GXemul: Introduction

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## Introduction

- Overview
- Is GXemul Free software?
- How to compile/build the emulator
- How to run the emulator
- Which processor architectures does GXemul emulate?
- Which host architectures/platforms are supported?
- Emulation accuracy
- Which machines does GXemul emulate?

NetBSD/pmax 1.6.2 with X11
running in GXemul

### Overview:

GXemul is a framework for full-system computer architecture emulation. Several processor architectures and machine types have been implemented. It is working well enough to allow unmodified "guest" operating systems to run inside the emulator, as if they were running on real hardware.

The emulator emulates (networks of) real machines. The machines may consist of ARM, MIPS, Motorola 88K, PowerPC, and SuperH processors, and various surrounding hardware components such as framebuffers, busses, interrupt controllers, ethernet controllers, disk controllers, and serial port controllers.

GXemul, including the dynamic translation system, is implemented in portable C, which means that the emulator will run on practically any 64-bit or 32-bit Unix-like systems, with few or no modifications.

Devices and processors are not simulated with 100% accuracy. They are only "faked" well enough to allow guest operating systems to run without complaining too much. Still, the emulator could be of interest for academic research and experiments, such as when learning how to write operating system code.

The emulator contains code which tries to emulate the workings of CPUs and surrounding hardware found in real machines, but it does not contain any ROM code. You will need some form of program (in binary form) to run in the emulator. For some emulation modes, PROM calls are handled by the emulator itself, so you do not need to use any ROM image at all.

You can use pre-compiled kernels (for example NetBSD kernels, or Linux), or other programs that are in binary format, and in some cases even actual ROM images. A couple of different file formats are supported: ELF, a.out, COFF/ECOFF, SREC, and raw binaries.

If you do not have a kernel as a separate file, but you have a bootable disk image, then it is sometimes possible to boot directly from that image. This works for example with DECstation emulation, Dreamcast emulation, or when booting from generic ISO9660 CDROM images if the kernel is included in the image as a plain file.

Thanks to (in no specific order) Joachim Buss, Olivier Houchard, Juli Mallett, Juan Romero Pardines, Carl van Schaik, Miod Vallat, Alec Voropay, Göran Weinholt, Alexander Yurchenko, and everyone else who has provided me with feedback.

### Is GXemul Free software?

Yes. I have released GXemul under a Free license. The code in GXemul is Copyrighted software, it is not public domain. (If this is confusing to you, you might want to read up on the definitions of the four freedoms associated with Free software, http://www.gnu.org/philosophy/free-sw.html.)

The code I have written is released under a 3-clause BSD-style license (or "revised BSD-style" if one wants to use GNU jargon). Apart from the code I have written, some files are copied from other sources such as NetBSD, for example header files containing symbolic names of bitfields in device registers. They are also covered by similar licenses, but with some additional clauses. The main point, however, is that the licenses require that the original Copyright and license terms are included when you make a copy or modification.

If you plan to redistribute GXemul without supplying the source code, then you need to comply with each individual source file some other way, for example by writing additional documentation containing copyright notes. I have not done this, since I do not plan on making distributions without source code. You need to check all individual files for details. The "easiest way out" if you plan to redistribute code from GXemul is, of course, to let it remain Free Software and simply include the source code.

In case you want to reuse parts of GXemul, but you need to do that under a different license (e.g. the GPL), then contact me and I can probably re-license/dual-license files on a case-by-case basis.

### How to compile/build the emulator:

Uncompress the .tar.gz distribution file, and run

    	$ ./configure
    	$ make

This should work on most Unix-like systems. GXemul does not require any specific libraries to build, however, if you build on a system which does not have X11 libraries installed, some functionality will be lost.

The emulator's performance is highly dependent on both runtime settings and on compiler settings, so you might want to experiment with using different `CC` and `CFLAGS` environment variable values when running the `configure` script.

On some platforms, it is possible that building GXemul will fail because of too high optimization. When using GCC (the GNU compiler), `-O3` is the default optimization level. This works fine on e.g. amd64 and i386 systems, but on some more uncommon platforms, `-O3` might trigger internal bugs in GCC itself. If this happens, try setting `CFLAGS` to `-O2` before running `./configure`, and try again,

Note that there is no `make install` functionality; package maintainers for individual operating systems solve this for their corresponding OSes.

### How to run the emulator:

Once you have built GXemul, running it should be rather straight-forward. Running `gxemul` without arguments (or with the `-h` or `-H` command line options) will display a help message.

To get some ideas about what is possible to run in the emulator, please read the section about installing "guest" operating systems. The most straight forward guest operating system to install is NetBSD/pmax; the instructions provided here should let you install NetBSD/pmax in a way very similar to how it is done on a real DECstation.

If you are interested in using the emulator to develop code on your own, then you should also read the section about Hello World.

To exit the emulator, type CTRL-C to enter the single-step debugger, and then type `quit`.

If you are starting an emulation by entering settings directly on the command line, and you are not using the `-x` option, then all terminal input and output will go to the main controlling terminal. CTRL-C is used to break into the debugger, so in order to send CTRL-C to the running (emulated) program, you may use CTRL-B. (This should be a reasonable compromise to allow the emulator to be usable even on systems without X Windows.)

