What is correct syntax in boot.config fo GPT partitions?

[Peter Steele]

>I've used the syntax
>
>1:ad(1,a)/boot/loader
>
>in boot.config to specify the boot device. This doesn't work with GPT partitions. What's the correct syntax in boot.config for GPT partitions?

I looked at the source code to boot.c and there doesn't seem to be anything specifically related to GPT partitioning. So I'm puzzled: If I have a two drive system with BSD loaded on both drives and the drives are configured with GPT partitions, how can I force the system to boot from the second drive using boot.config? I've done with this MBR partitioning using either

0:ad(0,a)/boot/loader
1:ad(1,a)/boot/loader

to specify either the first disk or the second disk. I've tried various incarnations of this to select with drive to boot from in my GPT based system but nothing works. My impression is that it isn't supported, except for MBR partitioning. Is this true?