IBM 5170 Technical Reference page 341

If mod
=
01, then disp
=
disp-low sign-extended to 16 bits,
disp-high is absent.
If mod
=
10, then disp
=
disp-high:disp-low.
If
rim
=
000, then EA
=
(BX)
+
(SI)
+
disp
If
rim
=
001, then EA
=
(BX)
+
(SI)
+
disp
If
rim
=
010, then EA
=
(BP)
+
(SI)
+
disp
If
rim
=
011, then EA
=
(BP)
+
(01)
+
disp
If
rim
=
100, then EA
=
(SI)
+
disp
If
rim
=
101, then EA
=
(01)
+
disp
If
rim
=
110, then EA
=
(BP)
+
disp
If
rim
=
111, then EA
=
(BX)
+
disp
disp follows the second byte of the instruction (before data if
required).
Segment Override Prefix
Segment Override Prefix
I
001 reg001
~
reg is assigned as follows:
reg Segment Register
00 ES
01 CS
10 SS
11 OS
16-bit (w = 1) 8-bit (w=O)
000 AX
000 AL
001 CX 001 CL
010 OX
0100L
011 BX 011 BL
100 SP 100AH
101 BP 101 CH
110 SI 1100H
111 01
111 BH
Instruction Set 6-23