The Linux/m68k for Macintosh FAQ

Table of Contents

Section 0: Forward

This FAQ and the Linux/m68k for Macintosh project could not exist without the generous help and support of a large number of people. There's still a lot of work to be done however, and your help is appreciated now as much as ever. If you want to join the project, there is information below. Our thanks go to all of the programmers who have put their time into making everything happen, both past and present. Thanks also to everyone who actually tested the patches and things that those programmers put out, everyone who offered web space, the people that generously gave their resources and money to our new domain names, the "original" (and defunct) MacLinux team that started it all, the Mac BSD team for letting us use their booter for a while, and anyone that I might be forgetting. You all have my thanks for giving me something to do. :)

Section I: This FAQ

1. Is there a FAQ?

Yes, there's a FAQ and you're looking at it. This FAQ is based primarily on the FAQ written by Mike Comb. Additionally, it contains material from an alternate FAQ written by Alan Cox. Currently, it is maintained by the Linux/m68k for Macintosh development team using the same CVS repository as the web pages and the kernel source.

2. Is this FAQ "official?"

About as official as anything is around here.

3. I have a comment/complaint/suggestion about the FAQ. Who should I tell?

Tell the mailing list. Everyone who has the power to add things to the FAQ should be subscribed to it.

4. Where can I download a recent version of this document and where can I look for updates?

The FAQ has now been moved to its official site at http://www.mac.linux-m68k.org/docs/faq.php.

Section II: The Project.

1. Is there a FAQ?

Do you like repeat questions?

2. Is there a mailing list?

The original mailing list (linux-mac68k@mac.linux-m68k.org) is defunct.

There are two other lists, however: the debian-68k mailing list and the linux-m68k kernel mailing list. Neither of these lists is Mac-specific: they are read by users and developers of all of the 68k Linux sub-architectures (Amiga, Atari, etc).

3. Is there a mailing list archive?

The old Mac68k list is archived here and here. See the links given above for archives of the other two mailing lists.

4. Is there a FTP site?

No, it is defunct. The content has been moved to this web site. New downloads are now made available from the SourceForge file release page.

5. What resources are available over the web?

Quite a few, natch. The "official" web page is at http://www.mac.linux-m68k.org/. See the Resources page for more information.

6. What kinds of people are you looking at for the project right now?

Really, we're looking for anyone. :) In particular, people with in-depth knowledge of Mac hardware would be nice, and actually being proficient at C would also be very nice. Linux kernel experience would also be wonderful, but not necessary. There's also a demand for people testing and trying out new things on their machines; this is the only way that the developers can know if things work or not. :)

7. Are there any newsgroups or chat rooms?

We don't have any specific newsgroup, but the 68k Macintosh port seems to get a disproportionate amount of "airtime" on comp.os.linux.m68k, and many of the developers read that newsgroup regularly.

An IRC channel on OFTC has been created for development and support of Linux/m68k for Macintosh. The channel name is #linux-mac68k. See the OFTC webpages for more information on the network, the rules, and a list of servers near you.

8. What are the goals of the project?

The primary purpose of our efforts is to get Linux running on as many 68k based Macs as possible and to support as much of their hardware as possible. Both of these are formidable goals in and of themselves. In addition to this, we intend to do it without using any of Apple's ROMs that shipped with each Mac and was required for the MacOS. (This will make our OS implementation less dependent on potentially bug-filled and outdated roms that would otherwise slow the execution of the OS.)

Section IV: Hardware Requirements

1. What machines will never be supported?

In general, any computer with a 68000 processor won't be able to run Linux. Of the two 68020 machines, the Mac II supports the addition of a PMMU and can be made to run. (See the PMMU section, later.) The LC does not have this capability. Also some machines are fitted with a defective 68LC040 chip and cannot support Linux FPU emulation (or any FPU emulation, for that matter); before thinking of these machines as "supported" read the section on them later. For other unsupported machines, see the Power Mac section.

