GXemul: Miscellaneous

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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.


Writing operating system code, or developing firmware, using GXemul:

Is this a good idea? The answer is yes and no, depending on the level of detail you need in your simulations. Important things to keep in mind are:

The bottom line is that GXemul can be useful as a complement to testing your code on real hardware, but it should not be fully relied on.


Using GXemul in compiler contruction courses:

If you are learning how to write a compiler, and wish to target a realistic target platform, then MIPS or ARM (as emulated by GXemul) might be suitable choices.

Your compiler needs to output real assembly language code, which the assembler (e.g. gas, the GNU assembler) can then compile into object format, and then you need to link this into an executable image. This is much closer to how things work in real life than running assembly language listings in a simulator (e.g. SPIM).

However, GXemul does not simulate out-of-order execution, penalties related to instruction scheduling, or load-delays, so it cannot be used to evaluate optimizing compilers that take advantage of such processor features. GXemul keeps track of the number of instructions executed, but that's it.


How to start the emulator with a disk image:

Add -d [prefixes:]diskimagefilename to the command line, where prefixes are one or more single-character options. Run gxemul -h to get a list of possible options.

Here are some examples. If you want to run a NetBSD/pmax kernel on an emulated DECstation machine, you would use a command line such as this: 

	$ gxemul -e 3max -d pmax_diskimage.fs netbsd-pmax-INSTALL

NOTE: For some emulation modes, such as the DECstation mode, you do not actually have to specify the name of the kernel, if the disk image is bootable! Bootblocks are then read from the disk and executed, as on a real machine.

It is possible to have more than one disk. For each -d argument, a disk image is added; the first will usually (*) be SCSI target 0, the second will be target 1, and so on, unless you specify explicitly which ID number the devices should have. 

	$ gxemul -e 3max -d disk0.raw -d disk1.raw -d 5:disk2.raw netbsd-pmax-INSTALL

Note: In the example above, disk2.raw will get scsi id 5.

(*) = One exception is that in the LUNA88K machine, if no id is specified, the first disk image gets id 6, the second id 5 and so on.

If a filename has a 'c' prefix, or ends with ".iso", then it is assumed to be a CDROM device (this can be overridden with a 'd' prefix, to force a read/write disk). For example, the following command would start the emulator with two CDROM images, and one harddisk image: 

	$ gxemul -e 3max -d image.iso -d disk0.img -d c:second_cdrom.img netbsd-pmax-INSTALL

Usually, the device with the lowest id becomes the boot device. To override this, add a 'b' prefix to one of the devices: 

	$ gxemul -e 3max -d rootdisk.img -d bc:install-cd.iso name_of_kernel

If you have a physical CD-ROM drive on the host machine, say /dev/cd0c, you can use it as a CD-ROM directly accessible from within the emulator: 

	$ gxemul -e 3max -d rootdisk.img -d bc:/dev/cd0c name_of_kernel

It is probably possible to use harddisks as well this way, but I would not recommend it.


How to start the emulator with tape images:

Using emulated tape drives is a bit more complicated than disks, because a tape can be made up of several "files" with space in between. The solution I have choosen is to have one file in the host's file system space for each tape file. The prefix for using tapes is 't', and the filename given is for the first file on that tape (number zero, implicitly). For files following file nr 0, a dot and the filenumber is appended to the filename.

As an example, starting the emulator with 

	-d t4:mytape.img

will cause SCSI id 4 to be a tape device, using the following file number to name translation scheme:

File number: File name in the host's filesystem:
0 mytape.img
1 mytape.img.1
2 mytape.img.2
.. ..

If you already have a number of tape files, which should be placed on the same emulated tape, then you might not want to rename all those files. Use symbolic links instead (ln -s).

There is another advantage to using symbolic links for tape filenames: every time a tape is rewound, it is reopened using the filename given on the command line. By changing what the symbolic name points to, you can "switch tapes" without quiting and restarting the emulator.

Note: Tape support is most likely very buggy, because it has not been tested much, and has probably also suffered from bit-rot by now.


How to use disk image overlays:

This is most likely best understood by an example:

It is also possible to add multiple overlays. In that case, writes always go the the last added overlay.

