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To my knowledge, neither MS-DOS nor BIOS offers any kind of API for sound cards. Therefore the concept of a "driver" is absent, as we know it today. Apart from accessories and sample files and Windows-related stuff found in the setup package, what is the essentials needed for a DOS-program to use a sound card?
One thing I've read somewhere and cannot find anymore is that some sound cards are 'inactive' at power-up and need some sort of initialization to work. Can you please comment on this?
The typical way to provide "driver" services to other programs in DOS is to run a TSR (Terminate and Stay Resident) program installing a software interrupt vector such that running DOS programs could invoke this INT for services (see Ralph Brown's Interrupt List).
In the sound context, however, programs would typically do the device I/O directly by reading and writing I/O ports directly, handling interrupts and DMA transfers when relevant. Maybe due, in part at least, to the fact that multimedia services were rapidly evolving.
With no resource management provided by DOS, you would have to detect devices manually which could be a bit tricky and potentially trigger crashes depending on what happened to be installed in the I/O space. It is possible that some devices only monitored a minimal I/O footprint until an init sequence was performed to reduce this risk, but this is not behaviour I recognize from the sound routines I wrote myself for AdLib, Roland MPU-401 MIDI and classic SoundBlaster cards.
Mainly, detection was based on conventional I/O, IRQ and DMA allocations supplemented by environment variable conventions specifying these configuration points.
You basically wrote a piece of code that should only give a certain result in the presense of the expected device (fx setting up the on-board timers on the AdLib card) and ran it blindly against the conventional or specified I/O addresses and saw what dropped out of the sky.
Summary:
Real Sound Blasters don't need a driver to initialise or support them. Clones may need one-time initialisation. Exotic cards may need memory-resident translation layers.
Games use a collection of per-card drivers to 'talk' to the appropriate hardware interfaces. These can be hard-coded into the game, or a collection of external files like in HMI or Miles.
Depending on your situation, one or both of these may apply.
To play Halloween Harry on a real Sound Blaster, no extra files are needed. The SB support is hard-coded.
To play Theme Hospital on a SB Live PCI under MS-DOS, the game uses a third-party Miles driver to abstract the SB card. The card itself requires a Live driver to imitate the hardware interface of an older SB card.
All the answers given here so far are correct, for different scenarios. What may confuse is that there are two distinct phases that both could correctly be called 'drivers'. Let me describe what I mean:
Pretty much everything here applies to both digitised audio output and AdLib/OPL2/OPL3 support equally.
1) Initialisation and support to -provide- the interface
Legitimate, first-party Sound Blaster series of cards are programmed directly through I/O ports. There's a chip on-board known as the 'DSP'* which handles all the data movement to and from the card. If you have a real Sound Blaster, and the game knows how to 'talk Sound Blaster' to the DSP using the interface described in the Sound Blaster Series Hardware Programming Guide then that's all that's needed.
*(Not to be confused with a 'DSP' in later usage which typically provides a programmable effect like reverb.)
If you have a clone card or a third-party 'compatible card', then one of the following applies:
2) Game support to -consume- the interface
Completely independently of the above, is the 'driver' that provides the game's ability to talk to a specific sound card. This could be more precisely called an audio library, but since it also has to talk to the Sound Blaster / Windows Sound System etc DSP processors, it's also a driver. In this respect, a DOS game is sort of like a mini-operating-system into itself.
This driver takes the form of a library of routines that abstract the base primitives of the sound card interface into a useful, consistent set of commands for game developer.
By itself, a Sound Blaster provides a single output stream of audio and FM capability. A Gravis Ultrasound or SB AWE provides a wavetable interface to multiple short looping streams of sound-card-RAM-resident samples (in addition to the SB digitised stream and FM, for the AWE). The PC speaker goes beep.
The game programmer doesn't want to think about this level of detail - they want to start music, play an explosion, etc. It would be the driver's job to abstract these details: begin/stop output, start/stop sound effects, mix them, alter volumes, etc.
Early games would have these drivers coded directly into the game in a sort of ad-hoc fashion - Halloween Harry can only support original Sound Blasters, and the support is hard-coded into the game. Rise of the Triad has its own huge sound library; since RoTT is open source, you can see all of the different initialisation and support routines on Github.
For late, mature MS-DOS games like Theme Hospital, a library like Miles or HMI is used. If you've seen a sound card set up screen with dozens of sound cards available, then they most likely use one of these libraries. I point this out since the different sound card drivers can be shown in a directory listing as .386 or .ovl or .hmi files. Epic MegaGames Jensen-library games like One Must Fall 2097 and Jazz Jackrabbit store their sound card drivers in MDRV---R.MUS files.
The sound card drivers in 1) would be provided on an installation disk with your sound card, if required.
The sound card drivers in 2) would be provided with or part of the games themselves.
Most PCI soundcards do not have hardware support for games and other applications that expect a SoundBlaster or AdLib to be present. Older cards made a special effort to provide what's known as "register level compatibility", so they could be used with a wide range of existing games. By the time PCI arrived, Windows had become the PC operating system of choice, so compatibility with DOS games was less important at the hardware level.
The DOS "driver" for these newer cards is actually emulation software, which intercepts accesses to the I/O ports normally occupied by Adlib and SB hardware, and converts them into commands to the actual soundcard present. This may include performing audio synthesis and/or mixing in software.
Like any hardware, the hardware of a sound card needs to be “prepared for operation” after having powered up in an unconfigured state.
Usually this consists of writing certain values to certain hardware ports and/or memory addresses (after testing for the presence of said sound card). After this, the sound card is ready for operation.
In Windows or any other modern Operating Systems this is done by a driver upon booting the operating system and scanning for any hardware that’s present. In DOS this configuration is (usually) done by the game or application utilizing the sound card when it is started. Before the application starts, the hardware is not yet configured.
When the Sound Blaster was first introduced, the documented way to use the digitized audio features was to make use of a supplied blob of code which was supplied by Creative Labs. If memory serves, using this blob of code required reading it into RAM at a multiple-of-16 address, and invoke it with a normalized form of that address (offset zero of whatever segment it happened to start at). If memory serves, the MIDI interface was defined in terms of I/O port operations, and the documentation may have specified how code which was able to run fast enough could output individual samples to an I/O port without using DMA [which ended up being how many programs actually used the SoundBlaster], but I think Creative Labs' expectation was that people would use the supplied blob of code. I don't think it was clear, however, whether they expected that programmers would always put that blob in a file with a certain name in a certain place so as to allow it to be replaced with alternative implementations, or how much space they expected programmers to allocate for it.
