Original author(s) Fabrice Bellard
Developer(s) QEMU team:
Anthony Liguori, Paul Brook, et al.
Stable release 1.6.1 / October 9, 2013; 9 months ago (2013-10-09)[1]
Written in C
Operating system Linux, Microsoft Windows, and some UNIX platforms
Type Hypervisor
License GNU GPL version 2

QEMU (short for "Quick EMUlator") is a free and open-source hosted hypervisor that performs hardware virtualization.

QEMU is a hosted virtual machine monitor: It emulates central processing units through dynamic binary translation and provides a set of device models, enabling it to run a variety of unmodified guest operating systems. It also provides an accelerated mode for supporting a mixture of binary translation (for kernel code) and native execution (for user code), in the same fashion as VMware Workstation and VirtualBox do. QEMU can also be used purely for CPU emulation for user-level processes, allowing applications compiled for one architecture to be run on another.


QEMU was written by Fabrice Bellard and is free software and is mainly licensed under GNU General Public License (GPL). Various parts are released under BSD license, GNU Lesser General Public License (LGPL) or other GPL-compatible licenses.[2] There is an option to use the proprietary FMOD library when running on Microsoft Windows, which, if used, disqualifies the use of a single open source software license. However, the default is to use DirectSound.


QEMU has two operating modes:[3]

User-mode emulation
In this mode QEMU runs single Linux or Darwin/Mac OS X programs that were compiled for a different instruction set. System calls are thunked for endianness and for 32/64 bit mismatches. Fast cross-compilation and cross-debugging are the main targets for user-mode emulation.
Computer emulation
In this mode QEMU emulates a full computer system, including peripherals. It can be used to provide virtual hosting of several virtual computers on a single computer. QEMU can boot many guest operating systems, including Linux, Solaris, Microsoft Windows, DOS, and BSD;[4] it supports emulating several instruction sets, including x86, MIPS, ARM, PowerPC, SPARC, ETRAX CRIS and MicroBlaze.


User space
User space
User space
User space
Mac OS X
User space
Host system : Linux, Mac OS X, Windows
Hardware : CPU, main memory, storage memory, networking hardware, etc.


QEMU can save and restore the state of the virtual machine with all programs running. Guest operating systems do not need to be patched to run inside QEMU.

QEMU supports the emulation of various architectures, including IA-32 (x86) PCs, x86-64 PCs, MIPS R4000, Sun's SPARC sun4m, Sun's SPARC sun4u, ARM development boards (Integrator/CP and Versatile/PB), SH4 SHIX board, PowerPC (PReP and Power Macintosh), ETRAX CRIS and MicroBlaze architectures. The QEMU homepage provides a complete list of supported architectures.

The virtual machine can interface with many types of physical host hardware. Some of these are: hard disks, CD-ROM drives, network cards, audio interfaces, and USB devices. USB devices can be completely emulated (mass storage from image files, input devices), or the host's USB devices can be used (however, this requires administrator privileges and does not work with all devices).

Virtual disk images can be stored in a special format (qcow or qcow2) that only take up disk space that the guest OS actually uses. This way, an emulated 120 GB disk can occupy only several hundred megabytes on the host. The QCOW2 format also allows the creation of overlay images that record the difference from another base image file which is not modified. This provides the possibility for reverting the emulated disk's contents to an earlier state. For example, a base image could hold a fresh install of an operating system that is known to work, and the overlay images are used. Should the guest system become unusable (virus attack, accidental system destruction, ...), the overlay can be deleted and an earlier emulated disk image version recreated.

QEMU can emulate network cards (of different models) which share the host system's connectivity by doing network address translation, effectively allowing the guest to use the same network as the host. The virtual network cards can also be connected to network cards of other instances of QEMU or local TAP interfaces. Network connectivity can also be achieved by bridging a TUN/TAP interface used by QEMU with a non-virtual Ethernet interface on the host OS using the host OS's bridging features.