GXemul uses Unix' way of supporting files with "holes", so even if ls -l overlay.img says that the overlay is several gigabytes large, du overlay.img should reveal that only the blocks that have actually been written to have been stored in the overlay, e.g.:

	$ ls -l
	..
	-rw-r--r--  1 debug  wheel  3072319488 Mar 24 11:59 nbsd_cats.img
	-rw-r--r--  1 debug  wheel     2465354 Mar 24 11:44 netbsd.aout-GENERIC.gz
	-rw-r--r--  1 debug  wheel  2930841600 Mar 24 14:02 overlay.img
	-rw-r--r--  1 debug  wheel      715538 Mar 24 14:02 overlay.img.map
	$ du overlay.img
	864     overlay.img

The .map file is simply a raw bitmap telling which blocks of the overlay file that are in use.

As a short-hand for creating overlay files, running the emulator once with those overlay files, and then immediately removing the overlay files, the disk image prefix R (uppercase) can be used instead. In other words, instead of typing something like:

touch overlay.img overlay.img.map
gxemul -e ..... -d disk.img -d V0:overlay.img
rm overlay.img overlay.img.map
one would type:
gxemul -e ..... -d R:disk.img


Transfering files to/from the guest OS:

If the emulated machine supports networking (see this section for more info), then the easiest way to transfer files is probably to use FTP or similar methods.

There is another way of transfering files which works for any kind of emulated machine which supports disks (either SCSI or IDE). Any file can be supplied as a disk image. For example, consider the following:

	$ gxemul -XEcats -d nbsd_cats.img -d archive.tar.gz netbsd-GENERIC

This will start NetBSD/cats with nbsd_cats.img as IDE master on controller 0 (wd0), and archive.tar.gz as IDE slave on controller 0 (wd1). From inside NetBSD, it is now possible to extract the files using the following command:

	(inside emulated NetBSD/cats)
	# tar zxvf /dev/wd1c

Don't worry if NetBSD complains about lack of disklabel; it doesn't matter. On some machines, NetBSD uses wd1d instead of wd1c for the entire disk. There is also a minor problem: reading the end of the disk image. If you experience problems untaring archives like this, then pad out the archive first with some zeroes.

Transfering files out from the emulated operating system to the host can be done the same way. First, prepare an empty archive file:

	$ dd if=/dev/zero of=newarchive.tar bs=1024 count=1 seek=10000

This example created a 10 MB empty file. Then, start the emulator like this:

	$ gxemul -XEcats -d nbsd_cats.img -d archive.tar netbsd-GENERIC

and transfer files by creating an archive directly onto the disk image:

	(inside emulated NetBSD/cats)
	# tar cvf /dev/wd1c filenames

where filenames are the files or directories to transfer.


How to extract large gzipped disk images:

Unix filesystems usually support large files with "holes". Holes are zero-filled blocks that don't actually exist on disk. This is very practical for emulated disk images, as it is possible to create a very large disk image without using up much space at all.

Using gzip and gunzip on disk images can be very slow, as these files can be multiple gigabytes large, but this is usually necessary for transfering disk images over the internet. If you receive a gzipped disk image, say disk.img.gz, and run a naive 

	$ gunzip disk.img.gz

on it, you will not end up with an optimized file unless gunzip supports that. (In my experiments, it doesn't.) In plain English, if you type ls -l and the filesize is 9 GB, it will actually occupy 9 GB of disk space! This is often unacceptable.

Using a simple tool which only writes blocks that are non-zero, a lot of space can be saved. Compile the program cp_removeblocks in the experiments/ directory, and type: 

	$ gunzip -c disk.img.gz | cp_removeblocks /dev/stdin disk.img

This will give you a disk.img which looks like it is 9 GB, and works like the real file, but the holes are not written out to the disk. (You can see this by running for example du disk.img to see the physical block count.)


Using a PROM dump from a real machine:

In GXemul, a simple PROM/BIOS implementation is usually faked, so that guest operating systems can start up. For example, if the PROM has services which the guest OS can call, which tell how much memory there is in a machine, or to print simple characters to a terminal, those can be implemented in software without having to run the original PROM image from a physical machine.

Raw PROM images from real machines can, in a few cases, be used in the emulator. A few things are worth keeping in mind, though:

  1. ROM code is usually much more sensitive to correctness of the emulator than operating system kernels or userland programs are, so don't expect any PROM image to just magically work.
  2. If you are running a modified ROM/firmware image in GXemul, in order to see that it boots up and works, it will not automatically mean that it will behave the same way when you put it back in real hardware again.
  3. Most of the emulation modes of GXemul have been designed to run without the need for a PROM image, for example the load order (or mechanism) used to boot from a CDROM or SCSI disk, so running with actual machine firmware might not really be supported, more than for simple experiments.

Useful command line options:

Preparation:

The ROM image first needs to be extracted from your real machine. There are several ways to do this, and it depends on your available hardware, the specifics of the machine in question, and how much time you have.