2. What machines will be supported?

Any computers with a 68030 or 68040 will work, even better if they have an FPU. Also, 68020's with a PMMU will work (this seems to be only the Mac II). Other expansion boards and whatnot should work, but expect a fight.

3. What machines boot *now*?

Almost all better-than-68000 Macs should work in some form or another. Keep in mind, however, that not all hardware will be supported on all machines. For more information, see the machine status page.

4. What about FPU?

An FPU is not an essential piece of Mac hardware to have for day to day operations, but it does figure in rather prominently in the Linux world and, if you don't have one, you should probably emulate it to make sure all of your really complex applications work. Fortunately, FPUs shipped with all of the II series, except for the IIsi and IIvi. 68LC040's are another issue, they don't have built-in FPU and are (in many cases) very buggy. See the section on them later.

Thanks to the low-level wizardry of Roman Zippel, we now have a floating-point emulator that should be functional on all 68030 machines in the latest 2.2 and 2.6 kernels.

5. What about the 680LC40's?

This little chip can be quite a problem for the uninformed. If you have one of these chips, you may have a defective one that does not permit FPU (floating-point) emulation. According to Motorola's official errata, any chip with a mask revision less than 2E71M has the bug. Specifically, revision 2E23G, used in the LC 475, has been confirmed to be buggy. Running Linux on one of these machines is possible, but probably not worth it. It seems that the bug can be avoided by not using both swap and FPU emulation. But in practice memory is scarce, so really you need to recompile all of your binaries without floating point instructions (gcc's ‑msoft‑float option works nicely) and you'll also need to rebuild your kernel to cause it not to try to save FPU register states while task-swapping (in the 2.6 kernel, this can be specified in the kernel configuration). All in all, a difficult task and probably not worth it. At one point, it was possible to get a replacement from Apple and/or Motorola, but that was quite a long time ago, and given Apple's present state of complete disassociation from the 68k Macs, your best bet would be simply to upgrade to a real 68040. Sorry.

6. What about the Power Macs?

Power Macintoshes are not based on Motorola's 68k chips and therefore are not covered by this port. There are several ports available however. Initially, there was Apple's (now defunct) microkernel mkLinux port and the monolithic kernel Linux/PPC aka PenguinPPC. The latter port does not currently support non-PCI Macs, but there is a port available for NuBus PowerMacs.

7. Any machines in particular that we might have problems with?

In general, each Mac model brings in its own set of problems and we constantly have to rethink our drivers and interfaces to maintain the kind of device abstraction that makes Linux so portable. Therefore, each Mac will likely cause its own problems. For more information, see the machine status page.

8. What about MODE32?

Mode32 is a fix required because the ROMs in several machines are not 32-bit clean (SE/30, II, IIx, IIcx). Currently, I have no trouble booting my IIcx without Mode32, but this was not always the case. If you need it, it can be downloaded from download.info.apple.com. See the note later.

As Mode32 merely fixes a ROM problem, it is not required for the general operation of Linux on the Macintosh, just the bootup stage.

9. What about a PMMU?

In order to get Linux to run on your older 68020 based Macintoshes, you will need to get a PMMU chip to enable the paged memory that Linux (and A/UX) likes. Only the Mac II has a socket for this chip. Also, you should be aware that the 68851 steals an extra clock cycle for every instruction executed in order to do address translation, thus making an already pitifully slow machine by today's standards even slower.

10. Where can I buy a PMMU?

Since no 68020 Macs shipped with a 68851 PMMU, you'll need to buy one and install it yourself. Adam recommends Data Memory Systems (www.datamem.com) and he says that you can get it for $29 + shipping. Of course, $29 will probably buy you an '030 Mac somewhere. :)

11. How much RAM?

The Linux/m68k port requires about the same amount of RAM as the other Linux ports: about 4 megs at the minimum. Even if Linux supports less, the recent versions of the Penguin booter do not. 4 MB is no longer enough to load a 2.2 kernel and a ramdisk. Even 5 MB is stretching it. System 7.0.1 uses less memory than the other Mac systems; if you have a low memory machine, you might try running it. If you intend to run Debian 3.0, you are looking at a more usable minimum of 32 MB.