QEMU integrates several services to allow the host and guest systems to communicate; for example, an integrated SMB server and network port redirection (to allow incoming connections to the virtual machine). It can also boot Linux kernels without a bootloader.

QEMU does not depend on the presence of graphical output methods on the host system. Instead, it can allow one to access the screen of the guest OS via an integrated VNC server. It can also use an emulated serial line, without any screen, with applicable operating systems.

Simulating multiple CPUs that can be used like a real SMP system is possible.

QEMU does not require administrative rights to run, except if additional kernel modules for improving speed are used (like KQEMU), or when some modes of its network connectivity model are utilized.

Tiny Code Generator

The Tiny Code Generator (TCG) aims to remove the shortcoming of relying on a particular version of GCC or any compiler, instead incorporating the compiler (code generator) into other tasks performed by QEMU at run time. The whole translation task thus consists of two parts: blocks of target code (TBs) being rewritten in TCG ops - a kind of machine-independent intermediate notation, and subsequently this notation being compiled for the host's architecture by TCG. Optional optimisation passes are performed between them.

TCG requires that there be dedicated code written to support every architecture it is being run on. It also requires that the target instruction translation be rewritten to take advantage of TCG ops, instead of the previously used dyngen ops.

Starting with QEMU Version 0.10.0, TCG ships with the QEMU stable release.[5]


KQEMU was a Linux kernel module, also written by Fabrice Bellard, which notably sped up emulation of x86 or x86-64 guests on platforms with the same CPU architecture. This was accomplished by running user mode code (and optionally some kernel code) directly on the host computer's CPU, and by using processor and peripheral emulation only for kernel mode and real mode code. KQEMU could execute code from many guest OSes even if the host CPU did not support hardware-assisted virtualization. KQEMU was initially a closed-source product available free of charge, but starting from version 1.3.0pre10,[6] it was relicensed under the GNU General Public License. QEMU versions starting with 0.12.0 (as of August 2009) support large memory which makes them incompatible with KQEMU.[7] Newer releases of QEMU have completely removed support for KQEMU.

QVM86 was a GNU GPLv2 licensed drop-in replacement for the then closed-source KQEMU. The developers of QVM86 ceased development in January, 2007.

Kernel-based Virtual Machine (KVM) has mostly taken over as the Linux-based hardware-assisted virtualization solution for use with QEMU in the wake of the lack of support for KQEMU and QVM86.

Intel's Hardware Accelerated Execution Manager (HAXM) is a free (but not open source) alternative to KVM for x86-based hardware-assisted virtualization on Windows and Mac OS X. Intel currently mostly solicits its use with QEMU for Android development.[8]

Supported disk image formats

The following disk image formats are supported by QEMU:[9]

Hardware-assisted emulation

The MIPS-compatible Loongson-3 processor adds 200 new instructions to help QEMU translate x86 instructions; those new instructions lower the overhead of executing x86/CISC-style instructions in the MIPS pipeline. With additional improvements in QEMU by the Chinese Academy of Sciences, Loongson-3 achieves an average of 70% the performance of executing native binaries while running x86 binaries from nine benchmarks.[10]

Parallel emulation

Virtualization solutions that use QEMU are able to execute multiple virtual CPUs in parallel. QEMU is also able to run multiple threads in user-mode emulation mode.

For full system emulation, QEMU uses a single thread to emulate all the virtual CPUs and hardware. COREMU[11] is a patch to QEMU to eliminate this limitation. Each core uses a separate instance of QEMU binary translation engine, with a thin library layer to handle the inter-core and device communication and synchronization.

Integration with other virtualization solutions


VirtualBox, released in January 2007, uses some of QEMU's virtual hardware devices, and has a built-in dynamic recompiler based on QEMU. As with KQEMU, VirtualBox runs nearly all guest code natively on the host via the VMM (Virtual Machine Manager) and uses the recompiler only as a fallback mechanism, e.g., when guest code executes in real mode .[12] In addition, VirtualBox does a lot of code analysis and patching using a built-in disassembler in order to minimize recompilation. VirtualBox is free and open-source (available under GPL), except for certain features.


