Here's the scoop on the porting of Linux to the Motorola processors.
by Chris Lawrence
``Linux is NOT portable (uses 386 task switching etc.), and it probably never will support anything other than AT-hard disks, as that's all I have.'' --Linus Torvalds, August 25, 1991.
In the five years since Linus wrote those words, Linux has been ported from its Intel roots to a number of other architectures: the ports to the Alpha and Sparc processors are probably the most familiar to readers of Linux Journal. In all the attention given to ports to ever more exotic hardware, it's easy to overlook the first production quality port: Linux/m68k.
The ``m68k'' stands for the Motorola 68000 series of processors, found at the heart of popular computers like the Apple Macintosh, the Amiga, the Atari ST and its successors (the Atari TT, Medusa and Falcon), as well as the Sun 3, NeXT, Hewlett-Packard/Apollo Domain workstations and others. The MC68020 (with the MC68851 memory management unit), MC68030, MC68040, MC68LC040 and MC68060 are the only CPUs in the 68000 family supported by Linux/m68k, because Linux (like other Unix-like operating systems) requires a memory management unit (MMU) for protected and virtual memory support. A floating point unit is optional, but recommended. Floating point emulation is not distributed in the main kernel tree, since its copyright conflicts with the GNU General Public License.
Like Linux/i386, 4MB of RAM is the absolute minimum, with 8MB being sufficient for most uses. The X Window System requires a minimum of 12MB of RAM for a usable system. A minimal installation currently requires about 55MB of hard drive space, plus at least a few MB of swap space. My personal system currently has about 830MB of hard drive space devoted to Linux (one SCSI hard drive and most of two IDE hard drives). When it comes to RAM and hard drive space, you can never have too much.
Linux/m68k started out as a port of Linux to work only on the Amiga. Hamish Macdonald and Greg Harp released their first version, which they called 0.05, to the public on July 1, 1993. This version was based on Intel Linux 0.99pl9. Soon after that release, several groups of Atari users working independently made the first efforts to adapt the port to that platform. The two ports were merged into one tree starting with 0.9 in July of 1994, with many new features like Ethernet, frame buffer and X Window System support arriving with 0.9pl5 later that year. Further efforts were made to combine some of the advances of the Linux/i386 1.0 and 1.1 series, including ELF support, into the Linux/m68k 0.9 series, culminating in 0.9pl13. Roman Hodek took over maintenance of 0.9 while Hamish started work on catching up with Linux/i386, then approaching the 1.2 release.
The adaptation of Linux/m68k to the general 1.2 kernel was a difficult process. From the first public release (1.2.10), there were 13 patch levels in 11 months (the final release was known as 1.2.13pl10). The format of ext2fs used under Linux/m68k was changed twice (once from 0.9.13 to 1.2.10, and again in 1.2.13pl4). Overall, the 1.2 series saw Linux/m68k mature into a usable system; major improvements were made in X support, and a color display was implemented on the Amiga.
By the time Linux/m68k 1.2 became stable, however, the rest of the Linux community was moving ahead at a rapid pace. Hamish again turned over a usable kernel to Roman and did some very preliminary work on the 1.3 series; Jes Degn Sòrensen adopted the 1.3 source tree in the Autumn of 1995 and began coordinating the work on it. After the initial hurdles of getting the basic code working, progress came quickly. The first working 1.3 series kernel (1.3.23) was released in late February 1996, and was brought into sync by early April (to 1.3.86, one day after the release of Linux).
The current, stable Linux/m68k version is 2.0.28. Development of Linux/m68k continues unabated, with the recent 2.1.17 development release of the main kernel integrating over several hundred kilobytes of changes from the Linux/m68k tree.
As of Linux/m68k 2.0.28, the latest release of the production 2.0 kernel, the Amiga and Atari are directly supported. Users of Motorola VMEbus systems (the MVME 162, 166 and 167) can use an earlier release, 2.0.8. Porting efforts are underway for the Sun 3 and Hewlett-Packard/Apollo Domain workstations and the Apple Macintosh. There has also been some interest in a port to the NeXT workstation.
Compatibility between Linux/m68k and Linux/i386 is very high at the source level. Almost all programs that don't use Intel-specific optimizations (like -m486), assembler code, SVGAlib or /dev/port should work ``out of the box''. Notable exceptions are programs that expect the proc file system's data to be in a specific format (such as /proc/interrupts, which on Linux/m68k can contain any number of interrupts, including shared interrupts). Almost all of the GNU project's software has been tested and used successfully on Linux/m68k, as have the popular Perl, Python and Tcl programming languages and free Web browsers including Arena, Chimera, Grail, Lynx and Mosaic.
As of this writing, no commercial software available for Linux/i386 has been recompiled for Linux/m68k, nor has most other software released without source (with the notable exception of the XForms library). The primary obstacles are as follows:
Unlike Linux on Intel and Alpha, there is no standard video hardware under Linux/m68k. The Amiga and Atari video chip sets are fundamentally different, as are the various graphics adapters available for both systems. Linux/m68k gets around this problem by using the Universal Framebuffer (UFB) device. This term is misleading, since it is used only on Linux/m68k at this point; but there are plans to merge it with the SparcLinux Framebuffer later. The UFB device abstracts the hardware interface to support a relatively simple, device- and system-independent programming interface. An easy-to-use user-mode library, known as oFBsis, is under development as part of the OSIS project to emulate the Atari TOS environment. One side effect of the UFB approach is virtually all Linux/m68k binaries are compatible with all Linux/m68k platforms. For example, the XFree68 server binary can operate all of the display hardware supported by Linux/m68k on both the Atari and Amiga. Even the kernel can be compiled to run on both Ataris and Amigas, and was distributed this way until the 2.0 series, when the number of devices needed for each OS made the combined kernel too large for users with only 4 MB of RAM. More programs supporting the UFB interface are forthcoming.
