Coldfire Emulation

I have this also working but need to document.

--[[User:Strauman|Strauman]] 16:43, 18 July 2011 (CDT)

LEON3 Emulation

See RTEMS/LEON3 on Qemu email thread.