|For a long time now it has been possible to select the boot drive from the BIOS without changing jumpers regardless of whether the drives are P-ATA or S-ATA. |
The necessity to boot from drive C: was a throw back to MS-DOS and with the demise of MS-DOS With Windows 2000/XP, it is no longer a necessity.
To be strictly accurate the BIOS never boots from drive C: becasue the BIOS has no knowledge of drive letters, That is strictly something determined by the operating system at boot time. The BIOS boots from the the first primary bootable partition it finds on the designated drive . That nearly always just happens to be drive C: although it could be any drive letter that Windows allocates to it.
In the days a MS-DOS and MS-DOS based operating system like Windows 98, the BIOS attempted to to boot from the first primary partition in the system, which was invariably drive C:, and it would ignore any other primary partitions.
In those days a drive could only have one active primary partition. No so with modern computers. These days you can have four primary partitions on the same drive, all bootable if you chose to make it so. In that case you would need some kind of boot manager in order to determine which partition to boot from.