Section V: Specific Hardware Issues

1. What input devices are supported?

Amazingly enough, we have keyboards (and mice)!

Some third-party 3 button mice are also supported. I have a Logitech MouseMan that works perfectly on the console and in X. All ADB mice are currently accessed in "cooked" mode through /dev/adbmouse. You should use the "busmouse" protocol for gpm and X. With 2.6 kernels, use /dev/input/mice and "imps2" protocol instead.

2. What about serial support?

Serial support is working and in the 2.2 kernel. Until recently the Quadra 900 and 950 and the IIfx required that you set the serial ports to "compatible" mode in MacOS before booting. However, we've recently learned how to do this from Linux, so with the very latest kernels, this won't be necessary.

Note that the overall bad design of 68k Mac hardware prevents reliable operation of the serial ports at speeds greater than 38400 bps in Linux. (This limitation applies to MacOS as well, except on some high-end models.)

As a side note, obviously the Mac serial plugs won't fit into your PC's serial ports and there are some other technical incompatibilities. Fear not! You can buy an adapter for $10 or so from a catalog or a larger computer store. You can also construct a workable null-modem cable in order to use your PC for a serial console by plugging together a PC null-modem cable and a Macintosh modem cable. With the correct cable, the Mac's RS422 serial port can be made electrically compatible with a PC's RS232 serial port.

3. Output?

Yes, we have framebuffers! The Macintosh framebuffer driver was recently rewritten for Linux 2.2, and now will give at least a workable text console on all models with internal video, as well as the most common NuBus video adaptors. This does not, however, imply that the colormaps will be even vaguely correct. Read on...

The video driver has trouble changing the color map on some models. If your Mac has this problem, and you're handy with ResEdit, you can now (since Penguin-16 at least) tweak the Penguin colormap in order to get something resembling decent color in X. (FIXME: There's a web page about this somewhere, but I've lost the URL...) However, if you fix the colors in X, the console will lose.

If you have a Quadra, Centris, or 68040-based LC or Performa (except for the 660AV and 840AV read on), you are in luck, as our framebuffer driver has recently learned how to change the colormaps on both types of internal video chipset used there. Furthermore, the PowerPC driver for the Valkyrie chipset used on the 580 and 630 series Quadras, LCs, and Performas has been ported to 68k, and it supports changing the video mode and colour depth on the fly (using fbset, or the X server). You will need the most recent 2.2 kernel or later to do this. To enable it under older kernels, add "video=valkyriefb:" (the colon is important) to the kernel command line in Penguin, and enjoy!

Finally, the Linux 2.2 fbcon subsystem supports 16 and 24-bit truecolor modes on the text console, and the XFree68_FBDev X server also supports them. Therefore, even on machines such as the AV series Quadras where we don't support colormap setting, you can switch to one of these modes in MacOS before booting (Penguin-16 will still complain about this - click "Skip" to continue anyway), and have plenty of pretty colours.

If you're handy with MacsBug, you can also help us add support for colormap changing to the framebuffer...

4. Ethernet?

Several ethernet cards based on the National Semiconductor 8390 chip (also used in many ISA ethernet cards, the NE2000 being the most well-known) are working. These include Apple cards and several clones including Daynaport, Asante, and Farallon. Not all clones are known to work "as-advertised" and adding a new driver to this list may be easy, but might also involve some clone specific bugs that will make problems rather difficult to track down. Also, the Asante MacCon CS card for the "comm-slot" on the 575, 580, and 630 models is supported, and the driver will probably be adaptable to support other cards based on the SMC91C94 and SMC91C92 chipsets.

