Stabilizer of CPU core voltage powers from 5v rail !Gigabyte 7VAXP Ultra MB
ATTENTION AMD Athlon XP 2800+ (333FSB) powers from 5v rail !!! I wrote about it in this thread Amd Athlon Xp 2200 also there i have described how to determine it.
Only new mobos powers AMD 64 from 12V
All socket A mobos powers cpu from 5v rail - it's ATX 1.1 standard