Retro-Porting to NT 3.1

In another useless project, I decided to find out why even trivial programs created with the Open Watcom compiler refuse to run on Windows NT 3.1. Attempting to start an executable failed with foo.exe is not a valid Windows NT application. But this time, the surgery was successful.

Failure and success running on NT 3.1

There turned out to be several problems, some related and some not. The biggest issue was the problem previously discussed in this post. To recap, the Open Watcom runtime calls the GetEnvironmentStringsA API, which does not exist on NT 3.1. This also causes trouble on Win32s; although newer Win32s versions do implement GetEnvironmentStringsA, it just always fails.

The solution is to use the original GetEnvironmentStrings API (which is equivalent to GetEnvironmentStringsA) . But that’s not enough.

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Posted in Development, Microsoft, NT, Watcom | 12 Comments

Retro-Porting to OS/2 1.0

A few weeks ago I embarked on a somewhat crazy side project: Make the Open Watcom debugger work on OS/2 1.0. This project was not entirely successful, but I learned a couple of things along the way.

The Open Watcom debugger components for OS/2 are 32-bit modules, but didn’t use to be. At least up to and including Watcom C/C++ 11.0, the OS/2 debugger components were almost entirely 16-bit. That allowed them to work on 32-bit OS/2 but also on 16-bit OS/2 version 1.x.

Needless to say, debugging 32-bit programs with a 16-bit debugger required a bit of extra work and there was an additional interface module (os2v2hlp.exe) which called the 32-bit DosDebug API on 32-bit OS/2 and interfaced with the 16-bit debugger ‘trap file’ std32.dll.

Building the 16-bit components again was not particularly difficult. And they even mostly worked on OS/2 1.2 and 1.1. But I wanted to get the code working on OS/2 1.0 as well. Because why not.

Running the Open Watcom debugger on OS/2 1.1
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Posted in Development, IBM, Microsoft, OS/2, Watcom | 10 Comments

Failing to Fail

The other day I was going over various versions of the venerable DOS/16M DOS extender from Rational Systems (later Tenberry Software). The DOS/16M development kit comes with a utility called PMINFO.EXE which is meant to give the user some idea about the performance of a system running in protected mode.

I know that the utility has trouble on faster CPUs and I expected it to fail about like this:

DOS/16M PMINFO.EXE failing with a FP division by 0

But running the utility on an older laptop with an Intel Haswell processor, I instead got this:

The same PMINFO.EXE crashing with a #GP fault

Rather than cleanly exiting after catching a floating-point division by zero, the program crashed with a general protection fault. That looks like a bug, but why would it be happening? And where is the bug?

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Posted in Bugs, Intel, PC architecture, x87 | 11 Comments

Unintended Side Effects

I’ve been lately trying to understand and improve the idling behavior of DOS programs and one of my guinea pigs has been the Open Watcom vi editor.

While the 16-bit real-mode DOS variant of the editor idles nicely, the 32-bit DOS extended version that’s actually shipped with the Open Watcom tools just refuses to idle. What’s notable is that it won’t idle with DOS POWER.EXE, with DOSIDLE, and it also won’t idle when run under Windows 9x or XP. And it doesn’t idle with my DR-DOS $IDLE$ driver either.

So I decided to track down what’s going on in the DR-DOS case. What I could see was that the DRIDLE.SYS driver never went into idle mode because every time it decremented the INT 16h idle counter, it was already reset to the initial (maximum) value by a timer tick.

In other words, as far as the DRIDLE.SYS was concerned, the editor wasn’t idle because it was only calling INT 16h very rarely. But I know that the editor polls INT 16h in a tight loop!

While trying to characterize the problem, I also found another oddity: The refusal to idle happens with the (default) DOS/4GW extender. But when the same vi.exe program is run using the CauseWay or DOS/32A DOS extenders (both also shipped with Open Watcom), the editor idles just fine!

Initially I thought there could be some problem specific to the protected-mode version of the editor, since the code is necessarily slightly different from the real-mode variant. But if it were the editor itself, it would behave the same with all DOS extenders, wouldn’t it?