On-board SONIC ethernet, found on most of the Quadras, is also supported, as are SONIC-based comm-slot cards. However, we haven't yet devised a reasonable scheme for autoprobing for the existence of the SONIC. On-board MACE ethernet, found on the 660AV and 840AV, also works in the 2.2.18 and later kernels.

LC PDS cards based on the 8390 chipset generally work, as does Apple's SONIC-based LC ethernet board.

5. SCSI?

As of right now, SCSI is working on nearly all machines and you can mount and play with your Mac harddrives from within Linux. If you want to set up an ext2 partition, you'll need to use the installer. (See below.)

6. Sound?

I think that there are far better things to worry about than sound support. :) The sound chip isn't well documented and might be a pain to implement correctly. (Is there already a working driver for this?) In addition, some (older) kernels will make a LOUD noise for debugging when you start them. Do not interpret this noise as working sound... consider it more along the lines of a debugging message that hurts.

7. Floppy drives?

There are two main kinds of floppy controllers on the Macintosh models that we're working with: the IWM and the SWIM. A driver for most models was contributed by Laurent Vivier and merged into the linux-2_2 CVS branch.

8. Other devices?

It depends on the device. If you want something bad enough, you could always write a driver for it. :) In general, if it is already supported by a particular Linux port, you won't have as much trouble getting it working as if you need to develop drivers from scratch. The "Valkyrie" framebuffer, SMC9194 Ethernet, and CUDA ADB drivers are examples of this, and reading them might be instructive in the sort of things you have to deal with when porting to 68k Macintosh.

Section VI: Testing and Playing.

1. Can I use Linux without damaging my important data?

Yes. There's a simple process you can use to get a "booting" Linux running (or not running, as the case may be) on your system without sacrificing any of your data.

First, download Penguin, initrd and a kernel. Follow the instructions on booting, given below in the section on Installing, EXCEPT select the ramdisk that you want to use in the configuration section and put 'root=/dev/ram' in the command line. If it comes up to a login prompt, great. Otherwise, you'll need to look into Troubleshooting, later. At this point, you have a very minimal setup that isn't any good for anything more than testing.

2. Can I boot linux from an EXTERNAL harddisk that I want to make only for Linux?

Yes, and this is only a bit more complicated. You'll need to read the bits on installing, later. Still, follow the steps above to make sure that Linux will work for you in the first place.

3. How can I be sure Linux will work on my Mac?

Try it and see. Look two questions up. Can never hurt to try.

4. How can I boot into Linux?

Booting into Linux is easy, all you need is a recent copy of Penguin (the booter) and a kernel. (Both of which can be found on this site under "Downloads".) Before you can boot into Linux, you will need to turn off Virtual Memory and you may need Mode32 (see the section on Mode32, later). Start up Penguin, select the kernel that you want to boot from in the configuration screen, and then use the boot option from the file menu. If it works, you'll get a message about rarp requests. If not, see the troubleshooting section, later.

5. Why won't Linux boot on my machine?

You looked at the status web page and your machine was supported. But it just won't boot! Why not? Well, there are a few common problems that cause most of these failures. The most common one is that you're trying to use an ancient kernel that doesn't properly support your hardware. You should always be using the latest kernel, unless you have a really good reason not to.

Other common problems are less obvious. If you have a IIci or IIsi, you must boot with either Penguin 19 or Penguin 17, and you must be running system 7.5 or higher. This is due to a bug in the MacOS video driver, as well as a bug in Penguin that has not yet been found. If your machine has built-in ethernet, or even a NuBus ethernet card, try unplugging the ethernet before booting. If you have any NuBus cards installed, try removing them and booting again. If all else fails, ask the mailing list.

Section VII: Installing.

