gxemul - an experimental framework for full-system machine emulation
gxemul [options] -e name [file ...]
gxemul [options] configfile
gxemul [machine, other, and general options] [file ...]
gxemul [general options] @configfile
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
(e.g. NetBSD) to run inside the emulator, as if they were running on real
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
Note: Only very few emulated machine modes have been rewritten to use the
new emulation framework introduced in version 0.6.0, so there are two
different ways to use the emulator. (Most emulation modes and machines
are only valid in the old framework.) The options available for the new
-B Enables snapshotting (required for reverse execution/stepping).
Start with a machine based on template ’name’. The name may be
followed by optional arguments in parentheses, e.g.
gxemul -V -e ’testmips(cpu=R4400)’
Remember to use quotes if your shell gives special meaning to
-H Display a list of available machine templates.
-q Start up in Quiet mode (i.e. suppress debug messages). If an
error occurs during emulation which stops execution and drops the
user into the debugger, quiet mode is turned off.
-V Start up in interactive mode, paused. If this option is used, -q
Any files supplied on the command line when using the -e option will be
loaded into root.machine0.mainbus0.cpu0 (which is assumed to be the main
CPU in the machine created by the template configuration).
When not using the -e option, the config file should be one that was
previously saved from within the emulator using the ’save’ command.
Starting the emulator with the -V switch will bring you to the
interactive debugger environment, without starting the emulation.
RUNNING GUEST OPERATING SYSTEMS
Please read the HTML documentation for more details on how to run
complete guest operating systems in the emulator.
The emulator can be invoked in the following ways:
1. When emulating a complete machine, configuration options can be
supplied directly on the command line.
2. Options can be read from a configuration file.
The easiest way to use the emulator is to supply settings directly on the
The most important thing you need to supply is the file argument. This is
the name of a binary file (an ELF, a.out, COFF/ECOFF, SREC, or a raw
binary image) which you wish to run in the emulator. This file might be
an operating system kernel, or perhaps a ROM image file. If more than
one filename is supplied, all files are loaded into memory, and the entry
point (if available) is taken from the last file.
Apart from the name of a binary file, you must also use the -E and/or -e
options to select which emulation mode to use. This is necessary because
the emulator cannot in general deduce this from the file being executed.
For example, a MIPS-based machine from DEC (a DECstation) is very
different from a MIPS-based machine from SGI. Use gxemul -H to get a list
of available emulation modes.
There are three exceptions to the normal invocation usage mentioned
1. For DECstation emulation, if you have a bootable DECstation harddisk
or CDROM image, then just supplying the diskimage via the -d option is
sufficient. The filename of the kernel can then be skipped, as the
emulator runs the bootblocks from the diskimage directly and doesn’t need
the kernel as a separate file.
2. If you supply an ISO9660 CDROM disk image, then using the -j option to
indicate a file on the CDROM filesystem to load is sufficient; no
additional kernel filename needs to be supplied on the command line.
3. For Dreamcast emulation, when booting e.g. a NetBSD/dreamcast CDROM
image, it is enough to supply the disk image (with the correct ISO
partition start offset). Bootblocks will be read directly from the CDROM
image, and there is no need to supply the name of an external kernel on
the command line.
Gzipped kernels are automatically unzipped, by calling the external
gunzip program, both when specifying a gzipped file directly on the
command line and when loading such a file using the -j option.
Machine selection options:
-E t Try to emulate machine type t. This option is not always needed,
if the -e option uniquely selects a machine. (Use -H to get a
list of types.)
-e st Try to emulate machine subtype st. Use this together with -E.
(This option is not always needed, if a machine type has no
-C x Try to emulate a specific CPU type, x. This overrides the
default CPU type for the machine being emulated. (Use -H to get
a list of available CPU types.)
Add filename as a disk image. By adding one or more modifier
characters and then a colon (":") as a prefix to filename, you
can modify the way the disk image is treated. Available modifiers
b Specifies that this is a boot device.
d DISK (this is the default).
gH;S; Override the default geometry; use H heads and S sectors-
per-track. (The number of cylinders is calculated
i IDE. (This is the default for most machine types.)
oOFS; Set the base offset for an ISO9660 filesystem on a disk
image. The default is 0. A suitable offset when booting
from Dreamcast ISO9660 filesystem images, which are
offset by 11702 sectors, is 23965696.
r Read-only (don’t allow changes to be written to the
V Add an overlay filename to an already defined disk image.