There is no way to send an actual CTRL-B to the emulated program, when typing in the main controlling terminal window. The solution is to either use configuration files, or use `-x`. Both these solutions cause new xterms to be opened for each emulated serial port that is written to. CTRL-B and CTRL-C both have their original meaning in those xterm windows.

### Which processor architectures does GXemul emulate?

The architectures that are emulated well enough to let at least one guest operating system run (per architecture) are ARM, MIPS, Motorola 88K, PowerPC, and SuperH.

Please read the sections about emulation modes and guest operating systems for more information about the machines and operating systems, respectively, that can be considered "working" in the emulator.

(There is some code in GXemul for emulation of other architectures, but they are not stable or complete enough to be listed among the "working" architectures.)

### Which host architectures/platforms are supported?

GXemul should compile and run on any modern host architecture (64-bit or 32-bit word-length). I generally test it on FreeBSD/amd64 7.0, FreeBSD/alpha 4.9, sometimes also on Linux (various platforms), and every now and then also on NetBSD or OpenBSD inside the emulator itself (various platforms).

Note 1: The dynamic translation engine does not require backends for native code generation to be written for each individual host architecture; the intermediate representation that the dyntrans system uses can be executed on any host architecture.

Note 2: Although GXemul may build and run on non-Unix-like platforms, such as Cygwin, Unix-like systems are the primary platform. Some functionality may be lost when running on Cygwin.

### Emulation accuracy:

GXemul is an instruction-level emulator; things that would happen in several steps within a real CPU are not taken into account (e.g. pipe-line stalls or out-of-order execution). Still, instruction-level accuracy seems to be enough to be able to run complete guest operating systems inside the emulator.

The existance of instruction and data caches is "faked" to let operating systems think that they are there, but for all practical purposes, these caches are non-working.

The emulator is in general not timing-accurate, neither at the instruction level nor on any higher level. An attempt is made to let emulated clocks run at the same speed as the host (i.e. an emulated timer running at 100 Hz will interrupt around 100 times per real second), but since the host speed may vary, e.g. because of other running processes, there is no guarantee as to how many instructions will be executed in each of these 100 Hz cycles.

If the host is very slow, the emulated clocks might even lag behind the real-world clock.

### Which machines does GXemul emulate?

A few different machine types are emulated. The following machine types are emulated well enough to run at least one "guest OS":

- ARM
  - CATS (NetBSD/cats, OpenBSD/cats)
  - IQ80321 (NetBSD/evbarm)
  - NetWinder (NetBSD/netwinder)

- MIPS
  - DECstation 5000/200 (NetBSD/pmax, OpenBSD/pmax, Ultrix, Linux/DECstation, Sprite)
  - Acer Pica-61 (NetBSD/arc)
  - NEC MobilePro 770, 780, 800, 880 (NetBSD/hpcmips)
  - Cobalt (NetBSD/cobalt)
  - Malta (NetBSD/evbmips, Linux/Malta (<super>*1</super>))
  - Algorithmics P5064 (NetBSD/algor)
  - SGI O2 (aka IP32) (<super>*2</super>) (NetBSD/sgi)

- Motorola 88K
  - Motorola MVME187 (OpenBSD/mvme88k)

- PowerPC
  - IBM 6050/6070 (PReP, PowerPC Reference Platform) (NetBSD/prep)
  - MacPPC (generic "G4" Macintosh) (NetBSD/macppc)
  - Artesyn PM/PPC (<super>*2</super>) (NetBSD/pmppc)

- SuperH
  - Sega Dreamcast (NetBSD/dreamcast, Linux/dreamcast)
  - Landisk I-O DATA USL-5P (OpenBSD/landisk)

(<super>*1</super>) = Linux/Malta sometimes works as a guest OS, but running Linux/Malta in GXemul is much more experimental/unknown than NetBSD, so it is still on the "unsupported" page.
(<super>*2</super>) = The emulation is enough for root-on-nfs, but no disk controller (SCSI nor IDE) is emulated yet for this machine type.

Note that of all of the machines above, none of them is emulated to 100%. The most complete emulation mode is probably the DECstation 5000/200. Things that will most likely not work include running raw PROM images for most machines, SGI IRIX, MacOS X or Darwin, Windows NT, or Dreamcast games.

There is code in GXemul for emulation of several other machine types; the degree to which these work range from almost being able to run a complete OS, to almost completely unsupported, perhaps just enough support to output a few boot messages via serial console. (See the end of this section on the Guest OSes page for some examples, but remember that these do not necessarily work.)

In addition to emulating real machines, there is also a "test-machine". A test-machine consists of one or more CPUs and a few experimental devices such as:

- a console I/O device (putchar() and getchar()...)
- an inter-processor communication device, for SMP experiments
- a very simple linear framebuffer device (for graphics output)
- a simple disk controller
- a simple ethernet controller
- a simple interrupt controller
- a real-time clock device

This mode is useful if you wish to run experimental code, but do not wish to target any specific real-world machine type, for example for educational purposes.

You can read more about these experimental devices here.