1. Do I need to repartition my harddisk?

Maybe. If you have only one drive, then yes. Otherwise, if you have an external drive, maybe not. In either case, you *need* at least one partition devoted to Linux someplace. (Two if you need swap.) If you have Ethernet and want to boot your machine NFS-as-root, you won't need to repartition, but you will need to install things elsewhere.

2. How do I repartition my harddisk?

Warning! If you don't know that you are destroying any data on your disk by doing this, maybe you shouldn't be doing this. :)

First, backup anything important and make sure you have a bootable emergency disk and your original install floppies.

Second, put a copy of "Apple HD Setup" on your emergency disk if it isn't there already. Reboot the machine using this disk. Go into the Setup tool and click "Partition". Delete your main Mac partition, and create 3 more in its place. I recommend a large "A/UX Misc." root partition and a swap partition of size about double the RAM size. Oh, and don't forget to have enough room for a Mac partition. (About 10 meg will be the minimum.)

Third, reboot again with the install disks and reinstall MacOS on the Mac partition. Unbackup your data, if you saved any.

You're done.

3. Are there any other tools I can use to repartition?

You can use the MkLinux/Darwin pdisk utility, which has been renamed to mac-fdisk. As a previous version of this FAQ mentioned, it is a real pain to use (though no more so than the BSD fdisk and disklabel utilities). The "libfdisk.a" library used in the Debian installer can recognize every sort of partition table known to humankind, so it's only a matter of time before someone finally replaces cfdisk with another nice user-friendly program that actually works on Macs, Suns, and other renegade architectures.

Update: that replacement is parted.

4. How do I install to an external harddisk?

This *can* be easier than any other installation method, depending on your resources. If you don't need the disk for anything but Linux and you can plug into another (m68k or other) Linux computer that you have root on and access it, you are all set.

5. Are there any distributions that'll make this easier?

Yes. Debian 2.2 comes on CD for several architectures, including m68k. The m68k CDs are written in "Hybrid" format and include everything you need to install Debian on your 68k Macintosh.

There are installation guides available for older Debian releases. The latest Debian Linux install guide is here.

6. What if I repartitioned my disk but don't have Mac system disks?

Apple has released System 7.5.x to the public. You can find it at Apple's mirror.

7. How are the Mac partition tables usually laid out?

The Mac partition tables usually contain 3 entries, even with a base install of the MacOS (system 7.) The first partition is where the partition table is stored (weird, huh?) The second is a "driver" partition. The third (and often last) partition is the HFS partition where your files are stored. In addition, the MacOS installer tends to leave about a meg of unallocated space at the end of the drive... I don't know if it is aligning the last partition to a meg boundary, or if it stores something important there.

8. Will I still need MacOS once I have Linux?

There is now a project underway to create a bootloader that does away with MacOS completely: Emile.

For the time being, such installations are not well documented, so typically MacOS is used to boot the machine, then Penguin is run from MacOS to boot Linux. Because of this, kernels and ramdisks are kept on the MacOS side on an HFS partition.

9. How do I modify a ramdisk image?

Most Linux systems have the 'loopback device' driver compiled in, you will need to use it. If the system you are using doesn't have it, then you'll have to compile a new kernel with loopback support to modify any images.

The command line to mount the image is 'mount -o loop -t ext2 filesys mnt' where ext2 is the filesystem of the ramdisk (might also be minix), filesys is the image (ungzipped) and mnt is a directory for a mount point.

10. How do I run X?

(Note: the following applies to older XFree86 installations, not to current Xorg releases). First, make sure that you have the proper X packages installed, including the XF68_FBDev server. You don't run xf86config to configure X. Instead, do: cp /usr/doc/xserver-common/examples/XF86Config.eg /etc/X11/XF86Config to create the proper config file. Now, just use startx to run X.

Section VIII: The Kernel.

1. What kernel(s) should I try?

The 2.2 series has the best hardware support, and stability. That said, there's also the "unstable" 2.6 kernel, which mostly works OK on Quadras, etc. The 2.6 kernel is required by Debian 4.0.