One of the most exciting developments in recent months is the port of the Debian distribution to Linux/m68k. Debian/m68k is currently in beta testing and will be released in tandem with the next Debian release. Most users currently install Linux/m68k manually using a number of tar files known as the Watchtower-2 file system, a fairly complicated procedure for those not familiar with Unix. There is also an older distribution, based on the 1.2 series kernels, called ALD, available for Ataris on CD-ROM. A proper distribution for both platforms, with support from Amiga and Atari CD-ROM vendors, in addition to the Linux CD vending community, would help make Linux a viable alternative operating system for serious Amiga and Atari users. At present, the only CD-ROMs available are the ALD CD-ROM and Infomagic's quarterly Linux Developer's Resource 6 CD set.
With the disappearance of the Amiga and Atari commercial developer communities over the past few years, many users have turned to the Free Software Foundation's GNU project for the tools they need. Unfortunately, the Unix heritage of the FSF tools causes problems for Amiga and Atari users who must contend with conflicting file naming formats, weak integration with the underlying OS, and memory-hogging emulation libraries. Linux and other free Unix-like operating systems can provide an environment suited for running these tools, with features like memory protection and virtual memory built-in, at minimal cost.
Substantial progress is underway to run well-behaved Amiga and Atari programs under X. The OSIS project, mentioned above, is usable for many Atari TOS applications already; AmigaOS emulation is also available but slow (via the Un*x Amiga Emulator), with faster support for programs that run within AmigaOS rules being worked on under both UAE and the AmigaOS Replacement OS (AROS). Full-speed Macintosh emulation should also be possible as it is under AmigaOS, but as yet, no one has demonstrated it. Binary compatibility with other Unix-like operating systems on Motorola platforms (similar to iBCS on Intel) is another area that could be developed further and may follow with the Sun 3 port. More emulation support is expected once Linux/m68k becomes easily accessible to Amiga and Atari users and their large freeware authoring communities.
Support for expansion devices under Linux/m68k is rather limited at present. Virtually every Ethernet card ever designed for the Amiga and Atari is supported, but only a relative handful of other devices are supported at present. However, many of them--like the Commodore A2091 and GVP SCSI controllers--are among the most common or--like the SCSI options for the Phase 5 accelerators--the most recent. The relative lack of people with hardware knowledge in the Linux/m68k community has slowed development in this area. With the wider popularity of Linux/m68k that should result after the Debian distribution is released, the dearth of technical expertise should become less of a setback as more people with hardware knowledge join in the development process.
While it is difficult to judge the popularity of other Unix-like operating systems on the Amiga and Atari (primarily NetBSD and OpenBSD), the Linux/m68k Registration Site seems to be a fairly accurate measure of Linux/m68k users. According to the site, well over 400 people use Linux/m68k at least some of the time. Our Registration Site is prominently advertised at most of the web pages devoted to Linux/m68k, and Geert Uytterhoeven, its maintainer, posts regular messages to the Linux/m68k-related newsgroups with statistics and a registration form. Registrations can be made using a Web-based form at the site, through e-mail or via snail mail. Despite these efforts, many users of Linux/m68k who only occasionally have or do not have Internet access remain unregistered. It is hoped that vendors of Linux/m68k distributions, once they become available, will help publicize the registration site.
The Web has become an increasingly important source of Linux/m68k information. Over a four-day period around Christmas, 350 visits to the primary site of the Linux/m68k Home Pages were recorded. The registration site also receives hundreds of visits per week. The Frequently Asked Questions file and installation guides for Amigas, Ataris and VME systems are available on the Web. Other Linux/m68k pages are available in French, German, Italian and Portuguese. Coupled with the very active developers' mailing list and the Linux/m68k-related newsgroups (in both English and German), users are well-supported with solid information and quick responses from the Linux/m68k user and developer communities.
As most of the developers reside in northern Europe, they have met a couple of times in person. The Linux/m68k project is in many ways a microcosm of the larger Linux project, bringing together people from across the world in pursuit of a common goal. A recent poster to comp.os.linux.m68k commented that the 68000-series processor has many years of life ahead of it. Those of us who work to promote Linux/m68k hope to keep the Motorola 68000 a viable platform for serious computing. With Linux/m68k, you can put together a complete Linux system for well under $1,000. So before you rush out and buy that $8,000 Alpha, dig through your closet, find that old processor, install Linux/m68, and have a computer with the same functionality for a lot less money.
Chris Lawrence is a senior Computer Science major at the University of Memphis who has used Linux/m68k on his 68040-based Amiga since February of 1995. He moonlights as a salesman for an Internet service provider, is also known as LordSutch at New Moon (a multi-user dungeon at telnet://eclipse.cs.pdx.edu:7680/) and mostly leaves kernel hacking to the professionals. He can be reached at quango@themall.net or in care of Linux Journal.