Xen, a virtual machine monitor, can run in HVM (hardware virtual machine) mode, using Intel VT-x or AMD-V hardware x86 virtualization extensions. This means that instead of paravirtualized devices, a real set of virtual hardware is exposed to the domU to use real device drivers to talk to.

QEMU includes several components: CPU emulators, emulated devices, generic devices, machine descriptions, user interface, and a debugger. The emulated devices and generic devices in QEMU make up its device models for I/O virtualization.[13] They comprise a PIIX3 IDE (with some rudimentary PIIX4 capabilities), Cirrus Logic or plain VGA emulated video, RTL8139 or E1000 network emulation, and ACPI support.[14] APIC support is provided by Xen.

Xen-HVM has device emulation based on the QEMU project to provide I/O virtualization to the VMs. Hardware is emulated via a QEMU "device model" daemon running as a backend in dom0. Unlike other QEMU running modes (dynamic translation or KVM), virtual CPUs are completely managed to the hypervisor, which takes care of stopping them while QEMU is emulating memory-mapped I/O accesses.


KVM (Kernel Virtual Machine) is a FreeBSD and Linux kernel module that allows a user space program access to the hardware virtualization features of various processors, with which QEMU is able to offer virtualization for x86, PowerPC, and S/390 guests. When the target architecture is the same as the host architecture, QEMU can make use of KVM particular features, such as acceleration.

Win4Lin Pro Desktop

In early 2005, Win4Lin introduced Win4Lin Pro Desktop, based on a 'tuned' version of QEMU and KQEMU and it hosts NT-versions of Windows. In June 2006,[15] Win4Lin released Win4Lin Virtual Desktop Server based on the same code base. Win4Lin Virtual Desktop Server serves Microsoft Windows sessions to thin clients from a Linux server.

In September 2006, Win4Lin announced a change of the company name to


SerialICE is a QEMU-based firmware debugging tool running system firmware inside of QEMU while accessing real hardware through a serial connection to a host system. This can be used as a cheap replacement for hardware ICEs.[17]


  • Incomplete support for less frequently-used[weasel words] architectures
  • As of 2011 only supports the traditional BIOS boot model for the guest OSes, no UEFI boot model support yet on x86-64 systems
  • Few special device drivers (graphics, sound, I/O) for guests are available, thus there is quite a large overhead for multimedia applications. For example, a Cirrus Logic graphics chip and various popular sound cards (ES1370, Sound Blaster 16, Gravis Ultrasound and AdLib) are emulated, but they do not take advantage of hardware acceleration on the host system. Recently a virtual video device compatible with the VMWare video driver has been added; however, it does not support any scaled video or 3D features.
  • It runs significantly slower than alternatives on systems that lack hardware CPU virtualization.[18]
  • For full system emulation, QEMU up to version 1.0 could not utilize multiple-host CPUs but ran all the virtual CPUs and hardware from a single thread.[19]

Emulated hardware platforms


Besides the CPU (which is also configurable and can emulate the Intel Sandy Bridge[20]), the following devices are emulated:

The BIOS implementation used by QEMU starting from version 0.12 is SeaBIOS. The VGA BIOS implementation comes from Plex86/Bochs.


On the PowerPC target, Open Hack'Ware, an Open-Firmware-compatible BIOS, is used.


QEMU emulates the following PowerMac peripherals:

  • UniNorth PCI bridge
  • PCI-VGA-compatible graphics card which maps the VESA Bochs Extensions
  • Two PMAC-IDE-Interfaces with hard disk and CD-ROM support.
  • NE2000 PCI adapter
  • Non-volatile RAM
  • VIA-CUDA with ADB keyboard and mouse.


QEMU emulates the following PREP peripherals:

  • PCI bridge
  • PCI VGA-compatible graphics card with VESA Bochs Extensions
  • Two IDE interfaces with hard disk and CD-ROM support
  • Floppy disk drive
  • NE2000 network adapter
  • Serial interface
  • PREP non-volatile RAM
  • PC-compatible keyboard and mouse


QEMU emulates the iEmu, emulator of Apple's iPhone. Starting from version 2.0.0 of their Bada SDK, Samsung has also chosen QEMU to help development on emulated 'Wave' devices.

