Bone spi bus numbers

In the board support file (am335xevm.c) for the bone, the spi0 struct
has 1 as the bus number, and spi1 has 2?? I tried using just spi1
with bus 1 and nothing....but if I use 2 it works. So spi1 is using
bus 2? How are the bus numbers for spi_board_info.bus_num determined?

Thx!

Hardware starts at 0. SW starts at 1.

Gerald