SATA boot query with 2 drives installed with OS's

Hi All, new to your lovely forum.

Having only ever built PC's using IDE disks etc, I'm unfamiliar with sata disk selection.

I'm just building a system for my partners Gigabyte motherboard, and all is well so far, having managed to download the drivers from Gigabytes website.

My questions is -

As there is obviously no master/slave setting on SATA drives, how does the bios know which SATA drive to book from. I currently have both (older and newer) installed and both have XP SP3 installed,

The Advanced Setting in the bios only gives the boot option of "Hard Drive" with no selection as to which one to actually boot from when there are two installed.

Is it as simple as it will always look at a specific physical SATA port first ?

Many thanks................ebt