I am trying to find out if the Auxiliary carry flag only turns on when the 4th bit carries out from the 3rd bit or if it will turn on for the 7th to 8th bit aswell. This is what I did already:

mov ax, 0Fh ; this is the test for the 3rd to 4th
add ax, 1 ; AC=1


mov ax, 00F0h ; this is the test for the 7th to 8th
add ax, 0010h ; not sure if this should set AC=1

The Auxiliary Carry (AF bit of Status Flags) is used for the carry out of bit 3 (0...7) into bit 4 for use in BCD operations! Think of it as a nibble carry!