Bochs: A Portable PC Emulator For Unix/X
By far, the majority of my time involving a computer was spent working with my Sun SPARCstation. I could do almost everything computer related, including e-mail interaction, system administration, net surfing, network Doom (I confess—it was me who circulated that hack so you could run Doom on Solaris 2.3), all on the same machine I used for software development. Occasionally, though, I found myself firing up the ol' PC on my desk, using MS Word to crank out documentation or a memo, or to run one of a handful of smaller utility programs I purchased years ago. Well, that got old quick! Having two keyboards, two monitors, and two computers on one's desk consumes more than desktop real estate—it takes much more time to maintain and introduces frustrations from the disparate OS designs. So, I began looking for a software solution which would allow me to run on my SPARCstation the handful of PC programs I used.
The idea of paying a lot of money to run something I'd already purchased didn't sit well with me, and I was interested in finding something I could extend if necessary. A little searching on the Internet didn't turn up anything low- or no-budget that would run MS Windows 3.1 on a SPARC and for which source code was accessible. The Wine and DOSEMU projects were making great progress, but they would never run on non-x86 architectures. There was an emulator program called pcemu, which would run on a SPARC. It did what it was intended to very well—run DOS. Unfortunately, it was engineered to emulate an 8086, and didn't lend itself to be extended to support 80286 and 80386 features. My search also turned up another 8086 emulator, one which went along with an older version of MINIX, allowing MINIX to run on non-x86 platforms. Since it was geared toward allowing a real mode version of MINIX to run within the emulator, this didn't seem like a good candidate for enhancing either.
There just wasn't anything out there that would do what I wanted it to do or which would be a good starting point if I was willing to put the effort into enhancing it. At the same time, I noticed significant similar interest on the net, specifically in the comp.emulators.* newsgroups. At that point, I began working on Bochs (pronounced “box”), a portable software PC emulator project, with the goal of making it possible to run PC software on a Unix workstation.
Looking back, it's still hard to believe it all came together. The biggest hurdle often was documentation. Here we have a somewhat undocumented DOS (and later Windows) trying to use parts of a multitude of non-standardized and sometimes poorly documented BIOSs, using hardware devices which aren't always documented accurately or fully, and running on a non-open Intel x86 architecture, which has undocumented instructions and features (e.g., LOADALL). If it weren't for the likes of The Undocumented PC and Undocumented DOS, I would have quit a long time ago. The other large hurdle was (and will be) tracking down and fixing the bugs which seem nearly impossible to find in such a complex system. One wrong bit flipped here, and sometimes the ill effect shows up 10 million instructions later, when the wrong character is displayed on the screen! Bugs which depend on timings proved to be elusive, since they don't always show up, and timing is drastically affected by the debug print statements inserted in the code. I've had occasion to sift through debug files of some 50+ megabytes (which I refer to as stealth bug contrails), only to find I didn't print out the one piece of info I needed!
In the two and a half years I've worked on this software, Bochs has made it past some very important landmarks. The first was booting up MS DOS 5.0 to the A prompt! At that point, I had only very rudimentary text HGA (a monochrome video card) emulation and no keyboard support, so I had to hardwire keystrokes into the keyboard BIOS to get past the time and date prompts during boot and to run DOS commands afterwards. It wasn't pretty, but it showed there was potential.
I was then on my way to implementing the graphics mode of the HGA and basic keyboard support, in an attempt to get Windows 3.0 running in real mode. After many red-eye nights, I managed to get Windows 3.0 to boot up past the initial graphics banner screen. What a sight that was! I remember staring at the screen, soaking it all in, not daring to type any keys, fearing it would hang Windows. Indeed it did, and that's when I knew I was right on track...
Though I had designed the emulator to be 32-bit right from the start, at this point I had only 8086 emulation. It was time to take the plunge and implement the protection and memory management models of the 80286. Surprisingly, after only a few months, I had coded a large part of 80286 support and was able to get MS Windows 3.1 (and 3.0) running in standard mode (286 protected mode).
Though MS Windows and MS-DOS have been largely the software I focus on running within the emulator, I've had some other interesting experiences. Recently, I've been working with the MINIX project (Andrew S. Tanenbaum and friends) to get it to boot/work under the Bochs emulator. The idea is to allow MINIX (a Unix clone suited for learning how internals work) to run on platforms to which it has not been ported. Recently, I was able to perform a full MINIX installation within Bochs and to boot up MINIX in 286 protected mode (although more work needs to be done).
I worked with and integrated my source code with the DOSEMU project's source, for a while. However, they were so productive with their code changes, it was hard to keep up, especially before things began to solidify with Bochs, and I had soon written my own keyboard and video support.
When I get to 80386 emulation (I've recently began this trek), I look forward to talking with the Wine team to look at the potential of a compile-time, optional merge of Wine and Bochs, allowing Wine to run on non-x86 platforms. Wine contains an MS Windows-to-X Windows/Unix translation technology, but it lacks x86 emulation which would allow it to run on other platforms. A merger would mean a significant performance gain could be achieved while running Windows within Bochs, since the Windows GUI and OS calls would be mapped to native Unix/X functions by Wine, instead of Bochs emulating the whole process entailed by the the Windows call. This is the same basic concept behind Sun's Wabi product.
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- January 2015 Issue of Linux Journal: Security
Editorial Advisory Panel
Thank you to our 2014 Editorial Advisors!
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