Tak to máš nějaký úplně speciální BIOS. Pro normální BIOS píšou v manuálu pro desku ASUS A7V-M:

Boot Sequence
The Boot menu allows you to select among the four possible types of boot
devices listed using the up and down arrow keys. By using the <+> or <Space>
key, you can promote devices and by using the <-> key, you can demote
devices. Promotion or demotion of devices alters the priority which the sys-
tem uses to search for a boot device on system power up. Configuration fields
include Removable Devices, IDE Hard Drive, ATAPI CD-ROM, and Other
Boot Device.