I think I am using VIA SATA controller.
From Asus download page, I found following downloads related with VIA:
(1) VIA VT6420 (VT8237) SATA RAID Driver Package Version 2.20D WHQL.
I am using 2 SATA hard drives independently, not in raid, so I guess this is not useful for me.
(2) VIA 4 in 1 driver Package Service v4.49P3
Looks the extracted files are executable files running under windows. So where is the driver for DOS mode running? I suppose the floppy disk driver used for Window installation shouls be DOS application.