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Posted in Development, DOS Extenders | 1 Comment


I have a laptop. Sometimes I run VMs on it, and some of those VMs run DOS. When a DOS VM does not have any power management, the laptop quickly lets me know by kicking up the fan speed. It is Very Annoying. So annoying that I really want to avoid the situation.

In newer versions of PC DOS and MS-DOS, the included POWER.EXE utility takes care of it, for the most part. But DR-DOS does not ship with any power management utility. That is despite the fact that since version 5.0, DR-DOS has a very promising-sounding IDLE command built in. Sadly, it does nothing:

DR-DOS 6.0 refuses to even try idling

The online help in DR-DOS 5.0/6.0 or Novell DOS 7.0 does not even mention the IDLE command. Scouring the depths of the Internet revealed a potentially helpful document titled Implementing Power Management (BatteryMAX) in DR-DOS.

In short, said document explains that DR-DOS has hooks for power management, but requires a driver called (internally) $IDLE$ to be installed in order to actually do anything. OEMs were supposed to ship such a driver. DRI/Novell/Caldera had a development kit with sample drivers that OEMs could use as a starting point.

But alas, it appears that the development kit didn’t survive, and I failed to find a single release of DR-DOS that would include a power management driver. So close, and yet so far!

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Posted in Computing History, Development, Digital Research | 9 Comments

Tracking Down a Bug

When I first encountered the Unix vi editor many years ago, I recoiled in horror. It was nothing like the editors I was used to—Borland IDEs, DOS 5.0/6.x EDIT, or OS/2 and Windows editors. But over the years, I learned to use vi.

It turns out that on many exotic *nix systems, vi is the only game in town. Sure, there might be some way to install pico or nano or whatever, but even if that is possible, it might be very difficult to get there without editing a couple of files… with vi. And when the system in question is some ancient XENIX or PC/IX or Microport UNIX or whatever, the effort of installing some other editor vastly outweighs the effort of learning vi—and that’s if the system in question even comes with a compiler.

Some time later I started using the vi editor that comes with the (Open) Watcom compilers. It’s a decent vi clone, not great but workable. It has the nice property that there are console versions for DOS, OS/2 (both 32-bit and 16-bit), and NT available (and *nix, too). The DOS version that ships with the compiler is a 32-bit DOS-extended version, but it’s also possible to build a 16-bit version. Which will run on a 286 or even an 8086, and that’s occasionally useful.

Open Watcom vi/286, a 16-bit DOS version with EMS/XMS support
Open Watcom vi/os2, a 16-bit OS/2 version running in OS/2 1.1

I’ve been using the 16-bit DOS version of Open Watcom vi for some time, and it works well… except sometimes it hangs the system. But only sometimes and not very consistently. When it does hang, it usually happens when quitting the editor, but rarely also when starting it up.

I’ve never been able to track down the problem because it doesn’t happen frequently enough and when it does, all I can see that the system ends up in a corrupted state. Until yesterday.

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Posted in Debugging, Development, DOS, Watcom | 14 Comments

First ROM Shadowing

The other day I was asked an interesting question: What was the first BIOS with support for ROM shadowing? In the 1990s, ROM shadowing was common, at first as a pure performance enhancement and later as a functional requirement; newer firmware is stored compressed in ROM and must be decompressed into RAM first, and firmware may also rely on writing to itself before being locked down and write protected.

Old PCs did not use ROM shadowing because it made no sense. ROMs were only marginally slower than RAM, if at all, and RAM was too precious to waste on mirroring the contents of existing ROMs. Over the years, RAM speeds shot up while ROM remained slow. By about 1990, executing BIOS code from ROM incurred a noticeable performance penalty, and at the same time devoting 64 or 128 KB to ROM shadowing was no longer prohibitively expensive.

But who did it first?

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Posted in C&T, Compaq, IBM, PC architecture, PC history | 55 Comments

A wunderBAR Story

Or, what should be broken became solid, and what should be solid became broken.

