This is the broadcom softmac driver. The aim is to support the BCM43xx chipsets which use the v4 firmware. The earlier BCM43xx chipsets (BCM4301, BCM4303, BCM4306) have less RAM and need to use the bwi(4) driver with the v3 firmware.
There was a very large reverse engineering effort a few years ago by the linux development community.
A lot of new code needs to be reimplemented from the linux b43 driver to avoid GPL issues.
The rough order of things so far is: