Download Qemu - Processor/Operating System Emulator

[Dr. V]

Whats is Qemu?

QEMU is a generic and open source machine emulator and virtualizer.

1. When used as a machine emulator, QEMU can run OSes and programs made for one machine (e.g. an ARM board) on a different machine (e.g. your own PC). By using dynamic translation, it achieves very good performances.
   
2. When used as a virtualizer, QEMU achieves near native performances by executing the guest code directly on the host CPU. A host driver called the QEMU accelerator (also known as KQEMU) is needed in this case. The virtualizer mode requires that both the host and guest machine use x86 compatible processors.

Detailed :

QEMU is a processor emulator that relies on dynamic binary translation to achieve a reasonable speed while being easy to port on new host CPU architectures.

In conjunction with CPU emulation, it also provides a set of device models, allowing it to run a variety of unmodified guest operating systems, thus it can be viewed as a hosted virtual machine monitor. 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 Microsoft Virtual PC.

QEMU can also be used purely for CPU emulation for user level processes; in this mode of operation it is most similar to valgrind.

One feature exclusive to QEMU is that of portability: the virtual machines can be run on any PC, even those where the user has only Limited Rights with no Administrator access, making the 'PC-on-a-USB-stick' concept very real. Similar applications exist (such as MojoPac) but they currently require Administrator rights to run, making them useless in areas such as public libraries, internet cafes, and so on.

[Dr. V]

QEMU has two operating modes:

User mode emulation

QEMU can launch Linux or Darwin/Mac OS X processes compiled for one CPU on another CPU. Target OS system calls are thunked for endianness and 32/64 bit mismatches. Wine Windows API reimplementation and DOSEMU are the main targets for QEMU in user mode emulation. This mode also eases cross-compilation and cross-debugging.

Complete Computer System mode emulation

QEMU emulates a full computer system, including a processor and various 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; it supports emulating several hardware platforms, including x86, AMD64, Alpha, ETRAX CRIS, MIPS, and SPARC.

[Dr. V]

Features :

• QEMU offers many features that are also present in other emulators. For example, it can save and restore the state of the virtual machine with all programs running. As with many emulators, guest operating systems do not need to be patched to successfully run.
  
• QEMU supports the emulation of various architectures, including IA-32 (x86) PCs, AMD64 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), and ETRAX CRIS architectures. The QEMU homepage provides a complete list of supported architectures.
  
• The virtual machine can be equipped with many types of hardware. Some of these are: hard disks, CD-ROM drives, network cards, sound chips, 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 administator privileges and does not work with all devices).
  
• Virtual hard disk images can be stored in a special format (QCOW2) that only takes up disk space that the guest OS actually uses. This way, an emulated 120 GiB disk can still take up just several hundred megabytes on the host. The QCOW2 format also allows the creation of overlay images that record the difference to another base image file which is not modified. This can be useful to have the possibility of reverting the 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 worked with. Should the guest system be unusable, the overlay can be deleted and 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.
  
• It also has some features that are not present in many emulators. QEMU integrates several services to allow the host and guest systems to communicate, for example, a SMB server and network port redirection (to allow incoming connections to the virtual machine). It can also boot Linux kernels without having to prepare a bootable image with a bootloader.
  
• QEMU does not depend on the presence of graphical output methods on the host system. It allows to access the screen of the guest OS via VNC. Operating systems that support it can also use no screen at all and use an emulated serial line instead.
  
• Simulating multiple CPUs that can be used like a real SMP system is possible.
  
• Unlike some other emulators, QEMU does not require administrative rights to run, except if additional kernel modules for improving speed are used (like KQEMU).

User mode emulation

QEMU is able to not just emulate a full system, but also to run a normal program that was compiled for another architecture. This is done when it cannot be recompiled because the source code is unavailable, or when the program's architecture does not permit running it under all architectures [Other example is Wine]

[Dr. V]

Target CPU support :



Target CPU-----User emulation-----System emulation
x86-------------------OK-------------------OK
x86_64-------------Testing----------------OK
ARM------------------OK-------------------OK
SPARC----------------OK-------------------OK
SPARC64-----------Dev only------------Dev only
PowerPC-------------Testing-------------Testing
PowerPC64----------Testing-------------Dev only
MIPS-------------------OK-------------------OK
MIPS64-----------Not supported------------OK
m68k (Coldfire)--------OK------------------OK
SH-4-----------------Testing-------------Dev only
Alpha----------------Dev only------------Dev only
CRIS------------------Testing--------------Testing

Host CPU support :

Host CPU------Status
x86---------------OK
x86_64-----------OK
PowerPC----------OK
Sparc32-----------Testing
ARM---------------Testing
HPPA--------------Dev only
MIPS--------------Not supported
S390--------------Not supported
Sparc64-----------Not supported
ia64---------------Not supported
m68k-------------Not supported
Alpha--------------Not supported



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