(A ID number must also be specified when this flag is
used. See the documentation for an example of how to use
0-7 Force a specific ID number.
For SCSI devices, the ID number is the SCSI ID. For IDE
harddisks, the ID number has the following meaning:
0 Primary master.
1 Primary slave.
2 Secondary master.
3 Secondary slave.
Unless otherwise specified, filenames ending with ".iso" or
".cdr" are assumed to be CDROM images. Most others are assumed to
be disks. Depending on which machine is being emulated, the
default for disks can be either SCSI or IDE. Some disk images
that are very small are assumed to be floppy disks. (If you are
not happy with the way a disk image is detected, then you need to
use explicit prefixes to force a specific type.)
For floppies, the gH;S; prefix is ignored. Instead, the number of
heads and cylinders are assumed to be 2 and 80, respectively, and
the number of sectors per track is calculated automatically.
(This works for 720KB, 1.2MB, 1.44MB, and 2.88MB floppies.)
-I hz Set the main CPU’s frequency to hz Hz. This option does not work
for all emulated machine modes. It affects the way count/compare
interrupts are faked to simulate emulated time = real world time.
If the guest operating system relies on RTC interrupts instead of
count/compare interrupts, then this option has no effect.
Setting the frequency to zero disables automatic synchronization
of emulated time vs real world time, and the count/compare system
runs at a fixed rate.
-i Enable instruction trace, i.e. display disassembly of each
instruction as it is being executed.
-J Disable instruction combinations in the dynamic translator.
-j n Set the name of the kernel to n. When booting from an ISO9660
filesystem, the emulator will try to boot using this file. (In
some emulation modes, eg. DECstation, this name is passed along
to the boot program. Useful names are "bsd" for OpenBSD/pmax,
"vmunix" for Ultrix, or "vmsprite" for Sprite.)
-M m Emulate m MBs of physical RAM. This overrides the default amount
of RAM for the selected machine type.
-N Display the number of executed instructions per second on
average, at regular intervals.
-n nr Set the number of processors in the machine, for SMP experiments.
Note 1: The emulator allocates quite a lot of virtual memory for
per-CPU translation tables. On 64-bit hosts, this is normally not
a problem. On 32-bit hosts, this can use up all available virtual
userspace memory. The solution is to either run the emulator on a
64-bit host, or limit the number of emulated CPUs to a reasonably
Note 2: SMP simulation is not working very well yet; multiple
processors are simulated, but synchronization between the
processors does not map very well to how real-world SMP systems
-O Force a "netboot" (tftp instead of disk), even when a disk image
is present (for DECstation, SGI, and ARC emulation).
-o arg Set the boot argument (mostly useful for DEC, ARC, or SGI
emulation). Default arg for DEC is "-a", for ARC/SGI it is
"-aN", and for CATS it is "-A".
-p pc Add a breakpoint. pc can be a symbol, or a numeric value.
(Remember to use the "0x" prefix for hexadecimal values.)
-Q Disable the built-in (software-only) PROM emulation. This option
is useful for experimenting with running raw ROM images from real
machines. The default behaviour of the emulator is to "fake"
certain PROM calls used by guest operating systems (e.g. NetBSD),
so that no real PROM image is needed.
-R Use a random bootstrap cpu, instead of CPU nr 0. (This option is
only meaningful together with the -n option.)
-r Dump register contents for every executed instruction.
-S Initialize emulated RAM to random data, instead of zeroes. This
option is useful when trying to trigger bugs in a program that
occur because the program assumed that uninitialized memory
contains zeros. (Use with care.)
Gather statistics based on the current emulated program counter
value, while the program executes. The statistics is actually
just a raw dump of all program counter values in sequence,
suitable for post-analysis with separate tools. Output is
appended to filename.
The flags should include one or more of the following type
v Virtual. This means that the program counter value is
p Physical. This means that the physical address of where
the program is actually running is used.
i Instruction call. This type of statistics gathering is
practically only useful during development of the
emulator itself. The output is a list of addresses of
instruction call functions (ic->f), which after some
post-processing can be used as a basis for deciding when
to implement instruction combinations.
The flags may also include the following optional modifiers:
d Disabled at startup.
o Overwrite the file, instead of appending to it.
