I recently got a Thinkpad A21m as a gift, and having so many computers already, I couldn’t figure out what to do with it. Then I learned that its sound device, a Cirrus Logic Crystal SoundFusion CS4624 PCI, is Sound Blaster Pro-compatible, and actually has a DOS driver, which supports both sound effects and FM music in pure DOS. The DOS driver is included in the Thinkpad Audio driver package 4.12.2884 for Win95/98, but not in the newer 5.12.x packages.

To get audio in pure DOS, the only thing required is running the CWCDOS.EXE application. The driver installer will actually add this into AUTOEXEC.BAT for you. Apparently it also requires a CWCDOS.WCM file, with the contents “20 02 88 03 05 01” in hexadecimal, which is also created by the installer. The DOS audio emulation layer uses either PC/PCI or DDMA, both of which are supported by the Intel 440BX chipset on the A21m.

CWCDOS leaves a small 1K TSR after being run. A few times I’ve been in situations where the audio started glitching, and running CWCDOS again fixed it. For whatever reason, every time you run it, it leaves another copy of itself in the RAM, and I found no switches to uninstall it, so if you do it too often, you may need to reboot as your conventional memory will become too low.

The implementation and quality of FM synthesis varies between different sound cards, and is often a topic of heated debates among retro-gaming enthusiasts. Genuine Yamaha OPL3 chips are overwhelmingly preferred, and others have their pluses and minuses. The CS4624 does not have genuine Yamaha FM, and I was interested to learn how the FM sounds in a variety of different DOS games, to decide whether the A21m is a good choice for DOS retro-gaming or not.

The end results were more positive that I would initially expect. In most games, FM music worked fine, and although it was usually “rough” and often inaccurate, it did not ruin the experience. There were a few games, such as Lemmings, where it sounded very bad, distorted and with wrong tempo (even worse, the tempo seems to be inconsistent, speeding up and slowing down). In other games, it was missing notes badly and slowing down, until I realized that choosing “Adlib” music option instead of “Sound Blaster” fixes it. Apparently, this is a known limitation of the chip with stereo FM music. Fortunately, many problematic games do include a separate “Adlib” option, including the Blizzard titles Lost Vikings and Warcraft and the Build engine games Duke Nukem 3D and Blood. One downside is that in most games the FM is rather quiet (compared to the digital sound effects), and I found no way to adjust it besides whatever volume options are provided by each and every game.

The CS4624 supports MPU-401 pass-through as well, and the Windows 98 drivers include MPU-401 emulation, that allows music to be played back via the “General MIDI” option in games that have it available. It sounds better (or at least noticeably richer and louder than the FM), but does not work in pure DOS as far as I could see, and also there is no way that I know of to connect an actual external MIDI device to the CS4624 in the laptop.

Overall, the CS4624 provide to be a rather capable audio device for DOS games: you get digital effects + FM in pure DOS (assuming the chipset supports PC/PCI or DDMA), and playing from within Windows gives you General MIDI on top of that.

When it comes to music, listening is better than talking about it, so I present the semi-random selection of FM audio samples from various games, recorded from the A21m via direct line-in-to-line-out:
https://cloakedthargoid.wordpress.com/crystal … dfusion-dos-fm/