Taken from a post by Thomas Frieden on the Amigaworld.net community portal:
<i>Just to let you know: OS4 supports Altivec since today (I had nothing else to do today )
I went for the "simple" approach: When a task starts, it's launched with Altivec disabled, thus no vector registers are stored during context switch. The first altive instruction then generates an exception, and the enable bit is set. From then on, the task saves/restores Altivec context whenever it is scheduled. vrsave is used to keep track of the registers.</i>