The 2.0 and 2.1, and 2.4 kernels are pretty much defunct. Don't use them.

2. Where are the kernels?

The official download page for kernels is the SourceForge file release page. The Debian site has a 2.2.25 kernel.

Section IX: Compiling the Kernel Yourself

1. Why would I want to compile the kernel myself?

In order for you to do any major work of any kind, you'll probably have to have the source and be able to do things with it. (Unless you just happen to be a super ResEdit/Macsbugs hacker that can play with the assembly. :) ) If you just want to test the things that others have done, you won't need to.

2. What do I need to recompile the kernel?

First, you need a suitable toolchain. What this means is that basically you need versions of GCC and GNU binutils (but watch out - some old revisions of binutils are buggy and will build non-working kernels!) that are capable of building and linking m68k ELF object files. You will probably also need GNU make. If you're using GNU/Linux on m68k, this is no problem at all, obviously! On other operating systems or architectures, if you're not averse to compiling GCC yourself, there are some instructions in the top level directory of the GCC source which will tell you how to build a cross-compiler. Ignore anything in the documentation related to run-time libraries - obviously, these are irrelevant to building a kernel.

To build the latest 2.6 kernel requires gcc-3.3 or later. To build the 2.2 kernel you'll want gcc-2.95.

On Debian Linux, there are various methods to obtain cross-compilers, which are not documented here. Some cross-compilers are provided on this site, see the cross-development page.

Now, obviously, you need to compile the kernel. If you're cross-compiling, you should edit the top-level Makefile (unless you used CVS to obtain your kernel source, where this is already done for you) such that the ARCH variable is set to m68k, and the CROSS_COMPILE variable is set to m68k-linux- (this tells make that the cross compiler is called m68k-linux-gcc, though for some tool-chains that might be m68k-linux-gnu-gcc or m68k-unknown-linux-gnu-gcc). You can now configure the kernel by whatever method you like (menuconfig, xconfig, config, or the "BSD method", i.e. editing .config directly :-). Finally, to actually compile the kernel just type make vmlinux. Penguin knows about gzip-compressed kernels, so you might want to change that to make vmlinux && gzip -f vmlinux. And, of course, if you want to use modules, you'll have to figure out how to get them onto the machine where the kernel is going to boot.

3. How can I put together the source?

This section was completely obsolete, and has been removed. For now the only sure way to get source is via CVS.

4. How do I get the source from CVS?

If you would like the sources for the 2.6 kernel, you can find them at http://linux-m68k-cvs.ubb.ca/.

If you would like the sources for the 2.2 or 2.4 kernels, they are hosted on SourceForge. Use these commands to check out the 2.2 kernel sources:

• export CVSROOT=:pserver:anonymous@linux-mac68k.cvs.sourceforge.net:/cvsroot/linux-mac68k
• cvs login (The password is blank. Just hit enter.)
• cvs co -r linux-2_2 linux-mac68k
• cd linux-mac68k
• cp config-standard-2.2 .config
• make oldconfig

If you want the unstable 2.4 branch instead, omit the "-r linux-2_2" in the "cvs co" command.

Section X: Linux Utilities that Might Help

1. What utilities exist for Linux that might be of use?

This section was completely obsolete, and has been removed.

Section XI: Utilities for Macs that Might Help

1. What utilities exist for the Macintosh that might help?

• MacOS 7.0.1

  For those without System 7 (required by Penguin), they can download this version for free from Apple. You can also now get System 7.5.2, which is known to fix some problems with disabling the video interrupts on the IIsi and IIci that may prevent Linux from running correctly. However, 7.5.2 is very buggy, so if you actually plan to use MacOS, you will obviously want to upgrade to 7.5.5. Furthermore, System 7.5 is horribly slow on 68030-based Macs, and none of them require it to boot (unlike most 68040-based Macs), so unless you really need it you are better off with 7.0.1 or 7.1.

