IBM AT 5170 Technical Reference page 81

Low and High Expansion Memory Bytes (Hex 17 and 18)
Bit 7-Bit 0 Address hex 17-Low-byte expansion size
Bit 7-Bit 0 Address hex 18-High-byte expansion size
Valid Sizes:
0200H
0400H
0600H
3COOH
512K-1/0 adapter
1024K-I/0 adapter (2 adapters)
1536K-I/0 adapter (3 adapters)
through
15360K I/O adapter (I5M
maximum).
Drive C Extended Byte (Hex 19)
Bit 7-Bit 0
Defines the type of first fixed disk drive installed
(drive C):
00000000 through 00001111 are reserved.
00010000 to 11111111 define type 16
through 255 as shown in the following table
(see BIOS listing at label FD_TBL).
Drive D Extended Byte (Hex lA)
Bit 7-Bit 0 Defines the type of second fixed disk drive
installed (drive D):
00000000 through 00001111 are reserved.
00010000 to 11111111 define type 16
through 255 as shown in the following table
(see BIOS listing at label FD_TBL).
System Board
1-65