Hardware resources¶

Traditionally, DOS had no facilities to detect and auto-configure connected hardware — plug and play (PnP), familiar from modern operating systems, only started appearing at the very end of the DOS era. Expansion cards like sound cards and network adapters each occupy a set of hardware resources: an I/O base address, and often an IRQ and DMA channel. On real hardware, these values were most often set via physical jumpers, and since a typical machine had several expansion cards configured this way, care had to be taken to give each card its own resources and avoid conflicts — getting them wrong was one of the most common sources of frustration in DOS.

When a game or driver asks you to configure a device, these values must match the DOSBox Staging configuration exactly — otherwise you’ll get silence, garbled audio, or a crash. DOSBox Staging uses sensible defaults, but you still need to enter the right values when a game’s setup utility asks.

What these settings mean¶

I/O base address (also called port) is a hardware address the CPU uses to communicate with a device — think of it as a “mailbox number” for the card. The CPU sends commands to this address and reads data back from it. Each device needs its own unique address. Common examples: 220 for the Sound Blaster, 240 for the Gravis UltraSound, 300 for the NE2000 network card. I/O base addresses are hexadecimal numbers — letters from A to F can also appear in addition to decimal digits. 220 is sometimes also written as 220h, 220H, or 0x220.

IRQ (Interrupt Request) is how a device signals the CPU that it needs attention — for example, when a sound card has finished playing a buffer and needs more data. Each device needs its own IRQ number; sharing an IRQ between two active devices will cause one or both to malfunction. Unlike I/O base addresses, IRQ numbers are plain decimal numbers.

DMA (Direct Memory Access) lets a device transfer data directly to or from system memory without involving the CPU for every byte. Sound cards use DMA to stream audio data efficiently. Like IRQ numbers, DMA channel numbers are decimal. The original IBM PC provided DMA channels 0–3 for 8-bit transfers; the IBM PC AT added channels 5–7 for 16-bit transfers (channel 4 is reserved for internal use).

High DMA is used only by Sound Blaster 16 and later cards for 16-bit audio. These cards need two DMA channels: a low DMA channel for 8-bit audio backward compatibility with older games, and a high DMA channel for native 16-bit audio. If a game’s setup asks for “16-bit DMA”, “High DMA”, or “HDMA”, this is what it means. Only channels 5, 6, and 7 are usable as a high DMA channel.

Default resource assignments¶

Configurable devices¶

These devices have I/O, IRQ, and/or DMA settings you can change in the DOSBox Staging configuration:

Device	I/O base	IRQ	DMA	High DMA	Config settings
Sound Blaster	220	7	1	5	`sbbase`, `irq`, `dma`, `hdma`
Gravis UltraSound	240	5	3	—	`gusbase`, `gusirq`, `gusdma`
IBM Music Feature Card	2A20	3	—	—	`imfc_base`, `imfc_irq`
NE2000 Ethernet	300	3	—	—	`nicbase`, `nicirq`

If you’re using the defaults and a game’s setup utility asks for sound card settings, just enter the values from the table above. Most games only ask for Address, IRQ, and DMA; if one also asks for High DMA (sometimes labelled “16-bit DMA” or “HDMA”), use the High DMA column.

Devices with fixed settings¶

These devices use hardcoded resource assignments that cannot be changed in the configuration. If a game asks the settings of these devices, use the values from this table.

Device	I/O base	IRQ	DMA
AdLib / OPL (FM synth)	388	—	—
MPU-401 (MIDI interface)	330	9	—
Game port (joystick)	201	—	—
Tandy DAC	C4	7	1
Tandy / PCjr PSG	C0	—	—
PC speaker	61	—	—
Parallel port DACs (Covox, Disney, Stereo-on-1)	378	—	—
Innovation SSI-2001	280	—	—
IBM PS/1 Audio	200–205	—	—
Serial ports (COM1 / COM2 / COM3 / COM4)	3F8 / 2F8 / 3E8 / 2E8	4 / 3 / 4 / 3	—

Note

The AdLib’s fixed I/O address at 388 is separate from the Sound Blaster’s configurable base address. Games can access the OPL chip through either address; this is how the Sound Blaster maintains AdLib backward compatibility.

