Wikipedia
Busdma
In computing, busdma is a set of application programming interfaces designed to help make device drivers less dependent on platform-specific code, thereby allowing the host operating system to be more easily ported to new computer hardware.
Originally implemented as the "bus_dma" APIs by the developers of the NetBSD operating system, busdma has been adopted by OpenBSD, FreeBSD and their derivatives. Both NetBSD and OpenBSD have additional "bus_space" APIs that have been amalgamated into the version of busdma incorporated into FreeBSD. DragonFly BSD developers are also slowly converting their drivers to use busdma.