IBM AT 5170 Technical Reference page 78

Fixed Disk Type Byte (Hex 12)
Bit 7-Bit 4
Bit 3-Bit 0
Defines the type of first fixed disk drive installed
(drive C):
0000 No fixed disk drive is present.
0001 Define type 1 through type 14 as shown
to
in the following table (also see BIOS
1110 listing at label FD_TBL)
1111 Type 16 through 255. See "Drive C
Extended Byte (Hex 19)" on page 1-65 .
Defines the type of second fixed disk drive
installed (drive D):
0000 No fixed disk drive is present.
0001 Define type 1 through type 14 as shown
to
in the following table (also see BIOS
1110 listing at label FD_TBL)
1111 Type 16 through 255. See "Drive D
Extended Byte (Hex lA)" on page 1-65 .
1-62 System Board