While searching for something completely different, I came across a fascinating story of the “wunderBAR”. Since the story is very short, I’ll quote it here in full:

It seems that in the sixties, the intent was to have a vertical bar. But the keyboard standard was printed with a dirt on the camera-ready proof so that it was mass-reproduced as a broken bar. Then most manufacturer implementors did not read the book (…) and copied the drawings from the picture of the keyboard. And so we have this character for which nobody has a use, but many code tables contain it and compilers use its code point!

That’s a really interesting story! But… it doesn’t quite ring true. Then again, there could well be something to it, because no one quite seems to know what the broken bar is for.

In fact, the broken bar barely even exists anymore. In the days of DOS, the character used for the pipe symbol (on the DOS command line) or for logical OR (in C/C++, for example) used ASCII code 7Ch (124 decimal), which was rendered as a broken vertical bar by the fonts used at least by the IBM MDA, CGA, EGA, and VGA cards. But nowadays that is no longer the case. The same ASCII codepoint is rendered as a solid vertical bar in Windows 10 or Linux, and also shown as a solid vertical bar on contemporary keyboards. What happened?

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Posted in Computing History, Corrections | 21 Comments

Hash Tables FTW

Over the last few weeks I did a bit of tinkering on a hobby software project. The code was written by two other people during the last several years and I finally managed to find some time to fix a couple of bugs.

The project is writing a functional clone of WGML 4.0 aka Watcom SCRIPT/GML, a text processor that came out of mainframe SCRIPT tradition (which goes back to 1968 or so). WGML is used by the Open Watcom project to produce documentation in a large number of formats: Windows and OS/2 native help files, homegrown Watcom help format, HTML, CHM, and last but not least PostScript/PDF.

Several slightly different WGML executables exist, and run either under OS/2 or 32-bit extended DOS. Running those on modern systems is possible (via DOSBox and such), but quite painful, and creates undesirable dependencies. The source code to WGML was presumably lost decades ago.

“Why don’t you just convert the documentation to some other format” is what a lot of people said, and when asked to show how, the discussion was over. Because it’s actually not that easy. I’m sure it’s not impossible but it’s much harder than it seems. As it turns out, SCRIPT includes a fairly flexible language with macros, variables (symbols), built-in functions, conditionals, and loops, and the documentation (several thousand pages) makes use of all that.

So we’re looking at either a huge amount of extremely boring and mind-numbing work converting the documentation source files, or a huge amount of interesting and challenging work rewriting the WGML text processor. Given that no one is paid for it, only the latter is actually a realistic option.

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Posted in Development | 2 Comments

Windows 3.x VDDVGA

While working on my Windows 3.x display driver, I ran into a vexing problem. In Windows 3.1 running in Enhanced 386 mode, I could start a DOS session and switch it to a window. But an attempt to set a mode in the DOS window (e.g. MODE CO80) would destroy the Windows desktop, preventing further drawing from happening properly. It was possible to recover by using Alt+Enter to switch the DOS window to full screen again and then returning to the desktop, but obviously that wasn’t going to cut it.

Oddly enough, this problem did not exist in Windows 3.0. And in fact it also didn’t exist in Windows 3.1 if I used the Windows 3.0 compatible VDDVGA30.386 VxD shipped with Windows 3.1 (plus the corresponding VGA30.3GR grabber).

There was clearly some difference between the VGA VDD (Virtual Display Driver) in Windows 3.0 and 3.1. The downside of the VDD is that its operation is not particularly well explained in the Windows DDK documentation. The upside is that the source code of VDDVGA.386 (plus several other VDD variants) was shipped with the Windows 3.1 DDK.

First I tried to find out what was even happening. Comparing bad/good VGA register state, I soon enough discovered that the sequencer registers contents changed, switching from chained to planar mode. This would not matter if the driver used the linear framebuffer to access video memory, but for good reasons it uses banking and accesses video memory through the A0000h aperture.

But how could that even happen? The VDD is meant to virtualize VGA registers and not let DOS applications touch the real hardware. Something had to be very wrong.

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Posted in 386, Development, Documentation, Graphics, PC history, Windows | 52 Comments