• Mode32

  This fix for "dirty" ROMS in several machines. (see the section on it, above) I don't know if Penguin needs it, but it has been known to spell the difference between booting and not in cases.

• Stuffit Expander

  This is mostly useful for reading Apple documentation that has been compressed using the proprietary StuffIt format. If you want to compress and decompress files with gzip instead, you can get MacGzip, which is Free Software, from any decent Mac archive site.

• Apple HD SC setup

  This program will let you partition your Mac harddisk so that you can install Linux on it. By default, it only works on SCSI drives with Apple firmware, because Apple wants to protect you from yourself. It can very easily be binary-patched (by changing a single byte in a resource) to allow it to recognize and initialize other models of SCSI drives. Instructions on how to do this (along with a BinHex archive of a program which will perform the patching for you) can be found at http://www.euronet.nl/users/ernstoud/patch.html

• MacLinux installer

  This utility copies files to an ext2 partition on a Mac. Maintained by Christiaan Welvaart. It is no longer very useful now that the Debian installer generally works on all Macs. However, you may still find it handy if you want to use something more "lightweight" than Debian.

• ext2 Utilities

  This set of utilities lets you format and test ext2 partitions on a Mac disk. Also maintained by the same person as the installer.

• MacsBug

  This utility will help you to debug those pesky Mac apps and (if you are really good), figure out a ton of Mac hardware information.

• SUntar

  This utility can be thought of as a kind of dd for MacOS. Use it to read and write disk partitions and raw floppy or filesystem images. It can be used to create a rescue floppy disk, for example.

Section XII: FTP Site Information

1. Can I upload to the FTP site (ftp.mac.linux-m68k.org)?

No, sorry. The FTP site is no longer used for new releases.

2. How is the FTP site arranged?

Most of what was on the old FTP site is in /pub/linux-mac68k. You should use HTTP instead of FTP to access it.

3. Can I mirror the FTP site?

Yes, if you like. Please be sure to use an intelligent mirroring program that will not download anything that hasn't changed.

Section XIII: Contributing to the project

Join the mailing list (see section II-2).

Section XIV: Compatibility

1. Will Linux/m68k for Macintosh be binary compatible with the other m68k Linux projects?

It already is, aside from the various Amiga and Atari-specific programs. That's what one major purpose of the kernel is in a real operating system: to present a uniform interface to user-space so it doesn't have to worry about the details of the hardware.

2. Will Linux/mac68k be source compatible with other Linux ports?

The short answer is yes, for the same reasons listed above. The long answer is yes, but there are certain differences between processor architecures that can't be reasonably abstracted away by the kernel. The major things to watch for are alignment issues (m68k is slightly stricter about having things be word-aligned than i386), endianness issues, and, for kernel drivers, the total absence of all sorts of low-level i386 brain-damage (having a tiny port I/O space and separate instructions for accessing it sticks out as the big one). Also, a lot of software authors seem to be under the mistaken impression that Linux is for i386 only, and make stupid assumptions about things as simple as the strings returned by uname(1) in Makefiles and the like.

3. Will Linux/mac68k be able to run MacOS or A/UX binaries?

Not right now, definitely not. It has been discussed that in the future, we'll probably be able to make a Mac-in-a-Mac using some features of the 68030+ processor to allow running of the MacOS as if the machine was a 68020 (since the 68k architecture is virtualizable, unlike i386). Honestly, it's more important to get as many systems as we can running Linux right now than to get them running MacOS. If you want the MacOS that bad, what are you doing here? :)

A/UX is uncertain. We haven't looked into it enough to make any judgments. Part of the reason for this is that A/UX is very hard to find. Personally, I (dhd) am much, much more interested in the possibility of being able to run 68k HP/UX and SunOS binaries like NetBSD and OpenBSD can.

In general, if you want binary emulation that actually works, NetBSD is your best option on any architecture.