ASUS seems to have enabled AMD Smart Access Memory on the first generation Ryzen CPUs on its B450 lineup of motherboards.
ASUS and MSI demonstrate Resizeable BAR (AMD Smart Access Memory
AMD Smart Access Memory (Resizable BAR) Guide
Asus brings AMD Smart Access Memory to Intel platforms - Mainboard
AMD Ryzen 5000 CPU & Smart Access Memory Support on X470 & B450
ASUS Prime B450M-A II AMD AM4 (Ryzen 5000, 3rd/2nd/1st
Radeon Software Adrenalin 21.9.1 Enables Smart Access Memory On
MSI Confirms Resizable-BAR (AMD Smart Access Memory) Support For
Smart Access Memory on 400 series Mobo + 2700x + 5700xt - Wait
GIGABYTE's Latest BIOS Update on AMD 500 Series Motherboards
AMD Smart Technologies
ASUS Prime B450M-A II AMD AM4 (Ryzen 5000, 3rd/2nd/1st