IBM AT 5170 Technical Reference page 389

The 2-bit and 3-bit reg fields are defined as follows:
2-Bit reg Field
reg Segment Register
reg
Segment Register
00 ES 10 SS
01 CS
11
DS
3-Bit reg Field
16-bit
(w
=
1)
a-bit
(w
=
0)
000 AX
000 AL
001 CX
001 CL
010 DX
010 DL
011 BX 011 BL
100 SP 100 AH
101 BP
101 CH
110 SI 110 DH
111 D I
111 BH
The physical addresses of all operands addressed by the BP
register are computed using the SS segment register. The physical
addresses of the destination operands of the string primitive
operations (those addressed by the DI register) are computed
~
using the ES segment, which may not be overridden.
Instruction Set 6-21