DMA and IRQ conflict resolution¶

DOSBox Staging automatically resolves DMA conflicts between the Gravis UltraSound, Sound Blaster, and Tandy DAC. If enabling a device would make it share a DMA channel with a device that’s already active, the newly-enabled device takes over that channel and the previously-active device is automatically disabled — the last device enabled wins. The logs inform you about this.

The most common case is the Tandy DAC and Sound Blaster, which share the same default DMA channel (1) and IRQ (7). For example, with machine = tandy, the Tandy DAC uses DMA 1. If you then enable sbtype = sb1, the Sound Blaster claims DMA 1 and the Tandy DAC is automatically disabled, leaving the Sound Blaster active. The same last-enabled-wins behaviour applies to conflicts between the Sound Blaster and the Gravis UltraSound.

If you’d rather keep the Tandy 3-voice PSG music working alongside the Sound Blaster — rather than relying on auto-disable to pick one — set tandy = psg in the [speaker] section. This keeps the Tandy PSG (which has no DMA or IRQ requirements) but disables the Tandy DAC outright, freeing DMA 1 and IRQ 7 for the Sound Blaster from the start.

[speaker]
tandy = psg

[sblaster]
sbtype = sbpro2

Additionally, the Tandy PSG occupies I/O ports starting at C0, which overlaps with the second DMA controller’s address range. DOSBox Staging handles this by shutting down the second DMA controller (channels 4–7) when Tandy sound is active. This means high DMA is unavailable on Tandy/PCjr machines, so Sound Blaster 16 16-bit audio will not work. Use sbtype = sbpro2 or earlier when combining Sound Blaster with Tandy sound.

Common issues¶

Game asks for settings not shown in its setup¶

Some games only offer a limited set of IRQ or DMA values. If the DOSBox defaults aren’t among the options, change the DOSBox Staging config to match what the game expects. For example, if a game only offers IRQ 5 for the Sound Blaster, set irq = 5 in the [sblaster] section.

The same applies to I/O base addresses. If a game only supports address 240 for its Sound Blaster setup, set sbbase = 240 in the [sblaster] section.

No sound despite correct settings¶

Double-check that the I/O address, IRQ, and DMA channel all match between the game’s configuration and the DOSBox Staging config. Many games silently fail if even one value is wrong. Also verify that the correct sbtype is selected — some games only work with specific Sound Blaster models.

For Sound Blaster 16 games with no 16-bit audio, make sure the high DMA channel is also set correctly in both the game and DOSBox Staging config.

Garbled or stuttering audio¶

This can happen when two emulated devices share the same IRQ or DMA channel. Ensure each enabled device uses unique values. Check the tables above for potential conflicts, especially if you have multiple sound devices enabled.

Games that hardcode settings¶

A number of early DOS games (roughly 1988–1992) assume fixed Sound Blaster settings and don’t provide a proper setup utility, or only partially detect the card. The most commonly hardcoded values are I/O 220, IRQ 7, DMA 1 — the Sound Blaster’s original factory defaults, and also the DOSBox Staging defaults. If a game with no setup option produces no sound, these assumptions are usually the reason, and the defaults should work. Some games and demos assume the factory defaults on the Gravis UltraSound, too.

This assumption became a lasting problem because Creative Labs later changed the Sound Blaster’s factory-default IRQ from 7 to 5, partly to avoid conflicts with the parallel port (also commonly on IRQ 7). By then hundreds of games had already shipped assuming IRQ 7, and many never gained a way to reconfigure it. Some games only check for a couple of hardcoded IRQ values instead of reading the actual configuration — Stellar 7, for instance, only auto-detects IRQ 3 or 7.

Symptoms aren’t always total silence — some games drop specific samples or effects rather than failing outright. Gods, for example, is known to lose samples if the Sound Blaster IRQ isn’t set to 7.

If a game with no setup option produces no sound, or drops samples or music unexpectedly, try DOSBox Staging’s defaults before troubleshooting further.