IBM AT 5170 Technical Reference page 388

The effective address (EA) of the memory operand is computed
according to the mod and
rim
fields:
If mod
=
11, then
rim
is treated as a reg field.
If mod
=
00, then disp
=
0, disp-low and disp-high are
absent.
r-..,
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)
+
(DI)
+
DISP
If
rim
= 100, then EA = (SI)
+
DISP
If
rim
=
101, then EA
=
(DI)
+
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).
Note: An exception to the above statements occurs when
mod=OO and
r/m=
110, in which case EA
=
disp-high;
disp-low.
Segment Override Prefix
00lregOOl
6-20 Instruction Set