Statistics gathering can be enabled/disabled at runtime by using
the "statistics_enabled = yes" and "statistics_enabled = no"
When gathering instruction statistics using the -s option,
instruction combinations are always disabled (i.e. an implicit -J
flag is added to the command line).
-T Halt if the emulated program attempts to access non-existing
-t Show a trace tree of all function calls being made.
-U Enable slow_serial_interrupts_hack_for_linux.
-X Use X11. This option enables graphical framebuffers.
-x Open up new xterms for emulated serial ports. The default
behaviour is to open up xterms when using configuration files, or
if X11 is enabled. When starting up a simple emulation session
with settings directly on the command line, and neither -X nor -x
is used, then all output is confined to the terminal that gxemul
-Y n Scale down framebuffer windows by n x n times. This option is
useful when emulating a very large framebuffer, and the actual
display is of lower resolution. If n is negative, then there will
be no scaledown, but emulation of certain graphic controllers
will be scaled up by -n times instead. E.g. Using -2 with VGA
text mode emulation will result in 80x25 character cells rendered
in a 1280x800 window, instead of the normal resolution of
-Z n Set the number of graphics cards, for emulating a dual-head or
tripple-head environment. (Only for DECstation emulation so far.)
Add disp as an X11 display to use for framebuffers.
-c cmd Add cmd as a command to run before starting the simulation. A
similar effect can be achieved by using the -V option, and
entering the commands manually.
-D Causes the emulator to skip a call to srandom(). This leads to
somewhat more deterministic behaviour than running without this
option. However, if the emulated machine has clocks or timer
interrupt sources, or if user interaction is taking place (e.g.
keyboard input at irregular intervals), then this option is
-H Display a list of available CPU types and machine types. (Most
of these don’t work. Please read the HTML documentation included
in the gxemul distribution for details on which modes that
-h Display a list of all available command line options.
-k n Set the size of the dyntrans cache (per emulated CPU) to n MB.
The default size is 48 MB.
-K Force the single-step debugger to be entered at the end of a
-q Quiet mode; this suppresses startup messages.
-V Start up in the single-step debugger, paused. If this option is
used, -q is ignored.
-v Increase verbosity (show more debug messages). This option can be
used multiple times.
Configuration file startup:
Start an emulation based on the contents of configfile.
For more information, please read the HTML documentation in the doc/
subdirectory of the gxemul distribution.
The following command will start NetBSD/pmax on an emulated DECstation
gxemul -e 3max -d nbsd_pmax.img
nbsd_pmax.img should be a raw disk image containing a bootable
The following command will start an emulation session based on settings
in the configuration file "mysession". The -v option tells gxemul to be
gxemul -v @mysession
If you have compiled the small Hello World program mentioned in the
gxemul documentation, the following command will start up an emulated
test machine in "paused" mode:
gxemul -E testmips -V hello_mips
Paused mode means that you enter the interactive single-step debugger
directly at startup, instead of launching the Hello World program.
The paused mode is also what should be used when running "unknown" files
for the first time in the emulator. E.g. if you have a binary which you
think is some kind of MIPS ROM image, then you can try the following:
gxemul -vv -E baremips -V 0xbfc00000:image.raw
You can then use the single-stepping functionality of the built-in
debugger to run the code in the ROM image, to see how it behaves. Based
on that, you can deduce what machine type it was actually from (the
baremips machine is not a real machine), and perhaps try again with
another emulation mode.
In general, however, real ROM images require much more emulation detail
than GXemul provides, so they can usually not run.
Please read the HTML documentation for more details.
There are many bugs. Some of the known bugs are mentioned in the TODO
file in the gxemul source distribution, some are marked as TODO in the
source code itself.
Most emulation modes are LEGACY modes, which have not yet been rewritten
to use the new framework. The documentation is most likely buggy, in the
sense that it describes things from the old framework when the new one
applies, and/or vice versa.
gxemul is in general not cycle-accurate; it does not simulate individual
pipe-line stages or penalties caused by branch-prediction misses or cache
misses, so it cannot be used for accurate simulation of any actual real-
gxemul is in general not timing-accurate. Many emulation modes try to
make the guest operating system’s clock run at the same speed as the host
clock. However, the number of instructions executed per clock tick can
obviously vary, depending on the current CPU load on the host.
GXemul is Copyright (C) 2003-2009 Anders Gavare <email@example.com>
See http://gxemul.sourceforge.net/ for more information. For other
Copyright messages, see the corresponding parts of the source code and/or