In 1.5.0 and 1.6.0 Samsung Exynos 4210 (dual-core Cortex a9) and Versatile Express ARM Cortex-A9 ARM Cortex-A15 are emulated. In v1.6.0, 32 bits functions of the ARMv8 (aarm64 64 bits architecture) are emulated, still not 64 bits functions.

The Xilinx Cortex A9-based Zynq SoC is modelled, with the following elements:

  • Zynq-7000 ARM Cortex-A9 CPU
  • Zynq-7000 ARM Cortex-A9 MPCore
  • Triple Timer Counter
  • DDR Memory Controller
  • DMA Controller (PL330)
  • Static Memory Controller (NAND/NOR Flash)
  • SD/SDIO Peripheral Controller (SDHCI)
  • Zynq Gigabit Ethernet Controller
  • USB Controller (EHCI - Host support only)
  • Zynq UART Controller
  • SPI and QSPI Controllers
  • I2C Controller


QEMU has support for both 32 and 64-bit SPARC architectures.

When the firmware in the JavaStation (sun4m-Architecture) became version 0.8.1 Proll,[22] a PROM replacement used in version 0.8.2, was replaced with OpenBIOS.


QEMU emulates the following sun4m/sun4c/sun4d peripherals:

  • TCX Frame buffer (graphics card)
  • Lance (Am7990) Ethernet
  • Non-volatile RAM M48T02/M48T08
  • Slave I/O: timers, interrupt controllers, Zilog serial ports, keyboard and power/reset logic
  • ESP SCSI controller with hard disk and CD-ROM support
  • Floppy drive (not on SS-600MP)
  • CS4231 sound device (only on SS-5, not working yet)


Emulating Sun4u (UltraSPARC PC-like machine), Sun4v (T1 PC-like machine), or generic Niagara (T1) machine with the following peripherals:

  • UltraSparc IIi APB PCI Bridge
  • PCI VGA compatible card with VESA Bochs Extensions
  • PS/2 mouse and keyboard
  • Non-volatile RAM M48T59
  • PC-compatible serial ports
  • 2 PCI IDE interfaces with hard disk and CD-ROM support
  • Floppy disk


Supported peripherals:

  • MicroBlaze with/without MMU, including
  • AXI Timer and Interrupt controller peripherals
  • AXI External Memory Controller
  • AXI DMA Controller
  • Xilinx AXI Ethernet
  • AXI Ethernet Lite
  • AXI UART 16650 and UARTLite
  • AXI SPI Controller


Supported peripherals:

From the Milkymist SoC

  • UART
  • VGA
  • memory card
  • Ethernet
  • pfu
  • timer


Main article: ETRAX CRIS


Main article: OpenRISC

External patches

External trees exist supporting the following targets:

See also

Free software portal
  • qcow - qcow and qcow2 disc image formats
  • Comparison of platform virtual machines
  • Emulator
  • Mtools - MS-DOS disc manipulation tools
  • OVPsim - commercially supported emulator providing over 100 open source models of embedded processors, many verified by vendors. Models are provided under an Apache license
  • Q (emulator) - Mac OS X port of QEMU
  • SPIM - MIPS processor simulator
  • GXemul - a BSD licensed alternative


External links

  • Systems emulation with QEMU an IBM developerWorks article by M. Tim Jones
  • QVM86 project page
  • Debian on an emulated ARM machine
  • Fedora ARM port emulation with QEMU
  • The Wikibook "QEMU and KVM" (in German, or computer translated to English)
  • QEMU on Windows
  • Microblaze emulation with QEMU
  • QEMU speed comparison
  • UnifiedSessionsManager - An unofficial QEMU/KVM configuration file definition
  • Unofficial mailing list for qemu users
  • Couverture, a code coverage project based on QEMU

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